Ignition Control Module: Description and Operation
Electronic Ignition (EI) System With Ignition Control (IC):
PURPOSE
The Ignition Control Module (ICM) performs several functions:
^ It powers the 3X crankshaft sensor circuit.
^ It determines the correct ignition coil firing sequence, based on 3X pulse. This coil sequencing occurs at Start-up. After the engine is running, the module determines the sequence, and continues triggering the ignition coils in proper sequence.
^ It sends a "3X crankshaft reference" (fuel control) signal to the PCM. The PCM determines engine RPM from this signal. It is also used by the Powertrain Control module (PCM) to determine crankshaft position for ignition control spark advance calculations. The signal sent to the PCM by the ICM is an "ON-OFF' pulse occurring 3 times per crankshaft revolution.
Operation
To properly control timing, the 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 ICM), 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 (CKT) 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 - CKT 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-CKT424. 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 - CKT 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.