SECTION 1F
ENGINE CONTROLS
DESCRIPTION AND OPERATION
Ignition System Operation
This ignition system does not use a conventional distributor and coil. It uses a crankshaft position sensor input to the engine control module (ECM). The ECM then determines Electronic Spark Timing (EST) and triggers the direct ignition system ignition coil.
This type of distributorless ignition system uses a "waste spark" method of spark distribution. Each cylinder is paired with the cylinder that is opposite it (1-4 or 2-3). The spark occurs simultaneously in the cylinder coming up on the compression stroke and in the cylinder coming up on the exhaust stroke. The cylinder on the exhaust stroke requires very little of the available energy to fire the spark plug. The remaining energy is available to the spark plug in the cylinder on the compression stroke.
These systems use the EST signal from the ECM to control the electronic spark timing. The ECM uses the following information:
- Engine load (manifold pressure or vacuum).
- Atmospheric (barometric) pressure.
- Engine temperature.
- Intake air temperature.
- Crankshaft position.
- Engine speed (rpm).
Electronic Ignition System Ignition Coil
The Electronic Ignition (EI) system ignition coil provides the spark for two spark plugs simultaneously. The EI system ignition coil is not serviceable and must be replaced as an assembly.
Crankshaft Position Sensor
This direct ignition system uses a magnetic crankshaft position sensor. This sensor protrudes through its mount to within approximately 0.05 inch (1.3 mm) of the crankshaft reluctor. The reluctor is a special wheel attached to the crankshaft or crankshaft pulley with 58 slots machined into it, 57 of which are equally spaced in 6 degree intervals. The last slot is wider and serves to generate a "sync pulse." As the crankshaft rotates, the slots in the reluctor change the magnetic field of the sensor, creating an induced voltage pulse. The longer pulse of the 58th slot identifies a specific orientation of the crankshaft and allows the engine control module (ECM) to determine the crankshaft orientation at all times. The ECM uses this information to generate timed ignition and injection pulses that it sends to the ignition coils and to the fuel injectors.
Camahaft Position Sensor
The Camshaft Position (CMP) sensor sends a CMP sensor signal to the engine control module (ECM). The ECM uses this signal as a "sync pulse" to trigger the injectors in the proper sequence. The ECM uses the CMP sensor signal to indicate the position of the #1 piston during its power stroke. This allows the ECM to calculate true sequential fuel injection mode of operation. If the ECM detects an incorrect CMP sensor signal while the engine is running, DTC P0341 will set. If the CMP sensor signal is lost while the engine is running, the fuel injection system will shift to a calculated sequential fuel injection mode based on the last fuel injection pulse, and the engine will continue to run. As long as the fault is present, the engine can be restarted. It will run in the calculated sequential mode with a 1-in-6 chance of the injector sequence being correct.
Idle Air System Operation
The idle air system operation is controlled by the base idle setting of the throttle body and the Idle Air Control (IAC) valve.
The engine control module (ECM) uses the IAC valve to set the idle speed dependent on conditions. The ECM uses information from various inputs, such as coolant temperature, manifold vacuum, etc., for the effective control of the idle speed.
Fuel Control System Operation
The function of the fuel metering system is to deliver the correct amount of fuel to the engine under all operating conditions. The fuel is delivered to the engine by the individual fuel injectors mounted into the intake manifold near each cylinder.
The two main fuel control sensors are the Manifold Absolute Pressure (MAP) sensor, the Front Heated Oxygen Sensor (HO2S1) and the Rear Heated Oxygen Sensor (HO2S2).
The MAP sensor measures or senses the intake manifold vacuum. Under high fuel demands the MAP sensor reads a low vacuum condition, such as wide open throttle. The engine control module (ECM) uses this information to richen the mixture, thus increasing the fuel injector on-time, to provide the correct amount of fuel. When decelerating, the vacuum increases. This vacuum change is sensed by the MAP sensor and read by the ECM, which then decreases the fuel injector on-time due to the low fuel demand conditions.
HO2S Sensors
The HO2S sensor is located in the exhaust manifold. The HO2S sensor indicates to the ECM the amount of oxygen in the exhaust gas and the ECM changes the air/fuel ratio to the engine by controlling the fuel injectors. The best air/fuel ratio to minimize exhaust emissions is 14.7 to 1, which allows the catalytic converter to operate most efficiently. Because of the constant measuring and adjusting of the air/fuel ratio, the fuel injection system is called a "closed loop" system.
The ECM uses voltage inputs from several sensors to determine how much fuel to provide to the engine. The fuel is delivered under one of several conditions, called "modes."
Starting Mode
When the ignition is turned ON, the ECM turns the fuel pump relay on for two seconds. The fuel pump then builds fuel pressure. The ECM also checks the Engine Coolant Temperature (ECT) sensor and the Throttle Position (TP) sensor and determines the proper air/fuel ratio for starting the engine. This ranges from 1.5 to 1 at -97 °F (-36 °C) coolant temperature to 14.7 to 1 at 201 °F (94 °C) coolant temperature. The ECM controls the amount of fuel delivered in the starting mode by changing how long the fuel injector is turned on and off. This is done by "pulsing" the fuel injectors for very short times.
Clear Flood Mode
If the engine floods with excessive fuel, it may be cleared by pushing the accelerator pedal down all the way. The ECM will then completely turn off the fuel by eliminating any fuel injector signal. The ECM holds this injector rate as long as the throttle stays wide open and the engine is below approximately 400. If the throttle position becomes less than approximately 80 percent, the ECM returns to the starting mode.
Run Mode
The run mode has two conditions called "open loop" and "closed loop."
Open Loop
When the engine is first started and it is above 400 rpm, the system goes into "open loop" operation. In "open loop," the ECM ignores the signal from the HO2S and calculates the air/fuel ratio based on inputs from the ECT sensor and the MAP sensor. The sensor stays in "open loop" until the following conditions are met:
- The HO2S sensor has a varying voltage output, showing that it is hot enough to operate properly.
- The ECT sensor is above a specified temperature.
- A specific amount of time has elapsed after starting the engine.
Closed Loop
The specific values for the above conditions vary with different engines and are stored in the Electronically Erasable Programmable Read-Only Memory (EEPROM). When these conditions are met, the system goes into "closed loop" operation. In "closed loop," the ECM calculates the air/fuel ratio (fuel injector on-time) based on the signal from the oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7 to 1.
Acceleration Mode
The ECM responds to rapid changes in throttle position
and airflow and provides extra fuel.
Deceleration Mode
The ECM responds to changes in throttle position and
airflow and reduces the amount of fuel. When deceleration
is very fast, the ECM can cut off fuel completely for
short periods of time.
Battery Voltage Correction Mode
When battery voltage is low, the ECM can compensate
for a weak spark delivered by the ignition module by using
the following methods:
- Increasing the fuel injector pulse width.
- Increasing the idle speed rpm.
- Increasing the ignition dwell time.
Fuel Cut-Off Mode
No fuel is delivered by the fuel injectors when the ignition is OFF. This prevents dieseling or engine run-on. Also, the fuel is not delivered if there are no reference pulses received from the central power supply. This prevents flooding.
Evaporative Emission Control System Operation
The basic Evaporative (EVAP) Emission control system used is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors when the vehicle is not operating. When the engine is running, the fuel vapor is purged from the carbon element by intake airflow and consumed in the normal combustion process.
Gasoline vapors from the fuel tank flow into the tube labeled
TANK. These vapors are absorbed into the carbon.
The canister is purged by the engine control
module (ECM) when the engine has been running for a
specified amount of time. Air is drawn into the canister
and mixed with the vapor. This mixture is then drawn
into the intake manifold.
The ECM supplies a ground to energize the EVAP emission
canister purge solenoid valve. This valve is Pulse
Width Modulated (PWM) or turned on and off several
times a second. The EVAP emission canister purge
PWM duty cycle varies according to operating conditions
determined by mass airflow, fuel trim, and intake
air temperature.
Poor idle, stalling, and poor driveability can be caused by the following conditions:
- An inoperative EVAP emission canister purge solenoid valve.
- A damaged canister.
- Hoses that are split, cracked, or not connected to the proper tubes.
Evaporative Emission Canister
The Evaporative (EVAP) Emission canister is an emission
control device containing activated charcoal granules.
The EVAP emission canister is used to store fuel
vapors from the fuel tank. Once certain conditions are
met, the engine control module (ECM) activates the
EVAP canister purge solenoid, allowing the fuel vapors
to be drawn into the engine cylinders and burned.
Positive Crankcase Ventilation System Operation
A Positive Crankcase Ventilation (PCV) system is used to provide complete use of the crankcase vapors. Fresh air from the air cleaner is supplied to the crankcase. The fresh air is mixed with blowby gases which are then passed through a vacuum hose into the intake manifold.
Periodically inspect the hoses and the clamps. Replace any crankcase ventilation components as required.
A restricted or plugged PCV hose may cause the following conditions:
- Rough idle
- Stalling or low idle speed
- Oil leaks
- Oil in the air cleaner
- Sludge in the engine
A leaking PCV hose may cause the following conditions:
- Rough idle
- Stalling
- High idle speed
Engine Coolant Temperature Sensor
The Engine Coolant Temperature (ECT) sensor is a
thermistor (a resistor which changes value based on
temperature) mounted in the engine coolant stream.
Low coolant temperature produces a high resistance
(100,000 ohms at -40 °F [-40 °C]) while high temperature
causes low resistance (70 ohms at 266 °F [130 °C]).
The engine control module (ECM) supplies 5 volts to the
ECT sensor through a resistor in the ECM and measures
the change in voltage. The voltage will be high
when the engine is cold, and low when the engine is hot.
By measuring the change in voltage, the ECM can determine
the coolant temperature. The engine coolant
temperature affects most of the systems that the ECM
controls. A failure in the ECT sensor circuit should set a
diagnostic trouble code P0117 or P0118. Remember,
these diagnostic trouble codes indicate a failure in the
ECT sensor circuit, so proper use of the chart will lead
either to repairing a wiring problem or to replacing the
sensor to repair a problem properly.
Throttle Position Sensor
The Throttle Position (TP) sensor is a potentiometer
connected to the throttle shaft of the throttle body. The
TP sensor electrical circuit consists of a 5 volt supply
line and a ground line, both provided by the engine control
module (ECM). The ECM calculates the throttle
position by monitoring the voltage on this signal line. The
TP sensor output changes as the accelerator pedal is
moved, changing the throttle valve angle. At a closed
throttle position, the output of the TP sensor is low,
about 0.5 volt. As the throttle valve opens, the output increases
so that, at Wide Open Throttle (WOT), the output
voltage will be about 5 volts.
The ECM can determine fuel delivery based on throttle
valve angle (driver demand). A broken or loose TP sensor
can cause intermittent bursts of fuel from the injector
and an unstable idle, because the ECM thinks the
throttle is moving. A problem in any of the TP sensor circuits
should set a diagnostic trouble code (DTC) P0121
or P0122. Once the DTC is set, the ECM will substitute a
default value for the TP sensor and some vehicle performance
will return. A DTC P0121 will cause a high idle
speed.
Catalyst Monitor Oxygen Sensors
Three-way catalytic converters are used to control emissions
of hydrocarbons (HC), carbon monoxide (CO),
and oxides of nitrogen (NOx). The catalyst within the
converters promotes a chemical reaction. This reaction
oxidizes the HC and CO present in the exhaust gas and
converts them into harmless water vapor and carbon
dioxide. The catalyst also reduces NOx by converting it
to nitrogen. The engine control module (ECM) can monitor
this process using the HO2S1 and HO2S2 sensor.
These sensors produce an output signal which indicates
the amount of oxygen present in the exhaust gas entering
and leaving the three-way converter. This indicates
the catalyst's ability to efficiently convert exhaust
gasses. If the catalyst is operating efficiently, the
HO2S1 sensor signals will be more active than the signals
produced by the HO2S2 sensor. The catalyst monitor
sensors operate the same way as the fuel control sensors.
The sensor’s main function is catalyst monitoring,
but they also have a limited role in fuel control. If a sensor
output indicates a voltage either above or below the
450 mv bias voltage for an extended period of time, the
ECM will make a slight adjustment to fuel trim to ensure
that fuel delivery is correct for catalyst monitoring.
A problem with the HO2S1 sensor circuit will set DTC
P0131, P0132, P0133 or P0134 depending, on the special
condition. A problem with the HO2S2 sensor signal
will set DTC P0137, P0138, P0140 or P0141, depending
on the special condition.
A fault in the Rear Heated Oxygen Sensor (HO2S2) heater element
or its ignition feed or ground will result in lower oxygen
sensor response. This may cause incorrect catalyst
monitor diagnostic results.
Electronic Exhaust Gas Recirculation Valve
The Electronic Exhaust Gas Recirculation (EEGR) system is used on engines equipped with an automatic transaxle to lower NOx (oxides of nitrogen) emission levels caused by high combustion temperature. The EEGR valve is controlled by the engine control module (ECM). The EEGR valve feeds small amounts of exhaust gas into the intake manifold to decrease combustion temperature. The amount of exhaust gas recirculated is controlled by signal of ECM. If too much exhaust gas enters, combustion will not take place. For this reason, very little exhaust gas is allowed to pass through the valve, especially at idle.
The EEGR valve is usually open under the following conditions:
- Warm engine operation.
- Above idle speed.
Results of Incorrect Operation
Too much EGR flow tends to weaken combustion, causing the engine to run roughly or to stop. With too much EGR flow at idle, cruise, or cold operation, any of the following conditions may occur:
- The engine stops after a cold start.
- The engine stops at idle after deceleration.
- The vehicle surges during cruise.
- Rough idle.
If the EGR valve stays open all the time, the engine may not idle. Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause the following conditions:
- Spark knock (detonation)
- Engine overheating
- Emission test failure
Intake Air Temperature Sensor
The Intake Air Temperature (IAT) sensor is a thermistor, a resistor which changes value based on the temperature of the air entering the engine. Low temperature produces a high resistance (4,500 ohms at -40°F [-40°C]), while high temperature causes a low resistance (70 ohms at 266°F [130°C]).
The engine control module (ECM) provides 5 volts to the
IAT sensor through a resistor in the ECM and measures
the change in voltage to determine the IAT. The voltage
will be high when the manifold air is cold and low when the air is hot. The ECM knows the intake IAT by measuring
the voltage.
The IAT sensor is also used to control spark timing when the manifold air is cold.
A failure in the IAT sensor circuit sets a diagnostic trouble code P0112 or P0113.
Manifold Absolute Pressure Sensor
The Manifold Absolute Pressure (MAP) sensor measures the changes in the intake manifold pressure which result from engine load and speed changes. It converts these to a voltage output.
A closed throttle on engine coast down produces a relatively
low MAP output. MAP is the opposite of vacuum.
When manifold pressure is high, vacuum is low. The MAP sensor is also used to measure barometric pressure.
This is performed as part of MAP sensor calculations.
With the ignition ON and the engine not running,
the engine control module (ECM) will read the manifold
pressure as barometric pressure and adjust the air/fuel
ratio accordingly. This compensation for altitude allows
the system to maintain driving performance while holding
emissions low. The barometric function will update
periodically during steady driving or under a wide open
throttle condition. In the case of a fault in the barometric
portion of the MAP sensor, the ECM will set to the default
value.
A failure in the MAP sensor circuit sets a diagnostic trouble code P0107 or P0108.
The following tables show the difference between absolute pressure and vacuum related to MAP sensor output, which appears as the top row of both tables.
Main Throttle Idle Actuator
The aim of the MTIA (Main Throttle Idle Actuator) is to control the idle speed with the throttle body itself. The throttle is motorized for low opening angle (0°, 22.5°). The characteristics of the airflow are not the same for low and high opening angles. As a matter of fact, the gradient of the mass air flow function of TPS is lower for small angles that permits to be more precise during the idle speed control. Out of idle speed the throttle is actuated mechanically by a classical bowdencable.
The throttle position sensor (TPS) provides a voltage signal that changes in relation to the throttle plate angle. The signal voltage will vary from about nearly 5.0V at idles to about 0.2V to 0.4V at wide-open throttle. The TPS is one of the most important inputs used by the ECM for fuel control and other functions such as idle, wide open throttle, deceleration enleanment, and acceleration enrichment.
Engine Control Module
The engine control module (ECM), located inside the
passenger kick-panel, is the control center of the fuel injection
system. It constantly looks at the information
from various sensors and controls the systems that affect
the vehicle’s performance. The ECM also performs
the diagnostic functions of the system. It can recognize
operational problems, alert the driver through the Malfunction
Indicator Lamp (MIL), and store diagnostic
trouble code(s) which identify problem areas to aid the
technician in making repairs.
There are no serviceable parts in the ECM. The calibrations
are stored in the ECM in the Programmable Read-
Only Memory (PROM).
The ECM supplies either 5 or 12 volts to power the sensors
or switches. This is done through resistances in the
ECM which are so high in value that a test light will not
come on when connected to the circuit. In some cases,
even an ordinary shop voltmeter will not give an accurate
reading because its resistance is too low. You must use a digital voltmeter with a 10 megohm input impedance
to get accurate voltage readings. The ECM controls
output circuits such as the fuel injectors, the idle air
control valve, the A/C clutch relay, etc., by controlling
the ground circuit through transistors or a device called
a "quad-driver."
Fuel Injector
The Multiport Fuel Injection (MFI) assembly is a solenoid-
operated device controlled by the engine control
module (ECM). It meters pressurized fuel to a single engine
cylinder. The ECM energizes the fuel injector or the
solenoid to a normally closed ball or pintle valve. This allows
fuel to flow into the top of the injector, past the
ball or pintle valve, and through a recessed flow director
plate at the injector outlet.
The director plate has six machined holes that control
the fuel flow, generating a conical spray pattern of finely
atomized fuel at the injector tip. Fuel from the tip is directed
at the intake valve, causing it to become further atomized and vaporized before entering the combustion
chamber. A fuel injector which is stuck partially open will
cause a loss of fuel pressure after the engine is shut
down. Also, an extended crank time will be noticed on
some engines. Dieseling can also occur because some
fuel can be delivered to the engine after the ignition is
turned OFF.
Knock Sensor
The knock sensor detects abnormal knocking in the engine.
The sensor is mounted in the engine block near the
cylinders. The sensor produces an AC output voltage
which increases with the severity of the knock. This signal
is sent to the engine control module (ECM). The
ECM then adjusts the ignition timing to reduce the spark
knock.
G Sensor
The engine control module (ECM) receives rough road
information from the G sensor. The ECM uses the rough
road information to enable or disable the misfire diagnostic.
The misfire diagnostic can be greatly affected by
crankshaft speed variations caused by driving on rough
road surfaces. The G sensor generates rough road information
by producing a signal which is proportional to
the movement of a small metal bar inside the sensor.
If a fault occurs which causes the ECM to not receive rough road information between 30 and 70 mph (50 and 113 km/h), DTC P1391 will set.
Strategy - Based Diagnostics
Strategy-Based Diagnostics
The strategy-based diagnostic is a uniform approach to repair all Electrical/Electronic (E/E) systems. The diagnostic flow can always be used to resolve an E/E system problem and is a starting point when repairs are necessary. The following steps will instruct the technician on how to proceed with a diagnosis:
- Verify the customer complaint. To verify the customer complaint, the technician should know the normal operation of the system.
- Perform preliminary checks as follows:
- Conduct a thorough visual inspection.
- Review the service history.
- Detect unusual sounds or odors.
- Gather Diagnostic Trouble Code (DTC) information to achieve an effective repair.
- Check bulletins and other service information. This includes videos, newsletters, etc.
- Refer to service information (manual) system check(s).
- Refer to service diagnostics.
No Trouble Found
This condition exists when the vehicle is found to operate normally. The condition described by the customer may be normal. Verify the customer complaint against another vehicle that is operating normally. The condition may be intermittent. Verify the complaint under the conditions described by the customer before releasing the vehicle.
Re-examine the complaint.
When the complaint cannot be successfully found or isolated, a re-evaluation is necessary. The complaint should be re-verified and could be intermittent as defined in "Intermittents," or could be normal.
After isolating the cause, the repairs should be made. Validate for proper operation and verify that the symptom has been corrected. This may involve road testing or other methods to verify that the complaint has been resolved under the following conditions:
- Conditions noted by the customer.
- If a DTC was diagnosed, verify a repair by duplicating conditions present when the DTC was set as noted in the Failure Records or Freeze Frame data.
Verifying Vehicle Repair
Verification of the vehicle repair will be more comprehensive for vehicles with On-Board Diagnostic (EOBD) system diagnostics. Following a repair, the technician should perform these steps:
Important : Follow the steps below when you verify repairs on EOBD systems. Failure to follow these steps could result in unnecessary repairs.
- Review and record the Failure Records and the Freeze Frame data for the DTC which has been diagnosed (Freeze Fame data will only be stored for an A or B type diagnostic and only if the MIL has been requested).
- Clear the DTC(s).
- Operate the vehicle within conditions noted in the Failure Records and Freeze Frame data.
- Monitor the DTC status information for the specific DTC which has been diagnosed until the diagnostic test associated with that DTC runs.
EOBD Serviceability Issues
Based on the knowledge gained from On-Board Diagnostic (EOBD) experience in the 1994 and 1995 model years, this list of non-vehicle faults that could affect the performance of the EOBD system has been compiled. These non-vehicle faults vary from environmental conditions to the quality of fuel used. With the introduction of EOBD diagnostics across the entire passenger car and light-duty truck market in 1996, illumination of the MIL due to a non-vehicle fault could lead to misdiagnosis of the vehicle, increased warranty expense and customer dissatisfaction. The following list of non-vehicle faults does not include every possible fault and may not apply equally to all product lines.
Fuel Quality
Fuel quality is not a new issue for the automotive industry, but its potential for turning on the Malfunction Indicator Lamp (MIL) with EOBD systems is new.
Fuel additives such as "dry gas" and "octane enhancers" may affect the performance of the fuel. If this results in an incomplete combustion or a partial burn, it will set DTC P0300. The Reed Vapor Pressure of the fuel can also create problems in the fuel system, especially during the spring and fall months when severe ambient temperature swings occur. A high Reed Vapor Pressure could show up as a Fuel Trim DTC due to excessive canister loading. High vapor pressures generated in the fuel tank can also affect the Evaporative Emission diagnostic as well.
Using fuel with the wrong octane rating for your vehicle may cause driveability problems. Many of the major fuel companies advertise that using "premium" gasoline will improve the performance of your vehicle. Most premium fuels use alcohol to increase the octane rating of the fuel. Although alcohol-enhanced fuels may raise the octane rating, the fuel's ability to turn into vapor in cold temperatures deteriorates. This may affect the starting ability and cold driveability of the engine.
Low fuel levels can lead to fuel starvation, lean engine operation, and eventually engine misfire.
Non-OEM Parts
All of the EOBD diagnostics have been calibrated to run with Original Equipment Manufacturer (OEM) parts. Something as simple as a high-performance exhaust system that affects exhaust system back pressure could potentially interfere with the operation of the Exhaust Gas Recirculation (EGR) valve and thereby turn on the MIL. Small leaks in the exhaust system near the post catalyst oxygen sensor can also cause the MIL to turn on.
Aftermarket electronics, such as cellular phones, stereos, and anti-theft devices, may radiate electromagnetic interference (EMI) into the control system if they are improperly installed. This may cause a false sensor reading and turn on the MIL.
Environment
Temporary environmental conditions, such as localized flooding, will have an effect on the vehicle ignition system. If the ignition system is rain-soaked, it can temporarily cause engine misfire and turn on the MIL.
Refueling
A new EOBD diagnostic checks the integrity of the entire Evaporative (EVAP) Emission system. If the vehicle is restarted after refueling and the fuel cap is not secured correctly, the on-board diagnostic system will sense this as a system fault, turn on the MIL, and set DTC P0440.
Vehicle Marshaling
The transportation of new vehicles from the assembly plant to the dealership can involve as many as 60 key cycles within 2 to 3 miles of driving. This type of operation contributes to the fuel fouling of the spark plugs and will turn on the MIL with a set DTC P0300.
Poor Vehicle Maintenance
The sensitivity of EOBD diagnostics will cause the MIL to turn on if the vehicle is not maintained properly. Restricted air filters, fuel filters, and crankcase deposits due to lack of oil changes or improper oil viscosity can trigger actual vehicle faults that were not previously monitored prior to EOBD. Poor vehicle maintenance can not be classified as a "non-vehicle fault," but with the sensitivity of EOBD diagnostics, vehicle maintenance schedules must be more closely followed.
Severe Vibration
The Misfire diagnostic measures small changes in the rotational speed of the crankshaft. Severe driveline vibrations in the vehicle, such as caused by an excessive amount of mud on the wheels, can have the same effect on crankshaft speed as misfire and, therefore, may set DTC P0300.
Related System Faults
Many of the EOBD system diagnostics will not run if the engine controlmodule (ECM) detects a fault on a related system or component. One example would be that if the ECM detected a Misfire fault, the diagnostics on the catalytic converter would be suspended until the Misfire fault was repaired. If the Misfire fault is severe enough, the catalytic converter can be damaged due to overheating and will never set a Catalyst DTC until the Misfire fault is repaired and the Catalyst diagnostic is allowed to run to completion. If this happens, the customer may have to make two trips to the dealership in order to repair the vehicle.
Serial Data Communications
Class II Serial Data Communications
Government regulations require that all vehicle manufacturers establish a common communication system. This vehicle utilizes the "Class II" communication system. Each bit of information can have one of two lengths: long or short. This allows vehicle wiring to be reduced by transmitting and receiving multiple signals over a single wire. The messages carried on Class II data streams are also prioritized. If two messages attempt to establish communications on the data line at the same time, only the message with higher priority will continue. The device with the lower priority message must wait. Themost significant result of this regulation is that it provides scan tool manufacturers with the capability to access data from any make or model vehicle that is sold.
The data displayed on the other scan tool will appear the same, with some exceptions. Some scan tools will only be able to display certain vehicle parameters as values that are a coded representation of the true or actual value. On this vehicle the scan tool displays the actual values for vehicle parameters. It will not be necessary to perform any conversions from coded values to actual values.
ON-Board Diagnostic (EOBD)
On-Board Diagnostic Tests
A diagnostic test is a series of steps, the result of which is a pass or fail reported to the diagnostic executive. When a diagnostic test reports a pass result, the diagnostic executive records the following data:
- The diagnostic test has been completed since the last ignition cycle.
- The diagnostic test has passed during the current ignition cycle.
- The fault identified by the diagnostic test is not currently active.
When a diagnostic test reports a fail result, the diagnostic executive records the following data:
- The diagnostic test has been completed since the last ignition cycle.
- The fault identified by the diagnostic test is currently active.
- The fault has been active during this ignition cycle.
- The operating conditions at the time of the failure.
Remember, a fuel trim Diagnostic Trouble Code (DTC) may be triggered by a list of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition, etc.) when diagnosing a fuel trim fault.
Comprehensive Component Monitor Diagnostic Operation
Comprehensive component monitoring diagnostics are required to monitor emissions-related input and output powertrain components.
Input Components
Input components are monitored for circuit continuity and out-of-range values. This includes rationality checking. Rationality checking refers to indicating a fault when the signal from a sensor does not seem reasonable, i.e. Throttle Position (TP) sensor that indicates high throttle position at low engine loads or Manifold Absolute Pressure (MAP) voltage. Input components may include, but are not limited to, the following sensors:
- Vehicle Speed Sensor (VSS).
- Crankshaft Position (CKP) sensor.
- Throttle Position (TP) sensor.
- Engine Coolant Temperature (ECT) sensor.
- Camshaft Position (CMP) sensor.
- Manifold Absolute Pressure (MAP) sensor.
In addition to the circuit continuity and rationality check, the ECT sensor is monitored for its ability to achieve a steady state temperature to enable closed loop fuel control.
Output Components
Output components are diagnosed for proper response to control module commands. Components where functional monitoring is not feasible will be monitored for circuit continuity and out-of-range values if applicable. Output components to be monitored include, but are not limited to the following circuit:
- Idle Air Control (IAC) Motor.
- Control module controlled EVAP Canister Purge Valve.
- A/C relays.
- Cooling fan relay.
- VSS output.
- MIL control.
Passive and Active Diagnostic Tests
A passive test is a diagnostic test which simply monitors a vehicle system or component. Conversely, an active test, actually takes some sort of action when performing diagnostic functions, often in response to a failed passive test. For example, the Exhaust Gas Recirculation (EGR) diagnostic active test will force the EGR valve open during closed throttle deceleration and/or force the EGR valve closed during a steady state. Either action should result in a change in manifold pressure.
Intrusive Diagnostic Tests
This is any on-board test run by the Diagnostic Management System which may have an effect on vehicle performance or emission levels.
Warm-Up Cycle
A warm-up cycle means that engine temperature must reach aminimum of 160°F (70°C) and rise at least 72°F (22°C) over the course of a trip.
Freeze Frame
Freeze Frame is an element of the Diagnostic Management System which stores various vehicle information at the moment an emissions-related fault is stored in memory and when the Malfunction Indicator Lamp (MIL) is commanded on. These data can help to identify the cause of a fault.
Failure Records
Failure Records data is an enhancement of the EOBD Freeze Frame feature. Failure Records store the same vehicle information as does Freeze Frame, but it will store that information for any fault which is stored in onboard memory, while Freeze Frame stores information only for emission-related faults that command the MIL on.
Common EOBD Terms
Diagnostic
When used as a noun, the word diagnostic refers to any on-board test run by the vehicle's Diagnostic Management System. A diagnostic is simply a test run on a system or component to determine if the system or component is operating according to specification. There are many diagnostics, shown in the following list:
- Misfire
- Front Heated Oxygen Sensor (HO2S1)
- Rear Heated Oxygen Sensor (HO2S2)
- Exhaust Gas Recirculation (EGR)
- Catalyst monitoring
- Fuel System
Enable Criteria
The term "enable criteria" is engineering language for the conditions necessary for a given diagnostic test to run. Each diagnostic has a specific list of conditions which must be met before the diagnostic will run.
"Enable criteria" is another way of saying "conditions required."
The enable criteria for each diagnostic is listed on the first page of the Diagnostic Trouble Code (DTC) description under the heading "Conditions for Setting the DTC." Enable criteria varies with each diagnostic and typically includes, but is not limited to, the following items:
- Engine speed.
- Vehicle speed
- Engine Coolant Temperature (ECT)
- Manifold Absolute Pressure (MAP)
- Barometric Pressure (BARO)
- Intake Air Temperature (IAT)
- Throttle Position (TP)
- High canister purge
- Fuel trim
- A/C on
Trip
Technically, a trip is a key-on run key-off cycle in which all the enable criteria for a given diagnostic are met, allowing the diagnostic to run. Unfortunately, this concept is not quite that simple. A trip is official when all the enable criteria for a given diagnostic are met. But because the enable criteria vary from one diagnostic to another, the definition of trip varies as well. Some diagnostics are run when the vehicle is at operating temperature, some when the vehicle first starts up; some require that the vehicle be cruising at a steady highway speed, some run only when the vehicle is at idle; some diagnostics function with the Torque Converter Clutch (TCC) disabled. Some run only immediately following a cold engine startup.
A trip then, is defined as a key-on run key-off cycle in which the vehicle was operated in such a way as to satisfy the enables criteria for a given diagnostic, and this diagnostic will consider this cycle to be one trip. However, another diagnostic with a different set of enable criteria (which were not met) during this driving event, would not consider it a trip. No trip will occur for that particular diagnostic until the vehicle is driven in such a way as to meet all the enable criteria
Diagnostic Information
The diagnostic charts and functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are not multiple faults present.
There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complimented by the diagnostic procedures contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes. When a malfunction is detected by the control module, a diagnostic trouble code is set and the Malfunction Indicator Lamp (MIL) is illuminated.
Malfunction Indicator Lamp (MIL)
The Malfunction Indicator Lamp (MIL) is required by On-Board Diagnostics (EOBD) that it illuminates under a strict set of guide lines.
Basically, the MIL is turned on when the engine control module (ECM) detects a DTC that will impact the vehicle emissions.
The MIL is under the control of the Diagnostic Executive. The MIL will be turned on if an emissions-related diagnostic test indicates a malfunction has occurred. It will stay on until the system or component passes the same test, for three consecutive trips, with no emissions related faults.
Extinguishing the MIL
When the MIL is on, the Diagnostic Executive will turn off the MIL after three consecutive trips that a "test passed" has been reported for the diagnostic test that originally caused the MIL to illuminate. Although the MIL has been turned off, the DTC will remain in the ECM memory (both Freeze Frame and Failure Records) until forty (40) warm-up cycles after no faults have been completed.
If the MIL was set by either a fuel trim or misfire-related DTC, additional requirements must be met. In addition to the requirements stated in the previous paragraph, these requirements are as follows:
- The diagnostic tests that are passed must occur with 375 rpm of the rpm data stored at the time the last test failed.
- Plus or minus ten percent of the engine load that was stored at the time the last test failed. Similar engine temperature conditions (warmed up or warming up) as those stored at the time the last test failed.
Meeting these requirements ensures that the fault which turned on the MIL has been corrected.
The MIL is on the instrument panel and has the following functions:
- It informs the driver that a fault that affects vehicle emission levels has occurred and that the vehicle should be taken for service as soon as possible.
- As a system check, the MIL will come on with the key ON and the engine not running. When the engine is started, the MIL will turn OFF.
- When the MIL remains ON while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, an EOBD System Check must be performed. The procedures for these checks are given in EOBD System Check. These checks will expose faults which may not be detected if other diagnostics are performed first.
Data Link Connector (DLC)
The provision for communicating with the control module is the Data Link Connector (DLC). The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below:
- Identifying stored DTCs.
- Clearing DTCs.
- Performing output control tests.
- Reading serial data.
Reading Diagnostic Trouble Codes
The procedure for reading diagnostic trouble code(s) is to use a diagnostic scan tool. When reading Diagnostic Trouble Codes (DTCs), follow the instructions supplied by tool manufacturer.
DTC Modes
On On-Board Diagnostic (EOBD) passenger cars there are five options available in the scan tool DTC mode to display the enhanced information available. A description of the new modes, DTC Info and Specific DTC, follows. After selecting DTC, the following menu appears:
- DTC Info.
- Specific DTC.
- Freeze Frame.
- Fail Records (not all applications).
- Clear Info.
The following is a brief description of each of the sub menus in DTC Info and Specific DTC. The order in which they appear here is alphabetical and not necessarily the way they will appear on the scan tool.
DTC Information Mode
Use the DTC info mode to search for a specific type of stored DTC information. There are seven choices. The service manual may instruct the technician to test for DTCs in a certain manner. Always follow published service procedures.
To get a complete description of any status, press the "Enter" key before pressing the desired F-key. For example, pressing "Enter" then an F-key will display a definition of the abbreviated scan tool status.
DTC Status
This selection will display any DTCs that have not run during the current ignition cycle or have reported a test failure during this ignition up to a maximum of 33 DTCs. DTC tests which run and pass will cause that DTC number to be removed from the scan tool screen.
Fail This Ign. (Fail This Ignition)
This selection will display all DTCs that have failed during the present ignition cycle.
History
This selection will display only DTCs that are stored in the ECM's history memory. It will not display type CNL DTCs that have not requested the Malfunction Indicator Lamp (MIL). It will display all type A, B and E DTCs that have requested the MIL and have failed within the last 40 warm-up cycles. In addition, it will display all type C and type D DTCs that have failed within the last 40 warm-up cycles.
Last Test Fail
This selection will display only DTCs that failed the last time the test ran. The last test may have run during a previous ignition cycle if a type A or type B DTC is displayed. For type C and type D DTCs, the last failure must have occurred during the current ignition cycle to appear as Last Test Fail.
MIL Request
This selection will display only DTCs that are requesting the MIL. Type C and type D DTCs cannot be displayed using this option. This selection will report type B DTCs only after the MIL has been requested.
Not Run SCC (Not Run Since Code Clear)
This option will display up to 33 DTCs that have not run since the DTCs were last cleared. Since any displayed DTCs have not run, their condition (passing or failing) is unknown.
Test Fail SCC (Test Failed Since Code
Clear)
This selection will display all active and history DTCs that have reported a test failure since the last time DTCs were cleared. DTCs that last failed more than 40 warm-up cycles before this option is selected will not be displayed.
Specific DTC Mode
This mode is used to check the status of individual diagnostic tests by DTC number. This selection can be accessed if a DTC has passed, failed or both. Many EOBD DTC mode descriptions are possible because of the extensive amount of information that the diagnostic executive monitors regarding each test. Some of the many possible descriptions follow with a brief explanation.
The "F2" key is used, in this mode, to display a description of the DTC. The "Yes" and "No" keys may also be used to display more DTC status information. This selection will only allow entry of DTC numbers that are supported by the vehicle being tested. If an attempt is made to enter DTC numbers for tests which the diagnostic executive does not recognize, the requested information will not be displayed correctly and the scan tool may display an error message. The same applies to using the DTC trigger option in the Snapshot mode. If an invalid DTC is entered, the scan tool will not trigger.
Failed Last Test
This message display indicates that the last diagnostic test failed for the selected DTC. For type A and type B DTCs, this message will be displayed during subsequent ignition cycles until the test passes or DTCs are cleared. For type C and type D DTCs, this message will clear when the ignition is cycled.
Failed Since Clear
This message display indicates that the DTC has failed at least once within the last 40 warm-up cycles since the last time DTCs were cleared.
Failed This Ig. (Failed This Ignition)
This message display indicates that the diagnostic test has failed at least once during the current ignition cycle. This message will clear when DTCs are cleared or the ignition is cycled.
History
This message display indicates that the DTC has been stored in memory as a valid fault. A DTC displayed as a History fault may not mean that the fault is no longer present. The history description means that all the conditions necessary for reporting a fault have been met (maybe even currently), and the information was stored in the control module memory.
MIL Requested
This message display indicates that the DTC is currently causing the MIL to be turned ON. Remember that only type A and type B DTCs can request the MIL. The MIL request cannot be used to determine if the DTC fault conditions are currently being experienced. This is because the diagnostic executive will require up to three trips during which the diagnostic test passes to turn OFF the MIL.
Not Run Since CI (Not Run Since Cleared)
This message display indicates that the selected diagnostic test has not run since the last time DTCs were cleared. Therefore, the diagnostic test status (passing or failing) is unknown. After DTCs are cleared, this message will continue to be displayed until the diagnostic test runs.
Not Run This Ig. (Not Run This Ignition)
This message display indicates that the selected diagnostic test has not run during this ignition cycle.
Test Ran and Passed
This message display indicates that the selected diagnostic test has done the following:
- Passed the last test.
- Run and passed during this ignition cycle.
- Run and passed since DTCs were last cleared.
If the indicated status of the vehicle is "Test Ran and Passed" after a repair verification, the vehicle is ready to be released to the customer.
If the indicated status of the vehicle is "Failed This Ignition" after a repair verification, then the repair is incomplete and further diagnosis is required.
Prior to repairing a vehicle, status information can be used to evaluate the state of the diagnostic test, and to help identify an intermittent problem. The technician can conclude that although the MIL is illuminated, the fault condition that caused the code to set is not present. An intermittent condition must be the cause.
Primary System - Based Diagnostics
There are primary system-based diagnostics which evaluate system operation and its effect on vehicle emissions. The primary system-based diagnostics are listed below with a brief description of the diagnostic function:
Oxygen Sensor Diagnosis
The fuel control Front Heated Oxygen Sensor (HO2S1) is diagnosed for the following conditions:
- Slow response.
- Response time (time to switch R/L or L/R).
- Inactive signal (output steady at bias voltage approx. 450 mv).
- Signal fixed high.
- Signal fixed low.
The catalyst monitor Rear Heated Oxygen Sensor (HO2S2) is diagnosed for the following conditions:
- Heater performance (time to activity on cold start).
- Signal fixed low during steady state conditions or power enrichment (hard acceleration when a richmixture should be indicated).
- Signal fixed high during steady state conditions or deceleration mode (deceleration when a lean mixture should be indicated).
- Inactive sensor (output steady at approximately 438 mv).
If the oxygen sensor pigtail wiring, connector or terminal are damaged, the entire oxygen sensor assembly must be replaced. Do not attempt to repair the wiring, connector or terminals. In order for the sensor to function properly, it must have clean reference air provided to it. This clean air reference is obtained by way of the oxygen sensor wire(s). Any attempt to repair the wires, connector or terminals could result in the obstruction of the reference air and degrade oxygen sensor performance.
Misfire Monitor Diagnostic Operation
The misfire monitor diagnostic is based on crankshaft
rotational velocity (reference period) variations. The engine
control module (ECM) determines crankshaft rotational
velocity using the Crankshaft Position (CKP)
sensor and the Camshaft Position (CMP) sensor. When
a cylinder misfires, the crankshaft slows down momentarily.
By monitoring the CKP and CMP sensor signals,
the ECM can calculate when a misfire occurs.
For a non-catalyst damaging misfire, the diagnostic will be required to monitor a misfire present for between 1000-3200 engine revolutions.
For catalyst-damaging misfire, the diagnostic will respond to misfire within 200 engine revolutions.
Rough roads may cause false misfire detection. A rough road will cause torque to be applied to the drive wheels and drive train. This torque can intermittently decrease the crankshaft rotational velocity. This may be falsely detected as a misfire.
A rough road sensor, or G sensor, works together with
the misfire detection system. The G sensor produces a
voltage that varies along with the intensity of road vibrations. When the ECM detects a rough road, the misfire
detection system is temporarily disabled.
Misfire Counters
Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred. These "misfire counters" are basically a file on each engine cylinder. A current and a history misfire counter are maintained for each cylinder. The misfire current counters (Misfire Cur #1-4) indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counter will display real time data without a misfire Diagnostic Trouble Code (DTC) stored. The misfire history counters (Misfire Hist #1-4) indicate the total number of cylinder firing events which were misfires. The misfire history counters will display 0 until the misfire iagnostic has failed and a DTC P0300 is set. Once the misfire DTC P0300 is set, the misfire history counters will be updated every 200 cylinder firing events. A misfire counter is maintained for each cylinder.
If the misfire diagnostic reports a failure, the diagnostic executive reviews all of the misfire counters before reporting a DTC. This way, the diagnostic executive reports the most current information.
When crankshaft rotation is erratic, a misfire condition will be detected. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring.
Use diagnostic equipment to monitor misfire counter data on On-Board Diagnostic (EOBD) compliant vehicles. Knowing which specific cylinder(s) misfired can lead to the root cause, even when dealing with amultiple cylinder misfire. Using the information in the misfire counters, identify which cylinders are misfiring. If the counters indicate cylinders numbers 1 and 4 misfired, look for a circuit or component common to both cylinders number 1 and 4.
The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:
- Contaminated fuel.
- Low fuel.
- Fuel-fouled spark plugs.
- Basic engine fault.
Fuel Trim System Monitor Diagnostic
Operation
This system monitors the averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If both values are outside their thresholds, a rich or lean DTC will be recorded.
The fuel trim system diagnostic also conducts an intrusive
test. This test determines if a rich condition is being
caused by excessive fuel vapor from the Evaporative
(EVAP) Emission canister. In order to meet EOBD requirements,
the control module uses weighted fuel trim
cells to determine the need to set a fuel trim DTC. A fuel
trim DTC can only be set if fuel trim counts in the
weighted fuel trim cells exceed specifications. This
means that the vehicle could have a fuel trim problem
which is causing a problem under certain conditions
(i.e., engine idle high due to a small vacuum leak or
rough idle due to a large vacuum leak) while it operates
fine at other times. No fuel trim DTC would set (although
an engine idle speed DTC or HO2S2 DTC may set). Use a
scan tool to observe fuel trim counts while the problem is
occurring.
A fuel trim DTC may be triggered by a number of vehicle faults. Make use of all information available (other DTCs stored, rich or lean condition, etc.) when diagnosing a fuel trim fault.
Fuel Trim Cell Diagnostic Weights
No fuel trim DTC will set regardless of the fuel trim counts in cell 0 unless the fuel trim counts in the weighted cells are also outside specifications. This means that the vehicle could have a fuel trim problem which is causing a problem under certain conditions (i.e. engine idle high due to a small vacuum leak or rough due to a large vacuum leak) while it operates fine at other times. No fuel trim DTC would set (although an engine idle speed DTC or HO2S2 DTC may set). Use a scan tool to observe fuel trim counts while the problem is occurring.