DTC 42

Code 42 Chart:

Code 42 Wiring Diagram:

CIRCUIT DESCRIPTION:

The ignition module sends a reference signal to the ECM when the engine is cranking or running. While the engine is under 700 rpm, the ignition module controls the ignition timing. When the engine speed exceeds 700 rpm, the ECM sends a 5.0 volts signal on the "bypass" CKT 424 to switch the timing to ECM control through the EST CKT 423.

An open or ground in the EST or "bypass" circuit will set a Code 42 and cause the engine to run on module or "bypass" timing. This will result in poor performance and poor fuel economy.

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

1. Checks to see if ECM recognizes a problem. If it doesn't 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 500 ohms, which is the normal resistance of 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 the test light was "ON" when connected from 12.0 volts to ECM harness terminal "BC7," either CKT 423 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 12V through the test light. If the ignition module actually switches, the ohmmeter reading should shift to over 8000 ohms.

5. Disconnecting the ignition module should make the ohmmeter read as if it were monitoring an open circuit (infinite reading). If the ohmmeter has a reading other than infinite, 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. Inspect 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.