Part 2 of 2
PCM AND ECM OPERATING MODESAs input signals to the Engine Control Module (ECM) change, the ECM adjusts its response to output devices. There are several different modes of operation that determine how the ECM responds to the various input signals.
Ignition Switch ON (Engine OFF) Mode
When the ignition switch is in the "ON" position, the following actions occur:
1. When the PCM and ECM receive an ignition sense input, they power up. Battery voltage is supplied through the battery voltage input.
2. The wait-to start, water-in-fuel, check engine, check gauges, and transmission temperature (auto trans only) lamps are turned ON by the PCM, EGM or BCM for at least 2 seconds as a bulb test. This is done by direct wire or through the Bus to the smart cluster.
3. The ECM monitors the intake air temperature signal. If the intake air temperature is below a predetermined temperature, the ECM will start the air intake heater preheat cycle. The ECM will energize the intake heaters by grounding the air intake heater relays. The ECM will turn the wait-to-start lamp on until the preheat cycle is over. If the ECM receives a crank signal before the preheat cycle is over, the preheat cycle will be aborted.
4. The ECM will request the water-in-fuel lamp ON if a signal is received from the water-in-fuel sensor.
NOTE: IF THE WATER-IN FUEL LAMP REMAINS ILLUMINATED AFTER THE 2-SECOND BULB CHECK, THE VEHICLE SHOULD NOT BE STARTED UNTIL THE FUEL FILTER HAS BEEN DRAINED AND PRIMED.
Engine Start-up Mode
When the ignition key is held in the "start" position, the following occurs:
1. The ECM receives the crank signal input from the crank position sensor.
2. The air intake heaters are not energized during cranking.
3. The ECM monitors engine speed and when the engine speed is above 650 rpm, the ECM determines that the engine is running.
4. If the ECM receives a crank signal before the preheat cycle is over, the heaters will be deenergized. However, the post-heat cycle will begin if the engine is in the run state or speed is above 1200 rpm.
Engine Warm-up Mode
1. The ECM may start the air intake heater post-heat cycle depending on intake air temperature. If manifold air temperature was at 66 °F or less when the ignition was turned ON, the cycle is started.
2. If the transmission temperature sensor shows it is below 59 °F, the automatic transmission will not be allowed to enter overdrive.
3. If the transmission temperature sensor shows it is below 69 °F, the transmission's torque converter clutch will not be allowed to engage.
4. The ECM will request the water-in-fuel lamp ON if a signal is received from the water-in-fuel sensor.
Cruise Or Idle Mode
During the cruising speed, the following inputs are received by the ECM:
1. The ECM monitors intake manifold air temperature through the intake air temperature sensor.
2. The intake air heater post-heat cycle will be complete, if it is not already over.
3. The vehicle speed, and throttle position sensors are used by the PCM to control the transmission overdrive and torque converter clutch operation.
4. If the transmission temperature sensor shows it is below 59 °F or above 275 °F, the automatic transmission will not be allowed to enter overdrive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmisssion temperature drops to 240 °F.
5. The water-in-fuel sensor signal is used by the ECM to request the water-in-fuel lamp ON, if water is sensed in the fuel.
Acceleration Mode
When there is an abrupt demand for increased engine output and vehicle acceleration, the following occurs:
1. The vehicle speed, and throttle position signals are used to control transmission overdrive and torque converter clutch operation.
2. If the transmission temperature sensor shows it is below 59 ° F, the torque converter clutch will not be allowed to engage.
3. If the transmission temperature sensor shows it is below 59° F or above 275 °F, the automatic transmission will not be allowed to enter overdrive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmission temperature drops to 240 °F.
4. If the speed control system resume/accelerate function is being used, the PCM will only allow the vehicle to accelerate at a predetermined rate. If a speed control has been set and the resume/accelerate button is momentarily pushed in, the PCM will increase vehicle speed by two miles per hour.
5. The PCM will disable the speed control when:
- the brakes are applied
- excessive engine speed
- the clutch is disengaged (manual transmission)
- the gear selector is shifted to neutral (automatic transmission)
- the speed control OFF switch is activated
Deceleration Mode
During deceleration, the following inputs are received by the PCM:
1. The vehicle speed, crank position, and throttle position signals are used to control transmission overdrive and torque converter clutch operation.
2. The torque converter clutch will disengage, if the brakes are applied.
3. If the transmission temperature sensor shows it is below 59 °F or above 275 °F, the automatic transmission will not be allowed to enter over-drive. If the transmission is in overdrive when the transmission temperature is above 275 °F, the transmission will downshift and not upshift again until the transmission temperature drops to 240 °F.
4. The PCM will disable the speed control when:
- the brakes are applied
- excessive engine speed
- the clutch is disengaged (manual transmission)
- the gear selector is shifted to neutral (automatic transmission)
- the speed control OFF switch is activated
Ignition Switch OFF Mode
When the ignition switch is turned to the "OFF" position, the PCM and ECM still stores in memory trouble codes and the throttle position sensor value from the previous key-ON. Non-Monitored Circuits (Diesel)
The PCM does not monitor the following circuits, systems, and conditions even though they could have malfunctions that result in driveability problems. A diagnostic code may not be displayed for the following conditions. However, problems with these systems may cause a diagnostic code to be displayed for other systems. For example, a fuel pressure problem will not register a diagnostic code directly, but could cause a rich or lean condition. This could cause a fuel system, or misfire monitor trouble code to be stored in the ECM.
Engine Timing - The ECM cannot detect incorrectly indexed timing gears.
Fuel Pressure - Fuel pressure is controlled by the fuel injection pump. The ECM cannot detect a clogged fuel pump inlet filter, clogged in-line filter, or a pinched fuel supply.
Fuel Injectors - The ECM cannot detect if a fuel injector is clogged, or if the pintle is sticking.
Fuel Requirements - Poor quality or air in fuel can cause problems such as hard starting, stalling, and stumble.
PCM Grounds - The PCM and ECM cannot detect a poor system ground. However, a diagnostic trouble code may be stored as a result of this condition.
Air Cleaner Air Flow - The PCM and ECM cannot detect a clogged or restricted air cleaner inlet or filter element.
Exhaust System - The PCM and ECM cannot detect a plugged, restricted, or leaking exhaust system.
NON-MONITORED CIRCUITS
The PCM does not monitor the following circuits, systems, and conditions even though they could have malfunctions that result in driveability problems. A diagnostic code may not be displayed for the following conditions. However, problems with these systems may cause a diagnostic code to be displayed for other systems. For example, a fuel pressure problem will not register a diagnostic code directly, but could cause a rich or lean condition. This could cause a fuel system, or misfire monitor trouble code to be stored in the ECM.
Engine Timing - The ECM cannot detect incorrectly indexed timing gears.
Fuel Pressure - Fuel pressure is controlled by the fuel injection pump. The ECM cannot detect a clogged fuel pump inlet filter, clogged in-line filter, or a pinched fuel supply.
Fuel Injectors - The ECM cannot detect if a fuel injector is clogged, or if the pintle is sticking.
Fuel Requirements - Poor quality or air in fuel can cause problems such as hard starting, stalling, and stumble.
PCM Grounds - The PCM and ECM cannot detect a poor system ground. However, a diagnostic trouble code may be stored as a result of this condition.
Air Cleaner Air Flow - The PCM and ECM cannot detect a clogged or restricted air cleaner inlet or filter element.
Exhaust System - The PCM and ECM cannot detect a plugged, restricted, or leaking exhaust system.
DIAGNOSTIC TROUBLE CODES
Each diagnostic trouble code is diagnosed by following a specific testing procedure. The diagnostic test procedures contain step-by-step instructions for determining the cause of trouble codes as well as no trouble code problems. It is not necessary to perform all of the tests in this book to diagnose an individual code.
Always begin by reading the diagnostic trouble codes using the DRB III.
Hard Code
A diagnostic trouble code that comes back within one cycle of the ignition key is a "hard" code. This means that the defect is there every time the powertrain control module checks that circuit or function. Procedures in this manual verify if the trouble code is a hard code at the beginning of each test. When it is not a hard code, an "intermittent" test must be performed.
Codes that are for OBDII monitors will not set with just the ignition key ON. Comparing these to non-emission codes, they will seem like an intermittent. These codes require a set of parameters to be performed (The DRB III pre-test screens will help with this for MONITOR codes), this is called a "TRIP". All OBDII DTCs will be set after one or in some cases two trip failures, and the MIL will be turned ON. These codes require three successful, no failures, TRIPS to extinguish the MIL, followed by 40 warm-up cycles to erase the code. For further explanation of TRIPS, Pre-test screens, Warm-up cycles, and the use of the DRB III, refer to the On Board Diagnostic training booklet # 81-699-97094.
Intermittent Code
A diagnostic trouble code that is not there every time the PCM/ECM checks the circuit is an "intermittent" code. Most intermittent codes are caused by wiring or connector problems. Defects that come and go like this are the most difficult to diagnose; they must be looked for under specific conditions that cause them. The following checks may assist you in identifying a possible intermittent problem:
- Visually inspect related wire harness connectors. Look for broken, bent, pushed out, or corroded terminals.
- Visually inspect the related harnesses. Look for chafed, pierced, or partially broken wire.
- Refer to any Hotline Newsletters or technical service bulletins that may apply.
- Use the DRB III data recorder or co-pilot.
Starts Since Set Counter
This reset counter counts the number of times the vehicle has been started since codes were last set or erased. This counter will count up to 255 start counts.
The number of starts helps determine when the trouble code actually happened. This is recorded by the PCM and can be viewed on the DRB as STARTS since set.
When there are no trouble codes stored in memory, the DRB will display "NO TROUBLE CODES FOUND" and the reset counter will show "STARTS since set = XXX."
HANDLING NO TROUBLE CODE PROBLEMS
Symptom checks cannot be used properly unless the driveability problem characteristic actually happens while the vehicle is being tested.
Select the symptom that most accurately describes the vehicle's driveability problem and then perform the test routine that pertains to this symptom. Perform each routine test in sequence until the problem is found. For definitions, see Glossary of Terms.
SYMPTOM DIAGNOSTIC TEST
Hard Start Checking Fuel Pressure
Checking ECT Sensor Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking the Minimum Idle Airflow
Checking Engine Mechanical Systems
Checking EGR System
Checking IAT Sensor
Start and Stall Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Sensor Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Hesitation/Sag/Stumble Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Sensor Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking Engine Mechanical Systems
Checking IAT Sensor
Checking PNP Switch
Surge Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Sensor Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Lack of Power/Sluggish Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Sensor Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Spark Knock/Detonation Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Sensor Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Cuts Out/Misses Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Backfire/Popback Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking MAP Sensor Calibration
Checking EGR System
Runs Rough,
Unstable/Erratic Idle Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Sensor Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking Engine Mechanical Systems
Checking IAT Sensor
Checking PNP Switch
Poor Fuel Economy Checking PCM Power and Ground Circuit
Checking Fuel Pressure
Checking ECT Sensor Calibration
Checking TPS Calibration
Checking MAP Sensor Calibration
Checking Engine Mechanical Systems
Checking IAT Sensor
Checking PNP Switch
NO START INFORMATION
Important Note:
If the Powertrain Control Module has been changed and the correct VIN and mileage have not been programmed, a DTC will be set in the ABS and Air Bag modules. In addition, if the vehicle is equipped with a Sentry Key Immobilizer Module (SKIM), be sure to transfer the Secret Key data to the PCM.
For ABS and Air Bag Systems:
1. Enter correct VIN and Mileage in PCM.
2. Erase codes in ABS and Air Bag modules.
For SKIM Theft Alarm:
1. Connect the DRB III to the data link connector.
2. Go to Theft Alarm, SKIM, Misc. and place the SKIM in secured access mode, by using the appropriate PIN code for this vehicle.
3. Select Update the Secret Key data, data will be transferred from the SKIM to the PCM (This is required to allow the vehicle to start with the new PCM).
4. If three attempts are made to enter secured access mode using the incorrect PIN, secured access mode will be locked out for one hour. To exit this lock out mode, leave the ignition key in the Run/Start position for one hour. Ensure all accessories are turned off. Also monitor the battery state and connect a battery charger if necessary.
After reading (System Description and Functional Operation), you should have a better understanding of the theory and operation of the on-board diagnostics, and how this relates to the diagnosis of a vehicle that may have a driveability related symptom or complaint.
WARNINGS AND CAUTIONS
Road Test Warnings
Some complaints will require a test drive as part of the repair verification procedure. The purpose of the test drive is to try to duplicate the diagnostic code or symptom condition.
CAUTION: BEFORE ROAD TESTING A VEHICLE, BE SURE THAT ALL COMPONENTS ARE REASSEMBLED. DURING THE TEST DRIVE, DO NOT TRY TO READ THE DRB III SCREEN WHILE IN MOTION. DO NOT HANG THE DRB III FROM THE REAR VIEW MIRROR OR OPERATE IT YOURSELF. HAVE AN ASSISTANT AVAILABLE TO OPERATE THE DRB III.
Vehicle Damage Cautions
Before disconnecting any control module, make sure the ignition is "OFF". Failure to do so could damage the module.
When testing voltage or continuity at any control module, use the terminal side (not the wire end) of the connector. Do not probe a wire through the insulation; this will damage it and eventually cause it to fail because of corrosion.
Be careful when performing electrical tests so as to prevent accidental shorting of terminals. Such mistakes can damage fuses or components. Also, a second code could be set, making diagnosis of the original problem more difficult.