Electronic Spark Timing

PURPOSE
The EST system uses several circuits which include the DIS reference circuit, reference ground, bypass and the EST circuit. In the DIS 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.

BYPASS CIRCUIT OPERATION
In the bypass circuit, the ECM applies approximately 5.0 volts at about 400 RPM to switch spark timing control from the DIS 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.

EST MODE OPERATION
EST mode is defined as any time during engine operation that the EST signal is controlling ignition spark timing. The BYPASS line is raised by the PCM. The DIS module shall release the EST line within 5 milliseconds after the BYPASS line is raised. The PCM shall pulse the EST signal with a minimum pulse width of 1.0 millisecond. The falling edge of the EST signal defines the desired timing for the ignition event.

In the EST circuit, the DIS module sends a reference signal to the ECM when the engine is cranking. While the engine is under 400 RPM, the DIS module controls the ignition timing. When engine speed exceeds 400 RPM, the ECM controls ignition timing and 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 OPERATION
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.