DTC 42

Chart - Code 42 Computer Controlled Coil Ignition:

Wiring Diagram for EST or Bypass Circuit Failure:

CODE 42 - C3-I EST OR BYPASS CIRCUIT FAILURE



Circuit Description: The Computer Controlled Coil Ignition (C-3 I) module sends a reference signal to the Electronic Control Module (ECM), when the engine is cranking. While the engine is under 400 rpm, the C-3 I 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, through the Electronic Spark Timing (EST) 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 engine speed must be greater than 600 rpm with no EST pulse for 200 ms (open or grounded CKT 423) or, ECM commanding by pass mode. (Open or grounded CKT 424)



Test Description: Numbers below refer to circled numbers o the diagnostic chart

1. Checks to see 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 ohm meter should be reading less than 200 ohms, which is the normal resistance of the EST circuit through the C-3 I module. A higher resistance would indicate a fault in CKT 423 a poor C-3 I module connection, or a faulty C-3 I module.

3. if test light was on, when connected from 12 volts to ECM harness terminal "D5", either CKT 423 is shorted to ground or the C-3 I module is faulty.

4. Checks to see if C-3 I module switches, when the bypass circuit is energized by 12 volts, through the test light. If the C-3 I 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, 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 ECM harness connectors for backed out terminals "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 checks out 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.