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Check Engine Light Issues

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86 MJ, 2.8L dog engine, getting code 12 which from what I read in the Chilton manual is just a self diagnostic code while the engine is off, key on. Start the engine, no codes but the light will not go off. Disconnectred the battery for a few minutes, still on. Pulled the ECU and reinstalled, still on. Any thoughts?

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Thank You for the info, I do not believe this truck has the emission timer. I did find a printed circuit board card with 5 diodes on it plugged into a black box, when I remove the card, no check engine light. So I am getting somewhere. Just need to figure out where or what is sending the 6 volts to make the light stay on. Any help is great.

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From my notes: Code 12 = Battery disconnected. Battery or power to the ECM disconnected in the last 50 key cycles.


This is applicable to your 86 MJ 2.8L dog engine, as well as the newer OBDI 91-96 engines. After a problem has gone the code will be removed after 50 on off key cycles or if the battery left disconnected over night which will cause a code 12.


This means as I interpret it that the code 12 will remain until 50 ON/OFF key cycles are completed. Discoing the ECU or Batt will just continue the code 12.

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