Question: ECM

[My81Vet]

Hi

This is my first post so bare with me.

My ECM does not work I have to leave it unpluged for my 1981 vet to run somewhat have way decent. Any ideas on how to solve my problem?

 

[cstarr89]

I'm by no means an expert on ECM's but I believe I have read before in other posts about this issue and I'm sure someone else will chime in with any corrections/additions on this. The 81 was (I believe) the first year the ECM was used on the corvette (standard in GM vehicles at this point) and from what i've pulled from the other threads they do not tend to last through the years for very long. The easiest fix would be to pick an ECM up from your local auto parts store for around $100 bucks and swap the old with the new. Since you've disconnected the old one you should already be half way there. I wouldn't wait to long before you pick one up because I believe they are in control of spark retarding? they use knock sensors on the block to detect and retard accordingly to engine detonation.
again just my .02 cents i'm sure someone else will chime in.
Also Welcome to CAC I'm sure you'll find plenty of knowledgeable/helpful people here.

 

[6880 Mike]

My advice is to purchase a ALDL scanner and read the codes in the ECM yourself, or take the car to a reputable shop and have them read the ECM. The error codes in the ECM may indicate other problems with the engine or confirm whether the ECM might need to be replaced.

You should not have to be disconnecting the ECM to get the car to run.

 

[GerryLP]

The shop manual also has the procedure for jumping the ALDL connector and count the number of flashes on the "engine check" light.

I know that the information is in the CAC website somewhere, and hopefully someone has the link to it...

http://www.corvetteactioncenter.com/forums/showthread.php?t=103399&highlight=computer+codes

http://www.corvetteactioncenter.com/forums/showthread.php?t=97288&highlight=computer+codes

http://www.corvetteactioncenter.com/forums/showthread.php?t=86711&highlight=computer+codes

This bottom one has answers from Mike (Stingray6974), who is a Corvette mechanic and worked them many years. I learned a whole lot from him...

http://www.corvetteactioncenter.com/forums/showthread.php?t=37263&highlight=computer+codes



GerryLP:cool

 

[Hib Halverson]

1980 was the first year for computers on Vettes but only cars sold in California had them. 81 was the first year for computers on all Vettes.

Scanning an 81 is tough because it's the only model year whih had a five-pin DLC connector. 82 had a 12-pin. Regardless of the no. of pins, this connector is located beneath the ash tray. Some scan testers still around today support '81 but few people who have the testers also have the 5-pin-to-12-pin adapter you need to connect the tester.

Unfortunately, getting codes from that computer is going to be impossible because when, you disconnect it you erase the memory, so you're going to have to road test the car with the ECM connected to get those codes to set.

Also, because my service manual coverage is only 82-08, I cannot tell you which pin in the 5-pin connector is the diagnostic pin...maybe someone else here can.

Here's how you're going to need to diagnose the problem.

First, buy a 1981 Factory Service Manual because it has the diagnostic info you will need...along with the wiring diagram which will tell you which DLC pin is the diagnostic pin.

Next, reconnect the ECM and drive the car for a while...at least 15 minutes after it fully warms up. The check engine light may come on but it might not if someone has removed the bulb.

Then, cycle the ignition but do not start the engine, ground the diagnostic pin and record the sequence of flashes of the check engine light. If the light does not flash, consult the FSM for repair instructions.