Powertrain Controls - ECM/PCM

DTC 42 Chart:

Ignition Control Timing Circuit:

Circuit Description:
Ignition control timing is provided by the PCM on CKT 423. Using inputs from both the 3X and 24X crankshaft position sensors, spark timing is calculated and delivered to the electronic ignition control module to sequence and energize the coil packs. If this ignition control circuit is grounded, the electronic ICM will control spark timing from a preset base timing schedule and a DTC 42 will be set.

The ignition control bypass circuit should always have 5 volts applied to it. This input controls the solid state switch internal to the ICM, which provides a complete circuit for timing signals delivered from the PCM to the electronic ICM. If the bypass circuit is open or grounded, timing signals from the PCM on the ignition control circuit are grounded inside the electronic ICM. The ICM will use its own base timing schedule and a DTC 42 will be set.

DTC 42 Will Set When:
No pulses are detected on the IC circuit by the PCM.

Action Taken (PCM will default to):
The PCM will allow the electronic ignition control module to control timing using its own base timing schedule. The MIL will become illuminated.

DTC 42 Will Clear When:
A current DTC 42 will clear after each ignition key cycle. A history DTC 42 will clear after 50 consecutive ignition key cycles without a current DTC 42 being stored.

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

1. DTC 42 indicates the PCM has detected an open or short to ground in the bypass circuit. This test confirms DTC 42 and that the fault causing the DTC is present.

2. Checks for a normal IC ground path through the ignition control module. An IC CKT 423 shorted to ground will also read less than 500 ohms; however, this will be checked later.

3. As the test light voltage touches CKT 424, the module should switch causing the ohmmeter to "overrange" if the meter is in the 1000-2000 ohms position. Selecting the 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched."

4. The module did not switch and this step checks for:

- IC CKT 423 shorted to ground.
- Bypass CKT 424 open.
- Faulty electronic ignition control module connection or module.

5. Confirms that DTC 42 is a faulty PCM and not an intermittent in CKT 423 or 424.

Diagnostic Aids:
The Tech 1 scan tool does not have any ability to help diagnose a DTC 42 problem.

Refer to "Intermittents" in "Diagnosis By Symptom." - Intermittent Malfunctions