Powertrain Controls - ECM/PCM

Code 42 Chart:

Ignition Circuit:

CIRCUIT DESCRIPTION:
The Direct Ignition System (DIS) module sends a reference signal to the Electronic Control Module (ECM) when the engine is cranking. While the engine speed is under 400 rpm, the DIS module controls the ignition timing. 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 Timing (EST) signal. The ECM expects to sense no voltage on the EST Line during this condition. If it senses a voltage, it sets a Code 42 and will not go into the EST mode. When the engine speed exceeds 400 rpm, the ECM applies 5.0 volts to the by-pass line to switch the timing to ECM control (EST). If the by-pass line is open or grounded, once the rpm for EST control is reached, the ignition module will not switch to EST mode. This results in low EST voltage 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.

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. This 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 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. 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:
The "Scan" tool does not have any ability to help diagnose a Code 42 problem. If a Code 42 is intermittent, - Intermittent Malfunctions