Cadillac Diagnostic Readouts

The 1984 Cadillac Electronic Control Module (ECM) can be accessed from the Electronic Climate Control (ECC)panel.

The following gives instructions particularly for the 1984 High Technology 4.1 liter (HT4100) motor, although they are pertinent for other surrounding years with similar Digital Fuel Injection (DFI) systems.

To enter diagnostic mode, proceed as follows:

1.  Turn the ignition ON
2.  Depress the OFF & WARMER buttons on the ECC panel simultaneously and hold until '..' appears.   '-1.8.8' will then be displayed which indicates the beginning of the diagnostic readout.
3.  Trouble codes may then be displayed on the ECC control head beginning with the lowest numbered code.   Note that the Fuel Data panel goes blank when the system is displaying in the diagnostic mode.   After the Trouble Codes have been displayed, if any, the display will then show '.7.0'.   This is a decision point.
4.  To clear Trouble Codes, when '.7.0' is displayed, depress OFF & HI simultaneously.   Hold until '.0.0' is displayed.  Once '.0.0' is displayed, the ECM will display '.7.0'.
5.  To exit the diagnostic mode, depress any of the ECC buttons except LO or Outside Temperature or turn the ignition off for 10 seconds. Trouble codes are not erased when this is done without clearing the codes.

The engine control solenoids, some dash warning lights and the ISC can be tested using the Output Cycling capabilities of the system.  Switch Tests enable the testing of many of the switches in the car, including a comprehensive test of the Cruise Control controls.  I have found both the Output Cycling and Switch Tests invaluable for fault finding the Cruise Control system.

One of the more common errors are related to the Throttle Position Sensor (TPS) and Idle Speed Control (ISC).  Some background information on the errors and adjustment are on the link below:

Diagnostic/Trouble Codes    Engine Data Display    Abbreviation Legend    TPS & ISC errors