Powertrain Controls - ECM/PCM

Ignition System Circuit Diagram (Type I):

CIRCUIT DESCRIPTION:
The ignition module sends a reference and a fuel control reference signal to the PCM when the engine is cranking. For the first two fuel control pulses, the ignition module controls ignition timing. Then the PCM sends a 5 volt 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 may be started but will run on module mode ignition timing.

Code 42 sets when the following conditions exist:
^ Open or grounded CKT 423 at the time of engine start-up, OR
^ ECM commanding bypass mode (open or grounded CKT 424) when the engine is started.

TEST DESCRIPTION

Code 42 Diagnostic Chart:

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 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 module connection, or a faulty ignition module.
3. If test light was "ON" when connected from 12 volts to PCM harness terminal "BC6" 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 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.