How can I check to see if the ECM in my 1986 Corvette is funcitoning properly? I am not sure if I have a bad ground somewhere or if the ECM needs replacement. :confused:

 

What symptoms is the car having which lead you to believe there is something wrong with the ECM?

 

I used to get Code 32. Now I consitently get a Code 21. I have set the timing but the car always idles around 1200 rpm once it warms up a little. Last night I was testing the EGR solenoidas per the Haynes manual. One of the leads turned the test light on while the other didn't. It did, however, engage the climate control fan. I also hear alot of whining through the speakers. This is why I am not sure if I have a bad ground or a bad ECM.
I just came inside to check message. I have been cleaning the IAC, Throttle Body, and checking how I should access the ECM. I want to do the job right but I would like to enjoy the car a few times before it goes away for the winter. That seasonal sence of urgency.
Any advise is very much appreciated.

 

That idling problem sounds like one of the problems I had on my 87 when my ECM failed.Wrong codes will also be set.If you can borrow a known good ECM try that it will tell you where you stand.The ECM's in our cars are prone to driver failure.There is an upgraded one availible from GM which is part#16198259.The old ones are #1227165.Make sure if it does need replacement you replace with upgrade.I got mine from the dealer for $325.00cdn haven't had a problem since.Good luck! :thumbs:

 

A scanner would be helpful, but you need to troubleshoot each code separately. For the 21, see what the signal voltage is at the ECM, Pin C13, dark blue wire. A 21 indicates it's above 2.5 volts and that's what your DVM should read at the ECM. Disconnect the TPS harness. If the voltage doesn't drop, the dark blue wire is shorted to a voltage source or the ECM is bad. If it does drop, something is up with the ground circuit. That's shared with the air temp and coolant sensors. You're not getting codes for those sensors, so you only need to be concerned with the ground from the TPS up to the splice. Check the connector and wiring by backprobing the disconnected harness at the ground circuit with a test light connected to battery voltage. If the ground & connector are good, the light will come on. An on light indicates that the connection isn't any good at the TPS or the TPS is bad. If the light doesn't come on, the ground is open (check wiring up to the first splice) or the ECM is bad.

Once the 21 is fixed, you will probably get the 32 back. In the interim, check vacuum supply to the solenoid.

The speaker whine, if Bose, is usually bad amps.

 

SunCr: Great advice -- looks like you read it from the Service Manual, the BEST source of diagnostic info.

1986Coupe: Haynes manual is OK for most stuff, but when you get into ECM or engine control diagnostics, you'll need a GM Service Manual. Ask here on the Forum -- someone who has one may be able to scan and post the relevant pages for you.

Would love to hear how you're progressing. Good luck :thumbs:

 

OK. In order to approach this in a calm way I decided to clean the throttle body. I just wanted to "tinker" and not lose my head. I haven't seen a colde 21 since. BUT...when the coolant temperature reaches 85 degrees C (sorry but I keep it on metric so I don't get pulled over for speeding) the coolant light comes on. This is another reason why I am not sure if it is a bad ground or something in the ECM. At 84 degrees it is fine but as soon as it gets to 85 the light comes on. Ground, ECM, or gremlins? :confused: