Electronic Spark Timing

In the EST mode, the ignition spark timing and ignition dwell time is fully controlled by the ECM/PCM. Above 450 rpm, the ECM/PCM applies 5.0 volts to the ignition module (bypass circuit) signaling the module to allow the ECM/PCM to control the dwell and spark timing. This is called EST mode. During EST mode, the ECM/PCM compensates for all driving conditions. The ECM/PCM relies on the following inputs to calculate the EST signal:

^ Crankshaft sensor
^ Coolant Temperature Sensor (CTS)
^ Engine speed
^ Knock Sensor
^ Park/Neutral switch
^ Throttle Position Sensor (TPS)
^ Vehicle Speed Sensor (VSS)
To diagnose any of the above components refer to COMPUTERIZED ENGINE CONTROLS.

The EST system utilizes several input circuits. These input circuits include the fuel control reference, spark reference, crankshaft reference ground, bypass signal, the EST output, ESC (knock sensor), and cam signal, all of which are described below.

Fuel Control Reference - From the ignition module, the ECM/PCM uses this signal to calculate engine rpm and crankshaft position. The ECM/PCM compares pulses on this circuit to any that are on ground CKT 453, ignoring any pulses that appear on both. The ECM/PCM also uses the pulses on this circuit to initiate injector pulses. If the ECM/PCM receives no pulses on this circuit, no fuel injection pulses will occur, the engine will not run.

Spark Reference - The spark reference signal is used to accurately control spark timing at low rpm and allow EST operation during crank. Below 1200 rpm, the ECM/PCM is monitoring CKT 647 and using it as the reference for ignition timing advance. When engine speed exceeds 1200 rpm, the ECM/PCM begins using CKT 430, fuel control reference to control spark timing.

Crankshaft Reference Ground - This is a ground circuit for the digital rpm counter inside the ECM/PCM, but the wire is connected to engine ground only through the ignition module. Although this circuit is electrically connected to the ECM/PCM, it is not connected to ground at the ECM/PCM. The ECM/PCM compares voltage pulses on the reference input CKT 430 to any on this circuit, ignoring pulses that ground on both. If the circuit is open, or connected to ground at the ECM/PCM, it may cause poor engine performance and possibly a SERVICE ENGINE SOON light with no code.

Bypass Circuit - The ECM/PCM either allows the ignition module to keep the spark advance at module mode 10° BTDC, or the ECM/PCM signals the ignition module that the ECM/PCM is going to control the spark advance EST mode. The ignition module determines the correct operating mode based on the level of voltage that the ECM/PCM sends to the ignition module on the bypass control circuit. The ECM/PCM provides 5.0 volts to the ignition module if the ECM/PCM is going to control spark timing EST mode. If the ECM/PCM does not turn ON the 5.0 volts, or if the 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 a code 42 and the ignition system will stay at module mode advance.

EST Circuit - The EST output circuitry of the ECM/PCM sends out timing pulses to the ignition module on this circuit. When in the module mode, the ignition module grounds these pulses. When in the EST mode, these pulses are the ignition timing pulses used by the ignition module to energize the ignition coils. Proper sequencing of the 3 ignition coils is always the job of the ignition module. If CKT 423 is open or grounded when the engine is started, a code 42 will set and the ignition system will stay in the module mode. If CKT 423 becomes open or grounded during EST mode operation, The engine will quit running but will restart. Upon restart, following an ignition cycle, a code 42 will be set, and the ignition will operate in module mode.

ESC Signal - The ESC system is comprised of a knock sensor and the ECM/PCM. The ECM/PCM monitors the ESC signal CKT 457 to determine when engine detonation occurs. When the knock sensor detects detonation, the ECM/PCM retards the timing EST to reduce detonation. Retarded timing can also be a result of excessive valve lifter, pushrod or other mechanical engine or transmission noise.

Cam Signal - The ECM/PCM uses this signal to determine the position of the #1 piston during its power stroke. This signal is used by the ECM/PCM to calculate true Sequential Fuel Injection (SFI) mode of operation. A loss of the is signal will set a code 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 fuel injection 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.

MODULE MODE
While the engine is being cranked the ignition module controls spark timing. This is known as bypass timing mode because the ignition module bypasses the ECM. The ignition system operates independently of the ECM, with module mode spark advance always at 10° BTDC. The ECM has no control of the ignition system when in this mode. In fact, the ECM could be disconnected from the car and the ignition system would still fire the spark plugs, as long as the other ignition system components were functioning. If an ECM/EST 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.