Results of Incorrect Operation

When the system is running on the Ignition Control Module (ICM), that is, no voltage on the bypass line, the ICM grounds (GND) the IC signal. The ECM expects to see no voltage on the Ignition Control (IC) line during this condition. If it sees a voltage, it sets Diagnostic Trouble Code (DTC) 42 and will not go into the IC mode.

When the rpm for IC is reached (about 400 rpm) the Engine Control Module (ECM) applies 5volts to the bypass line and the IC should no longer be grounded in the ICM so the IC voltage should be varying.

An open in the IC circuit while running, will result in the engine continuing to run, but in a back-up ignition timing mode (module timing) at a calculated timing value and the malfunction indicator lamp will not be "ON".

If the IC circuit is shorted to GND, the malfunction indicator lamp will be turned "ON", and a DTC 42 will be set and the engine will operate in module timing. Either condition may cause poor performance and reduced fuel economy.

If the bypass line is open or GND, the ICM will not switch to IC mode so the IC voltage will be low and DTC 42 will be set.

If the IC line is grounded, the ICM will switch to IC, but because the line is GND there will be no IC signal. A DTC 42 will be set.

DIAGNOSIS
If the engine cranks but will not run, CHART A-3 will be used to determine if the Electronic Ignition (EI) system is the cause of the problem or if it is another part of the fuel injection system.

If DTC 42 is set, that DTC chart should be used for diagnosis. If no DTC is set and the ignition system is suspected, use CHART C4-D ("EI" Misfire) for diagnosis.


DIAGNOSTIC TROUBLE CODE (DTC) 12
DTC 12 is used during the on-board diagnostic system check procedure to test the DTC display ability of the ECM. This diagnostic trouble code indicates that the ECM is not receiving the engine speed (REFERENCE) signal. This occurs with the ignition key "ON" and the engine not running.

The "reference" signal also triggers the fuel injection system. Without the "reference" signal the engine cannot run.