Spark Control System

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) spark timing 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
- IC mode

MODULE MODE
In the module mode, the ignition system operates independently of the PCM, with module mode spark advance always at 10 degrees 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 IC mode (PCM controlled spark advance) as soon as the engine begins cranking. Once the change is made to IC mode, it will stay in effect until either:

1. The engine is turned "OFF."
2. The engine quits running.
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.

IC MODE
In the IC 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 sensor).
- Throttle Position (Throttle Position (TP) sensor).
- Knock Signal (Knock sensor).
- Park/Neutral Position (Transaxle Range Switch (PRNDL) input).
- Vehicle Speed (Vehicle Speed Sensor).
- PCM and ignition system supply voltage.

COMPONENTS
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) spark timing:

- Electronic Ignition (EI) module.
- Engine Coolant Temperature (ECT) sensor.
- Intake Air Temperature (IAT) sensor.
- Mass Air Flow (MAF) sensor.
- PRNDL input from transaxle range switch.
- Throttle Position (TP) sensor.
- Vehicle Speed Sensor (VSS).
- ABS/TCS desired torque.

PCM TO IGNITION CONTROL MODULE CIRCUITS


Fuel control reference PCM input (CKT 430)
From the electronic ignition 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 and 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 IC 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 P0321 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 electronic ignition 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 electronic ignition module to keep the spark advance at "module mode" 10 degrees BTDC, or the PCM signals the ignition module that the PCM is going to control the spark advance (IC mode). The electronic ignition module determines correct operating mode based on the level of voltage that the PCM sends to the electronic ignition module on the bypass circuit. The PCM provides 5 volts to the electronic ignition 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 electronic ignition 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 DTC P1361 and the ignition system will stay at module mode advance.

Ignition Control (IC) output (CKT 423)
The IC output circuitry of the PCM sends out timing pulses to the electronic ignition module on this circuit. When in the "module mode," the electronic ignition module grounds these pulses. When in the IC mode, these pulses are the ignition timing pulses used by the electronic ignition 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 electronic ignition module. If CKT 423 is grounded when the engine is started, DTC P1361 will set and the ignition system will stay in the module mode. If CKT 423 becomes open or grounded during IC mode operation, DTC P1350 or P1361 may set and the engine will quit running but will restart. Upon restart, following an ignition cycle, DTC P1361 will be set, and the ignition system 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 (SFI) mode of operation. A loss of this signal will set DTC P0342. 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.

HOW DTCS P135O/P1361 ARE DETERMINED
The IC output circuitry in the PCM issues IC output pulses anytime crankshaft reference signal input pulses are being received. When the ignition system is operating in the module mode (no voltage on the bypass control circuit), the electronic ignition module grounds the IC pulses coming from the PCM. The electronic ignition module will remove the ground path for the IC pulses only after switching to the IC mode. (The PCM commands the switching between module and IC modes, via the bypass control circuit of the electronic ignition module.) The PCM monitors its own IC output, and expects to see no pulses on the IC circuit when it has not yet supplied the 5 volts on the bypass control circuit. When the second fuel control reference pulse at the start of crank is seen by the PCM, it applies 5 volts to the bypass control circuit and the IC pulses should no longer be grounded by the electronic ignition module. The PCM constantly monitors its IC output, and should "see" the IC pulses only when operating in the IC mode.

If IC output CKT 423 is open
The PCM will detect IC output pulses while attempting to start the engine (in the module mode) due to the electronic ignition module not being able to ground the IC pulses. Three things will occur:

1. DTC P1350 or P1361 will set.
2. The PCM will not apply 5 volts to the bypass circuit.
3. The engine will start and run in the module mode.

If IC output CKT 423 is grounded
The PCM will not detect a problem until the change to IC mode is commanded by the PCM. When the PCM applies 5 volts to the bypass control circuit, the ignition control module will switch to the IC mode. With IC CKT 423 grounded, there would be no IC pulses for the electronic ignition module to trigger the ignition coils, and the engine may falter. The PCM will quickly revert back to the module mode (turn "OFF" the 5 volts on the bypass control), DTC P1361 will set, and the ignition system will operate in the module mode until the fault is corrected and the engine is stopped and restarted.

If bypass control CKT 424 is open -or- grounded
The electronic ignition module will not switch to the IC mode. In this case, the IC pulses will stay grounded in the electronic ignition module, and DTC P1361 will be set. The engine will start and run in the module mode.

RESULTS OF INCORRECT OPERATION
An open or ground in the IC or bypass circuit will set DTC P1350 or P1361. If a fault occurs in the IC output circuit when the engine is running, the engine may falter or quit running but will restart and run in the module mode once the ignition has been cycled. A fault in either circuit will force the ignition system to operate on module mode timing (10 degrees BTDC), which will result in reduced performance and fuel economy.

The PCM uses information from the engine coolant temperature sensor in addition to RPM to calculate the main spark advance values as follows:

- High RPM = more advance
- Cold engine = more advance
- Low RPM = less advance
- Hot engine = less advance

Therefore, detonation could be caused by high resistance in the engine coolant temperature sensor circuit. Poor performance could be caused by low resistance in the engine coolant temperature sensor circuit.