Diagnostic Notes
DIAGNOSISIf the engine cranks but will not run, Chart A-3 will be used to determine if the Ignition Control (IC) 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 C-4F, "Misfire" for diagnosis.
HOW DTC 42 IS DETERMINED
An open in the IC circuit while running will result in the engine continuing to run although in a base timing mode (module timing). The timing values are stored in the ignition control module to allow for basic engine performance. If the IC circuit is shorted to ground, the MIL 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 grounded, the ignition control module will not switch to IC timing mode so the IC voltage will be low and DTC 42 will be set.
If the IC circuit is grounded, the ignition control module will switch to base timing and a DTC 42 will be set and MIL illuminated.
RESULTS OF INCORRECT OPERATION
An open or ground in the ignition control or bypass circuit will set a DTC 42. If a fault occurs in the ignition control output circuit when the engine is running, the engine may falter or quit running but will restart and run in base timing mode once the ignition has been cycled. A fault in either circuit will force the ignition system to operate on base timing (100 BTDC), which will result in reduced performance and fuel economy.
The PCM uses information from the engine coolant temperature sensor in addition to RPM to calculate the main spark advance values as follows:
High RPM - more advance
Cold engine - more advance
Low RPM - less advance
Hot engine - less advance
Therefore, detonation could be caused by high resistance in the engine coolant temperature sensor circuit. Poor performance could be caused by low resistance in the engine coolant temperature sensor circuit.