Powertrain Controls - ECM/PCM

DTC 42 Chart:

Wiring Diagram:

CIRCUIT DESCRIPTION:

When the system is running on the Distributor Ignition (DI) module, that is, no voltage on the by-pass line, the DI module grounds the Ignition Control (IC) signal. The ECM expects to sense low voltage on the IC line during this condition. If it senses a voltage, it sets DTC 42 and will not go into the IC mode.

When the engine speed for IC is reached (about 400 rpm), and by-pass voltage is applied, the IC should no longer be grounded in the DI control module so the IC voltage should be varying. If the by-pass line is open or grounded, the DI control 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 when the ECM applies bypass voltage to DI control module, but because the line is grounded there will be no IC signal. A DTC 42 will be set.


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

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

2. Checks for a normal IC ground path through the ignition module. An IC circuit 423 shorted to ground will also read less than 500 ohms. This will be checked later.

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

4. If the module did not switch, this step checks for:
^ IC circuit 423 shorted to ground.
^ Bypass circuit 424 open.
^ Faulty ignition module connection or module.

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


DIAGNOSTIC AIDS:

1. If a DTC 42 was stored and the customer complains of a "Hard start" condition, the problem is most likely a grounded IC line, circuit 423.

2. A PROM not fully seated in the ECM can result in a DTC 42.

3. The "Scan" tool does not have any ability to help diagnose a DTC 42 problem. Testing and Inspection