Ignition Control (IC) System

Crankshaft Sensor Operation:

Ignition System:

EI Fault Line Circuit:

DESCRIPTION
As the engine comes up to operating speed the ECM decides that it is ready to start control of timing. Its decision is based upon the time between falls of reference signal. The ECM raises the bypass line to approximately 5 volts. The ignition control module then lets the Ignition Control (IC) signal rise to the 5 volts level. Remember, in bypass mode, the ignition control module clamps the IC signal to less than 1.3 volts. When the ignition control module sees a 5 volts bypass signal, the unit operates under ECM spark control, or IC mode. The ignition control module will the the coil when the IC signal makes a transition from high-to-low. The turn "ON" of coil current is controlled by the "Closed Loop" dwell circuit of the ignition control module. As long as the ignition control module is receiving a high bypass signal and a normal IC signal, the system will operate in IC mode. The loss of the bypass signal will result in bypass mode operation. The loss of the IC signal will cause the ignition control module to revert to an internal spark calibration, and a lack of engine performance may be detected.

The ignition control module used is unique in that it compares the number of reference pulses sent out (1 per 90 degrees of crankshaft rotation) to the number of pulses received back from the ECM on the IC line (CKT 423). For example, if the ignition control module is operating in the IC mode (above 400 RPM), and no high to low transitions occur on the IC line all of a sudden, with the bypass line still at 4.5 to 5.0 volts for approximately two crankshaft revolutions, the ignition control module will switch to bypass mode (module timing advance only). To signal the ECM that a fault has been detected in the IC line transmission, the ignition control module will pull the EI fault line (CKT 1310) low.