Powertrain Controls - ECM/PCM

Code 42:

Ignition Circuit:

CIRCUIT DESCRIPTION:

The C3I module sends a reference signal to the PCM when the engine is cranking. Until the second fuel control pulse, the ignition module controls ignition timing (module mode). Then the PCM sends a 5.0 volts signal on the bypass CKT 424 to switch timing to PCM control. An open or ground in the EST or bypass circuit at the time the engine is started will 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:

^ Open or grounded CKT 423 at the time of engine start-up, or
^ PCM commanding bypass mode (open or grounded CKT 424) when the engine is started.

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

1. Checks to see if PCM 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 PCM disconnected, the ohmmeter should be reading less than 200.0 ohms, which is the normal resistance of the EST circuit through the C3I module. A higher resistance would indicate a fault in CKT 423, a poor C3I module connection, or a faulty C3I module.

3. If test light was "ON" when connected from 12.0 volts to PCM harness terminal "BC6" either CKT 424 is shorted to ground or the C3I module is faulty.

4. Checks to see if C3I module switches when the bypass circuit is energized by 12.0 volts through the test light. If the C3I module actually switches, the ohmmeter reading should shift to over 6,000.0 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 PCM harness connectors for backed out terminals "BC6" or "BC7," 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.

^ Refer to "DETONATION/SPARK KNOCK" in DIAGNOSIS BY SYMPTOM.