Powertrain Controls - ECM/PCM

Trouble Code 42 Chart:

Ignition Wiring Schematic:

CIRCUIT DESCRIPTION
Ignition control (IC) spark timing is provided by the Powertrain Control Module (PCM) on circuit 423. Inputs from a number of engine sensors allow the PCM to correctly calculate optimum spark timing. Engine load is calculated from the Manifold Absolute Pressure (MAP) sensor engine RPM is derived from the 3X crankshaft position sensor, engine operating temperature from Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT) sensors, and engine knock activity from the knock sensor. Other engine functions modify spark to a lesser degree such as Torque Converter Clutch (TCC) engagement and Exhaust Gas Recirculation (EGR) activity.

The bypass circuit controls an internal switch in the electronic Ignition Control Module (ICM). Unless the vehicle is in back-up spark (base timing mode), the bypass circuit will have 5 volts applied to it. This 5 volt signal from the PCM allows a solid state switch to direct the IC timing pulses to the ICM or sequencing and energizing the coil packs. If the bypass circuit is not energized, the IC pulses from the ICM are shunted to ground and the electronic ICM uses it's own base timing schedule to time and energize the coil packs.

DIAGNOSTIC TROUBLE CODE (DTC) 42 WILL SET WHEN
No pulses are detected on the IC circuit by the PCM.

ACTION TAKEN (PCM) WILL DEFAULT TO
The PCM will allow the electronic ICM to control timing using it's own base timing schedule. The Malfunction Indicator Lamp (MIL) will become illuminated.

DTC 42 WILL CLEAR WHEN
A current DTC 42 will clear after each ignition key cycle. A History DTC 42 will clear after 50 consecutive ignition key cycles without a current DTC 42 being stored.

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

1. DTC 42 indicates the PCM has detected an open or short to ground in the bypass circuit, or a grounded I/C circuit. This test confirms DTC 42 an that the fault causing the DTC is present.
2. Checks for a normal IC ground path through the ignition control module. circuit 423 shorted to ground will also read less than 500 ohms; however, this will be checked later.
3. As the test light voltage touches circuit 424, the module should switch causing the ohmmeter to "overrange" if the meter is in the 1000 - 2000 ohms position. Selecting the 10,000 - 20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched."
4. The module did not switch and this step checks for:
^ IC circuit 423 shorted to ground.
^ Bypass circuit 424 open.
^ Faulty electronic ICM connection or module.
5. Confirms that DTC 42 is a faulty PCM and not an intermittent in circuit 423 or 424.

DIAGNOSTIC AIDS
The Tech 1 scan tool does not have any ability to help diagnose a DTC 42 problem
Refer to Computers and Controls / System Diagnosis / Procedures / Diagnosis by Symptom / Intermittents. - Intermittent Malfunctions