I've swapped an '87 L98 (165 ECM) into my daughters '84 T/A. Car starts and runs fine except for high idle. 1000 RPM in drive. ECM controlled fan won't come on no matter how hot the temp gets. Check engine light is on with key on, goes off when engine running. My AX7000 won't make contact with the ECM and jumping A & B does nothing.

If the car runs, no check engine light running, check engine light works but no code 12 is this an ECM or a chip problem.

 

A couple times now the idle has dropped to the ECU controlled 600-650 RPM. The last time it did I grabbed my scanner and was able to pull data from the running engine. Everything seemed normal. No codes stored. Next morning, high idle and no communication.

Still wondering if this is an ECU or a PROM problem.

 

Odds are it is an ECM problem. Not much else to go on.

RBob.

 

New ECM in place with old PROM. So far so good. Idle is controlled and scanner is communicating.

Replacement's sticker recommended checking all relays and solenoids resistance for at least 20 Ohms. Relays were all in the 50's and solenoids were all 25 Ohms. Hope the old one was just old as I haven't found a cause.