Diagnostic Notes

CIRCUITS AFFECTING IGNITION CONTROL
To properly control timing, the Powertrain Control Module (PCM) relies on the following information:

^ Engine load (manifold pressure or vacuum).
^ Atmospheric (barometric) pressure.
^ Engine temperature.
^ intake air temperature.
^ Crankshaft position.
^ Engine speed (RPM).

The Ignition Control (IC) system consists of the ignition coil and module assembly (ignition coils, electronic Ignition Control Module), the 3X crankshaft position sensor and the 24X crankshaft position sensor, PCM and connecting wires. The electronic ICM connector terminals are lettered as shown in image. These circuits perform the following functions:

3X reference high - Circuit 430
The crankshaft position sensor sends a signal to the electronic ignition control module which generates a reference pulse which is sent to the PCM. The PCM uses this signal to calculate crankshaft position and engine speed (also used to trigger the injector).

3X reference low - Circuit 453
This wire is grounded through the module and makes sure the ground circuit has no voltage drop between the ignition module and the PCM, which if open, could affect performance.

IC bypass- Circuit 424
During initial cranking, the PCM will look for synchronizing pulses from the camshaft and 3X crankshaft position sensor indicating the position of #1 piston and intake valve. When 5 volts is applied to the bypass circuit the instant these signals are received by the PCM. This generally occurs within 1 or 2 revolutions of the crankshaft. An open or grounded bypass circuit will set a Diagnostic Trouble Code (DTC) 42 and the engine will run at base timing. A small amount of advance is built into the ICM to enhance performance.

IC - Circuit 423
The PCM uses this circuit to trigger the electronic ignition control module. The PCM uses the crankshaft reference signal to base its calculation of the amount of spark advance needed under present engine conditions.

24X reference signal
Additional to the electronic ignition system is the 24X crankshaft position sensor. It's function is to increase idle quality and provide good low speed driveability.

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 Malfunction Indicator Lamp (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 he set.
If the IC circuit is grounded, the ignition control module will switch to base timing and a DTC 42 will he 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 (10° 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.