The description and operation of the distributor ignition system is in Engine Electrical.
Refer to the Distributor Ignition (DI) System Check for an ignition system check.
Ignition Control (IC) System
The On-Board Diagnostic System Check procedure uses DTC 12 in order to test the self diagnostic ability of the ECM. This DTC indicates that the ECM does not receive the engine RPM (reference) signal.
The Reference signal also triggers the fuel injection system. Without the Reference signal, the engine cannot run.
Results of Incorrect IC Operation
The ECM uses the information from the MAP and the Engine Coolant Temperature (ECT) sensors in addition to the RPM in order 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. |
A high MAP output or low resistance in the ECT sensor circuit could cause detonation.
A high MAP output or low resistance in the ECT sensor circuit could cause poor performance.
DTC 42
A fault in the IC system will usually set a DTC 42.
When the system runs on the distributor ignition control module, there is no voltage on the bypass line and the module grounds the IC control line. The ECM expects to see no voltage on the IC control line during this condition. If the module sees 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 IC bypass line and the IC control should no longer be grounded in the distributor ignition control module, so the IC control voltage should vary.
If the IC bypass line is open, the module will not switch to the test mode, so the IC control voltage will be low and DTC 42 will set.
If the IC control line is grounded, the module will switch to IC, but because the line is grounded, there will be no IC control signal and the engine will not run. A DTC 42 may or may not set.
An open in the IC control circuit will set a DTC 42 and cause the engine to run on the distributor ignition control module timing. This will cause poor performance and poor fuel economy. A ground may set a DTC 42, but the engine will not run.
IC Performance Check
In order to check the IC operation, record the timing at 2,000 RPM with the diagnostic terminal not grounded. Then, open the Set Timing connector, and the timing should change at 2,000 RPM indicating that IC is operating.
