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GM OBD I ECM Problem?

Discussion in 'Non-Mopar Tech Support' started by AllanC, Jul 25, 2017.

  1. AllanC

    AllanC Well-Known Member

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    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.

    GM ALDL OBD I Diagnostic Connector.gif

    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.
     
  2. ImperialCrown

    Level III Supporter

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    If you unplug the ECM, does the 'ck eng' lamp stay off? This may verify that the lamp is not a wiring or instrument cluster problem.
    The cars that I remember jumping in order to read the fault codes had me connect the jumper in the connector before turning the ignition key to 'run'.
     
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  3. AllanC

    AllanC Well-Known Member

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    Excellent idea but I did not try that. Power for the CHECK ENGINE lamp passes through the ignition switch. The ECM controls the ground path and when to illuminate the bulb. If there is a short to ground on the return side of the lamp wire to the ECM , it will always be illuminated and the ECM will not be able to control it. I will definitely try this test.


    All the instructions I read indicated to turn the ignition switch to ON / RUN and then jumper terminals B & A in the diagnostic connector. But it could be just a simple sequencing event of connect first and then apply power. I know that I turned the ignition switch first and then applied the jumper wire.

    I found a somewhat crude wiring diagram from Chilton's online so I am thinking I should try and trace the B terminal wiring at the diagnostic connector back to the ECM and make sure it is continuous.

    It will be next week before I visit the vehicle again and try these tests. As always, thanks for your insights!
     
  4. AllanC

    AllanC Well-Known Member

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    I tried using the jumper wire in terminals A & B and then turning the ignition switch to the RUN position. That made no difference. I removed the diagnostic connector from its mounting under the dash and back probed the terminal pins. Still no luck. CHECK ENGINE lamp still would not flash.

    I found the ECM (engine control module) under the glove box on the driver side of the vehicle. It has 2 rather large connectors which appear to have about 20 wires in each. I am still searching for a wiring diagram p;inout at the ECM. I will try some GM / Oldsmobile forums and maybe I can find more information there.
     

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