Powertrain Control Module (PCM)
The PCM is responsible for maintaining proper spark and fuel injection timing for all driving conditions.To provide optimum driveability and emissions, the PCM monitors input signals from the following components in calculating Ignition Control (IC) timing:
^ Ignition control module.
^ Engine Coolant Temperature (ECT) sensor.
^ Intake Air Temperature (IAT) sensor.
^ Mass Air Flow (MAF) sensor.
^ Transmission range switch input.
^ Throttle Position (TP) sensor.
^ Vehicle Speed Sensor (VSS).
Modes of Operation
The ignition system uses the same four ignition module-to-PCM circuits as did previous Delco engine management systems using distributor-type ignition. Ignition Control (IC) is the PCM's method of controlling spark advance and ignition dwell when the ignition system is operating in the IC mode.
There are two "modes" of ignition system operation:
^ Module mode
^ Ignition control mode
In the module mode, the ignition system operates independently of the PCM, with module mode spark advance always at 10° BTDC. The PCM has no control of the ignition system when in this mode. In fact, the PCM could be disconnected from the vehicle and the ignition system would still fire the spark plugs, as long as the other ignition system components were functioning. (This would provide spark but no fuel injector pulses, and a no-start.) The PCM switches to ignition control mode (PCM controlled spark advance) as soon as the engine begins cranking. Once the change is made to ignition control mode, it will stay in effect until either: 1) the engine is turned "OFF," 2)the engine quits running, or 3) a PCM/IC fault is detected. If a PCM/IC fault is detected while the engine is running, the ignition system will switch to the module mode. The engine may quit running, but will restart and stay in the module mode with a noticeable driveability complaint.
In the ignition control mode, the ignition spark timing and ignition dwell time is fully controlled by the PCM. IC spark advance and ignition dwell is calculated by the PCM using the following inputs:
^ Engine speed (spark reference or fuel control reference)
^ Crankshaft position (spark reference or fuel control reference)
^ Engine Coolant Temp. ECT
^ Throttle Position TP sensor
^ Knock signal Knock sensor
^ Park/neutral position Transmission range switch
^ Vehicle speed Vehicle speed sensor
^ Diagnostic request input DLC diagnostic request terminal
^ PCM & ignition system supply voltage.
The following describes the PCM to ignition control module circuits:
Fuel Control Reference PCM Input (CKT 430) - From the ignition control module, the PCM uses this signal to calculate engine RPM and crankshaft position. The PCM compares pulses on this circuit to any that are on ground CKT 453, ignoring any pulses that appear on both. The PCM also uses the pulses on this circuit to initiate injector pulses. If the PCM receives no pulses on this circuit, no fuel injection pulses will occur, the engine will not run.
Spark Reference (CKT 647) - The spark reference signal is used to accurately control spark timing at low RPM and allow ignition control operation during crank. Below 1200 RPM, the PCM is monitoring CKT 647 and using it as the reference for ignition timing advance. When engine speed exceeds 1200 RPM, the PCM begins using CKT 430, fuel control reference to control spark timing. If the spark control reference circuit is not received by the PCM while the engine is running, a DTC 17 will be set and fuel control reference will be used to control spark advance under 1200 RPM, and module mode will be in effect at under 400 RPM. The engine will continue to run and start normally.
Reference Low (CKT 453) - This is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the ignition control module. Although this circuit is electrically connected to the PCM, it is not connected to ground at the PCM. The PCM compares voltage pulses on the reference input CKT 430 to those on this circuit, ignoring pulses that appear on both. If the circuit is open, or connected to ground at the PCM, it may cause poor engine performance and possibly a MIL (Service Engine Soon) with no DTC.
Bypass Signal (CKT 424) - The PCM either allows the ignition control module to keep the spark advance at "module mode" 10° BTDC, or the PCM signals the ignition control module that the PCM is going to control the spark advance (IC mode). The ignition control module determines correct operating mode based on the level of voltage that the PCM sends to the ignition control module on the bypass control circuit. The PCM provides 5 volts to the ignition control module if the PCM is going to control spark timing (IC mode). If the PCM does not turn "ON" the 5 volts, or if the ignition control module doesn't receive it, the module will keep control of spark timing (module mode). An open or grounded bypass control CKT 424 will set a DTC 42 and the ignition system will stay at module mode advance.
Ignition Control Output (CKT 423) - The ignition control output circuitry of the PCM sends out timing pulses to the ignition control module on this circuit. When in the "module mode," the ignition control module grounds these pulses. When in the IC mode, these pulses are the ignition timing pulses used by the ignition control module to energize one of the ignition coils. Proper sequencing of the 3 ignition coils, i.e.; which coil to "fire," is always the job of the ignition control module. If CKT 423 is open or grounded when the engine is started, a DTC 42 will set and the ignition system will stay in the module mode. If CKT 423 becomes open or grounded during ignition control mode operation, the engine will quit running but will restart. Upon restart, following an ignition cycle, a DTC 42 will be set, and the ignition will operate in "module mode."
Knock Sensor (KS) (CKT 496) The KS system is comprised of a knock sensor and the PCM. The PCM monitors the KS signal (CKT 496) to determine when engine detonation occurs. When the knock sensor detects detonation, the PCM retards the timing (IC) to reduce detonation. Retarded timing can also be a result of excessive valve lifter, pushrod or other mechanical engine or transmission noise.
Cam Signal (CKT 630)- The PCM uses this signal to determine the position of the #1 piston during its power stroke. This signal is used by the PCM to calculate true Sequential Fuel Injection (SF1) mode of operation. A loss of this signal will set a DTC 41. If the cam 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 cam pulse, and the engine will continue to run. The engine can be re-started and will run in the calculated sequential mode as long as the fault is present with a 1 in 6 chance of being correct.