Ignition Control Module: Testing and Inspection

IC SYSTEM
DTC 12 is used during the "On-Board Diagnostic System Check" to test the code display ability of the control module. This DTC indicates that the control module is not receiving the engine RPM (Reference) signal. The "Reference" signal also triggers the fuel injection system. Without the "Reference" signal, the engine cannot operate. Refer to Computers and Controls / System Diagnosis. Testing and Inspection

RESULTS OF INCORRECT IC OPERATION
The control module uses information from the MAP and coolant 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

Detonation can be caused by low MAP output or high resistance in the coolant sensor circuit.
Poor performance can be caused by high MAP output or low resistance in the coolant sensor circuit.

DTC 42
A fault in the IC system will usually set a DTC 42. Refer to DTC 42 in Computers and Controls / System Diagnosis. Testing and Inspection

When the system is operating on the distributor module, there is no voltage on the bypass line and the module grounds the IC signal. The control module expects to find no voltage on the IC line during this condition. If voltage is found, a DTC 42 will set and the system will not go into the IC mode.

When the RPM for IC is obtained (about 400 RPM), the control module applies 5 volts to the bypass line. The IC will no longer be grounded in the module and the IC voltage will be varying.
If the bypass line is open, the module will not switch to test mode, so the IC voltage will be low and DTC 42 will be set.
If the IC line is grounded, the module will switch to IC, there will be no IC signal and the engine will not operate. A DTC 42 may or may not set.

An open in the IC circuit will set a DTC 42 and cause the engine to operate on the distributor module timing. This will cause poor performance and poor fuel economy.
To check IC operation, place the vehicle in "Park" or "Neutral" and block the drive wheels. Start the engine and accelerate to 2000 RPM. Note the ignition timing. Disconnect the "Set Timing" connector and again note the timing. The timing will change if the IC system is operating.