DTC 42

Code 42 Chart:

Code 42 Wiring Diagram:

CIRCUIT DESCRIPTION:

The C3I module sends a reference signal to the ECM, when the engine is cranking. While the engine is under 400 rpm, the C3I module controls the ignition timing. When the engine speed exceeds 400 rpm the ECM sends a 5 volt signal on the bypass circuit 424 to switch the timing to ECM control. The ECM controls EST through circuit 423. 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 greater than 600 rpm, with no EST pulse for 200 mS. (Open or grounded circuit 423).

OR

^ ECM commanding bypass mode. (Open or Grounded circuit 424).

NOTE: Because of all the possible color code combinations used on electrical wiring diagrams, always refer to ECM CONNECTOR IDENTIFICATION under ELECTRICAL AND ELECTRONIC WIRING DIAGRAMS for correct color code identification of circuit.

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

1. Checks to see if the ECM recognizes a problem. If it does not set Code 42, at this point, 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 C3I module. A higher resistance would indicate a fault in circuit 423, a poor C3I module connection, or a faulty C3I module.

3. If the test light was "ON", when connected from 12 volts to the ECM harness terminal "D5", either circuit 423 is shorted to ground, or the C3I module is faulty.

4. Checks to see if the C3I module switches when the bypass circuit is energized by the 12 volts through the test light. If the C3I module actually switches, the ohmmeter reading should shift to over 8,000 ohms.

5. Disconnecting the ignition module should make the ohmmeter read as if it were monitoring an open circuit (infinite reading). Otherwise, circuit 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 a damaged Harness. Inspect ECM harness connectors for backed out terminal "B4" or "D5" improper mating, broken locks, improperly formed or damaged terminals, poor terminal to wire connection and damaged harness.

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