Powertrain Controls - ECM/PCM

Code 42:

Code 42 Wiring Diagram:

CIRCUIT DESCRIPTION:

The ignition module sends a reference signal to the ECM when the engine is cranking. Under 400 rpm, the ignition module controls ignition timing. When the engine speed exceeds 400 rpm, the ECM sends a 5.0 volt signal on the bypass CKT 424 to switch timing to ECM control.

An open or ground in the EST circuit will stall the engine and set a Code 42. The engine can be re-started but will run on module timing.

To set a Code 42 the following conditions must be met:

^ Engine speed must be greater than 6OO rpm with no EST pulse for 1OO mS (open or grounded CKT 423).

OR

^ ECM commanding bypass mode (open or grounded CKT 424).

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

1. Checks to see if ECM recognizes a problem. If it does not set Code 42, it is an intermittent problem and could be due to a loose connection.

2. With the ECM disconnected, the ohmmeter should be reading less than 200 ohms, which is the normal resistance of the EST circuit through the ignition module. A higher resistance would indicate a fault in CKT 423, a poor ignition module connection, or a faulty ignition module.

3. If test light was "ON" when connected from 12.0 volts to ECM harness terminal "BC7" either CKT 424 is shorted to ground or the ignition module is faulty.

4. Checks to see if ignition module switches when the bypass circuit is energized by 12.0 volts through the test light. If the ignition module actually switched the ohmmeter reading should shift to over 6,000 ohms.

5. Disconnecting the ignition module should make the ohmmeter read as if it were monitoring an open circuit (infinite reading). Otherwise CKT 423 is shorted to ground.

DIAGNOSTIC AIDS:

An intermittent may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation.

Check For:

^ Poor Connection or Damaged Harness - Inspect for ECM harness connectors for backed out terminals "BC7" or "BC8," improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection, and damaged harness.

^ Intermittent Test - If connections and harness check OK, a digital voltmeter connected from affected terminal to ground while moving related connectors and wiring harness. If the failure is induced, the voltage reading will change.