Distributor: Testing and Inspection

DIAGNOSTIC PROCEDURE
Refer to Chart C-4 Distributor Ignition System Check for ignition system diagnostic procedure. Chart C-4 Distributor Ignition (DI) System Check

INCORRECT IGNITION CONTROL OPERATION
An open in the Ignition Control (IC) circuit will set a Diagnostic Trouble Code (DTC) 41, and a ground in the IC circuit, will set a DTC 42 and cause the engine to run on the Distributor Ignition (DI) control module timing. This will cause poor performance and poor fuel economy.

The Engine Control Module (ECM) uses information from the Manifold Absolute Pressure (MAP) and engine coolant temperature sensors, in addition to rpm, to calculate spark advance as follows.

Low MAP output voltage = more spark advance cold engine = more spark advance

High MAP output voltage = less spark advance hot engine = less spark advance

Therefore, detonation could be caused by low MAP output or high resistance in the engine coolant temperature sensor circuit.

Poor performance could be caused by high MAP output or low resistance in the Engine Coolant Temperature (ECT) sensor circuit.

HOW DTC 42 IS DETERMINED
When the system is running on the DI control module, that is, no voltage on the bypass line, the DI control module grounds the IC signal. The ECM expects to see no voltage on the IC line during this condition. If it detects a voltage, it sets DTC 42 and will not go into the IC mode.

When the rpm for IC is reached (about 400 RPM), the ECM applies 5 volts to the bypass line and the IC should no longer be grounded in the DI control module, so the IC voltage should be varying.

If the bypass line is open, the DI control module will not switch to IC mode, so the IC voltage will be low and DTC 42 will be set.

If the IC line is grounded, the DI control module will switch to IC, but because the line is grounded there will be no IC signal and the engine will not run. A DTC 42 may, or may not be set.

DTC 12 APPLICATION
DTC 12 is used during the On-Board Diagnosis System Check procedure to test the DTC display ability of the ECM. This DTC indicates that the ECM is not receiving the engine RPM (reference) signal. This occurs with the ignition key "ON" and the engine not running.

The "Reference" signal also triggers the fuel injection system. Without the "Reference" signal, the engine cannot run.