Powertrain Controls - ECM/PCM

Diagnostic Trouble Code 42:

Ignition Circuit:

CIRCUIT DESCRIPTION
When the system is running on the Distributor Ignition (DI) control module, that is, no voltage on the bypass line, the distributor ignition control module grounds the Ignition Control (IC) signal. The Engine Control Module (ECM) expects to sense no voltage on the IC line during this condition. If it detects a voltage, it sets Diagnostic Trouble Code (DTC) 42 and will not go into the IC mode.

When the RPM for IC is reached (about 400 RPM), and bypass voltage applied, the IC should no longer be grounded in the DI control module so the IC voltage should be varying.

If the bypass line is open or grounded, the ignition module will not switch to IC mode so the IC voltage will be low and DTC 42 will be set.

If the IC line is grounded, the DI control module will switch to IC, but because the line is grounded there will be no IC signal. A DTC 42 will be set.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

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

2. As the test light voltage touches CKT 424, the module should switch. The important thing is that the module "switched."

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

^ IC CKT 423 shorted to ground.
^ Bypass CKT 424 open.
^ Faulty DI control module connection or module.

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

DIAGNOSTIC AIDS
If a DTC 42 was stored and the customer complains of a "Hard Start," the problem is most likely a grounded ignition control circuit (CKT 423).

Refer to Intermittents in Diagnosis By Symptom. Testing and Inspection