Powertrain Controls - ECM/PCM

Code Chart 42:

Ignition Circuit:

CIRCUIT DESCRIPTION:
The ignition control module sends a reference signal to the ECM via CKT 423 when the engine is cranking or running. Initially, the ignition control module controls ignition timing (module mode). 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 (IC mode). If an open or ground occurs in either circuit while is running in IC mode, the engine will stall and set DTC 42. If CKT 424 is open or grounded while the engine is cranking, the engine will start and remain in module mode timing.

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

^ Engine speed is greater than 600 rpm with no IC pulse for 100 mS (open or grounded circuit 423).
OR
^ ECM unable to command IC mode (open or grounded circuit 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 DTC 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 300 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 control module connection, or a faulty ignition control module.

3. If test light was "ON" when connected from 12 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 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.