Help!!! Ecm? Prom?

[silverbullet]

My stock 1981 (except for a hypertech chip) has been running great until yesaterday. I'm driving down the highway and come to a stop sign. The check engine light goes on. I smell smoke. I pull over and number 3 spark plug wire is on fire at the spark plug. The spark plug wires are Taylor 8mm spiral wound. They have about 100 miles on them. I get the car home and check the diagnostics and code 51 comes up. (a side note, I never get code 12 first, it just goes immediately to 51). I then replaced the spark plu wires and put the origanl stock prom back in and still get a code 51. I tried in numerous times each time disconnecting the battery in hopes of resetting the computer. I spoke with the local Chevy dealer and they said it sounds like a bad ECM. So I bought a rebuilt ECM from AC DELCO. I put it in the car and tried both chips with and still get a code 51. After my third try with the new ecm the car stopped running all together. I have put all new rotor, cap, wires, ignition module and still nothing. It won't even start with the computer disabled. The battery is fully charged and the starter tries to do something but nothing is firing. Does anyone have any ideas???

[mikejpss]

have no idea whats going on with yours, but heres some info to help u troubleshoot:

I unplugged my distributor from the computer, as well as my carb and the motor runs with a new vacuum advance distr. and non-computer carb. Perhaps I'm confusing you more than helping, but thought i would add that in. I believe the computer only advances/retards the timing after sensing the o2 sensor.That may be your problem.
what is a code 51?

[Tom Bryant]

It is possible to get bad rebuilt ECMs from Delco and others. We went through 3 or 4 one summer then finally replaced both the prom and ECM with New GM parts. Been right for many years now.

Here's what code 12 says in the manual:

No reference pulses to the ECM. This code is not stored in memory and will only flash while the fault is present.

Also says:

If your car won't flash a code 12 disconnect ground from "test" terminal. Check for blown fuse in circuit to ECM terminal "R".


Code 51 says:

Faulty calibration unit (PROM) or installation. It takes up to 30 seconds before this code will set.

If either of these parts fail your car should go into limp home mode and will only go about 30 mph and run rough but should start and run.
We were in limp home mode all that one summer it seems.

I'd closely inspect the prom and ECM contacts to look for anything damaged or out of alignment. Also go to Radio Shack and get some computer contacts spray cleaner. If you touched the contacts they may be tarnished enough to make a faulty connection. Also check the conections (4) on the EST distributor connector. Dirty or corroded connectors can really mess up this system.

Code 51 Faulty or mis-installed PROM simply says :

Check that all pins are fully inserted in the socket. If OK replace the PROM. I also saw a note that the 12 that flashes before the trouble code is flashed is different than the trouble code 12.

Tom

[vettenick]

Be careful...you can actually install the prom in reverse. Then when you turn the key on you'll fry the PROM. Just a thought.

[Chris Kennedy]

I don't think its PROM or computer. I would check the distributor electronic control pick-up device (don't know what it is called, but it is NOT the computer, but some electronic device that's part of/attached to the distributor). Frankly, I think the most likely caused might be an ignition harness failure, partial or total. The '81s, I am told, were susceptible to this. It's not a big deal to replace, but if it fails it shuts down lots of things. One final word of advice: Notwithstanding that I, too, love to tinker with things from the automotive aftermarket that talk about boosting power through electronic modifications such as proms, high output coils, etc., I have found that the best thing to do is to use the standard GM-Delco parts. Lots of this aftermarket stuff is a crock, and simply screws other things up, or, if it does one thing to improve performance, it messes-up other things that were fine before.

--Chris Kennedy
Houston, texas

[vettenick]

As far as the no start goes...Disconnect the 4 wire connector going to the distributor. This takes the ECM out of the picture. This car can run without the ECM. Next, check for spark at one of the plugs. If you have spark then remove the plugs to make sure they're not soaked with fuel. If the plugs are wet, then change them.

If you don't have spark, then there's a problem with the ignition system.
One other idea is to disconnect the tach plug from the cap. Iv'e seen situations when there was a problem with the tach circuit causing a no spark output from the ignition system.
Good luck! If you need further help...just post...Nick