Powertrain Controls - ECM/PCM

DTC 42 Ignition Control Error (Electrical Diagnosis):

Ignition Control Wiring Circuit:

CIRCUIT DESCRIPTION
When the system is running on the ignition module, that is, no voltage on the bypass line, the ignition module grounds the Ignition Control (IC) signal. The Powertrain Control Module (PCM) expects to see no voltage on the IC line during this condition. If it sees a voltage, it sets DTC 42 and will not go into the IC mode.
When the RPM for IC is reached (about 500 RPM), and bypass voltage applied, the IC should no longer be grounded in the ignition module so the IC voltage should be varying.
If the bypass line is open or grounded (as when timing is set), the ignition module will not switch to IC mode so the IC voltage will be low and Diagnostic Trouble Code (DTC) 42 will be set.
If the IC line is grounded, the ignition module will switch to IC, but because the line is grounded, there will be no IC signal. A DTC 42 will be set.

DTC 42 WILL SET WHEN
No signal is detected on the IC circuit by the PCM.

ACTION TAKEN (PCM will default to)
The PCM will allow the Distributor Ignition (DI) control module to control timing, using it's own base timing schedule. The MIL (Service Engine Soon) 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.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. DTC 42 means the PCM has seen an open or short to ground in the IC or bypass circuits. This test confirms DTC 42 and that the fault causing the DTC is present.
2. Checks for a normal IC ground path through the ignition module. An IC CKT 423 shorted to ground will also read less than 500 ohms; however, this will be checked later.
3. As the test light voltage touches CKT 424, the Module should switch causing the ohmmeter to overrange" if the meter is in the 1000-2000 ohms position. Selecting the 10-20,000 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 CKT 423 shorted to ground.
^ Bypass CKT 424 open.
^ Faulty ignition module connection or module.
5. Confirms that DTC 42 is a faulty PCM and not an intermittent in CKT 423 or CKT 424.

DIAGNOSTIC AIDS
If a DTC 42 was stored and the customer complains of a "Hard Start," the problem is most likely a grounded IC line (CKT 423).
A PROM not fully seated in the PCM can result in a DTC 42. A DTC 42 will set if base timing was checked or set by grounding the set timing connector.
Refer to Intermittents under Computers and Control Systems/System Diagnosis/Diagnosis By Symptom. - Intermittent Malfunctions