Vehicle is a 1985 Oldsmobile Delta 88 4 door sedan. Powertrain is 307 V8 with 4 bbl feedback carburetor and 3 speed automatic transmission with lockup torque converter clutch. The issue is that the CHECK ENGINE lamp is illuminated constantly with ignition ON / RUN and engine running or not running. I tried to connect an OBD I scan tool with proper, GM connector cabke but could not get the ECM (engine control module) to communicate with the scan tool. Scan tool is good and works though I have never used it with the GM specific cable to retrieve codes. I found a picture of the GM diagnostic port under the instrument panel. There is a way to use a jumper wire to connect between terminals B & A of the port (ignition switch in the ON / RUN position and the CHECK ENGINE lamp will enter diagnostic mode and flash any 2 digit trouble code. This is similar to the procedure with Chrysler OBD I vehicles where one would do the key dance routine (ON-OFF, ON-OFF, ON) and get the lamp to flash. Below is an image of the connector and the terminals I connected. However with this Oldsmobile it will do nothing. No blinking / flashing; the lamp remains illuminated. So now I am starting to wonder if something has failed in the ECM. The engine will start and run and drive well but it does seem to be running slightly rich. You can accelerate the engine and you get an occasional black smoke puff. Default position for the mixture control solenoid is RICH if the ECM has failed. Any thoughts or ideas on further tests to conduct before condemning the ECM as failed? I have been searching for a wiring diagram for the ECM but have not been successful. If I had a wiring diagram I would check the wiring between the diagnostic connector port and the ECM for proper continuity.