Powertrain Controls - ECM/PCM

Code 42 Chart:

Ignition Wiring Diagram:

CIRCUIT DESCRIPTION:

When the system is running on the ignition module, that is, no voltage on the by-pass line, the ignition module grounds the Electronic Spark Control (EST) signal. The ECM expects to sense no voltage on the EST line during this condition. If it senses a voltage, it sets Code 42 and will not go into the EST mode. When the engine speed for EST is reached (about 400 rpm), and by-pass voltage applied, the EST should no longer be grounded in the ignition module so the EST voltage should be varying. If the by-pass line is open or grounded, the ignition module will not switch to EST mode so the EST voltage will be low and Code 42 will be set. If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded there will be no EST signal. A Code 42 will be set. A Code 42 sets if there is an open or a short to ground in the EST or by-pass circuit.


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

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

2. Checks for a normal EST ground path through the ignition module. An EST 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:
^ EST circuit 423 shorted to ground.
^ Bypass circuit 424 open.
^ Faulty ignition module connection or module.

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


DIAGNOSTIC AIDS:

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

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

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