Powertrain Controls - ECM/PCM

Code Chart 42:

Ignition Circuit:

CIRCUIT DESCRIPTION:
The Electronic Ignition Control Module (ICM) sends a reference signal to the Engine Control Module (ECM) when the engine is cranking. While the engine speed is under 400 RPM, the ICM controls the ignition timing. When the system is running on the ICM (no voltage on the bypass line), the ICM grounds the Ignition Control (IC) signal. The ECM expects to sense no voltage on the Ignition Control (IC) line during this condition. If it senses a voltage, it sets DTC 42 and will not enter the ignition control mode.

When the engine speed exceeds 400 RPM, the ECM applies 5 volts to the bypass line to switch the timing to ECM control (ignition control). If the bypass line is open or grounded, once the RPM for ignition control is reached, the ICM will not switch to Ignition Control (IC) mode. This results in low Ignition Control (IC) voltage and the setting of DTC 42. If the IC line is grounded, the ICM will switch to Ignition Control (IC), but because the line is grounded, there will be no IC signal. A DTC 42 will be set.

TEST DESCRIPTION: Number(s) below refer to circled number(s) on the diagnostic chart.

1. DTC 42 means the ECM has sensed an open or short to ground in the IC or bypass circuits. This test confirms DTC 42 and that the fault causing the DTC 42 is present.

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

3. As the test light voltage contacts CKT 424, the ICM should switch, causing the ohmmeter to overrange if the meter is in the 1000 to 2000 ohms position. Selecting the 10,000 to 20,000 ohms position will indicate a reading 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 ICM connection or ICM.

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

DIAGNOSTIC AIDS:
If DTC 42 is intermittent, refer to Diagnosis By Symptom Testing and Inspection