Powertrain Controls - ECM/PCM

DTC 42 Chart Ignition Control Circuit:

Ignition System Wiring Diagram:

Circuit Description:

The ignition system provides two timing inputs to the ECM, high resolution (360 pulses per crankshaft revolution) and low resolution (4 pulses per crankshaft revolution). The ECM uses these two reference pulses to determine individual ignition spark timing for each cylinder.

Once the ECM calculates ignition timing, the timing signal will be sent to the ignition coil module on the IC circuit. Each timing pulse received by the ignition coil module, on the IC circuit, will trigger the ignition coil module to operate the ignition coil. Secondary ignition voltage is induced and is then sent to the distributor for distribution to each spark plug.

The IC signal voltage ranges from about 0.5 volt to 4.5 volts.

If a DTC 42 is detected, the ECM will disable the fuel injectors to prevent flooding of the engine. DTC 42 will set if the IC circuit problem occurs during cranking.


Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. DTC 42 will set if:

^ Engine speed is below 3000 RPM.
^ Voltage on CKT 423 is below about 0.5 volt. If the engine starts at this point, DTC 42 is intermittent.

2. This check determines if the IC signal from the ECM is available at the ignition coil module.

3. The remaining tests begin to check that the ignition coil module circuitry is OK. If the ignition coil module loses its voltage source, secondary voltage will not be produced and a DTC 42 will set.

4. Since the ignition coil module gets its power from the coil, check the ignition feed circuit to the ignition coil for opens.

5. A DTC 42 will set if the high and low resolution CKTs are shorted to each other. Before replacing any components, make sure these circuits are not shorted together.