Electronic Spark Timing

The EST system utilizes the same EST to ECM circuits that distributor type ignition systems use. These system circuits include the IDIS reference circuit, reference ground, bypass and the EST circuit.

In the IDIS reference circuit, the Crankshaft Sensor generates a signal to the ignition module which results in a reference pulse being sent to the ECM. The ECM uses this pulse to calculate crankshaft position, engine speed and injector pulse width.

In the reference ground circuit, the wire is grounded through the module and insures that the ground circuit has no voltage drop between the ignition module and the ECM, which could affect performance.

In the bypass circuit, the ECM applies approximately 5.0 volts at about 700 RPM to switch spark timing control from the IDIS module to the ECM. An open or grounded bypass circuit will set a code 42 and result in the engine operating in a backup ignition timing mode (module timing) at a calculated timing value. This may cause poor performance and reduce fuel economy.

In the EST circuit, the IDIS module sends a reference signal to the ECM when the engine is cranking. While the engine is under 700 RPM, the IDIS module controls the ignition timing. When engine speed exceeds 700 RPM, the IDIS module controls ignition timing. When engine speed exceeds 700 RPM, the ECM applies 5.0 volts to the bypass line to switch the timing to the ECM control (EST). An open or ground in the EST circuit will result in the engine continuing to run but in a backup ignition timing mode at a calculated timing value and the "SERVICE ENGINE SOON" lamp will not light. If the EST fault is present the next time the engine is restarted, a code 42 will be set and the engine will operate in module timing. This may cause poor performance and reduce fuel economy.

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.