Electronic Spark Timing

Ignition System Circuit:

DESCRIPTION
When engine speed exceeds 400 RPM, the PCM controls ignition timing and applies 5.0 volts to the bypass line to switch the timing to the PCM control (EST). The EST system uses several circuits which include the crank reference circuit (reference high), reference ground (reference low), bypass circuit, and the EST (ignition control) circuit. The Crankshaft Sensor creates an A/C voltage signal that the ignition control module read. the module then generates a square wave reference signal that is sent to the PCM.

Reference Signal Low
The crank reference low signal (also called reference ground) provides a ground to the PCM through the DIS module. The PCM uses this common ground to measure the reference "high" signal voltage. Without this ground through the ignition module, the PCM may not be able to detect ignition reference pulses. If this circuit is open, or grounded at any location other than through the DIS module, poor performance may result.

Reference Signal High
The "Crank Reference High" signal provides the PCM with engine speed and crankshaft position information from the DIS module. The signal is a square wave voltage output that toggles between 0 - 5 volts (if measured with the engine running, voltmeter would dislay approx. 2.4V). The PCM monitors the voltage in this circuit as compared with the reference low (reference ground). The voltage transitions to its high state (approx. 5 volts) at 6° BTDC of each cylinder. The PCM uses this signal to calculate crankshaft position and engine speed, then compares this with information from other engine sensors to determine optimum ignition timing advance for different engine speed and load conditions. The timing advance information is then sent back to the ignition control module over the EST signal line.

EST Signal
The EST signal is an output from the PCM to the Ignition Control Module. This signal provides ignition timing instructions to the ignition module. The ignition module sets base ignition timing according to signals from the crankshaft sensor plus a little preprogrammed advance based on engine speed. The EST signal tells the ICM to modify the base timing values for more advance or retard, depending on a number of other operating conditions.