I have a 1981 vette that has the computer controlled timing advance. I couldnt figure out why it wouldnt accelerate, so i put the timing light on it and and it was set a 12 btdc and when i rev the motor up the timing stayed the same. so i advanced the timing by turning the distributor so that it was at around 20 to start out with and it ran much better. Any ideas would be greatly appreciated.

 

Did you check for any ecm codes?

 

No i haven't yet. the check engine light isn't on but ill try to check it tomorrow.

 

not sure if it applies but learned this lesson on another car

be sure you disconnect the electronic advance BEFORE adjusting timing. I kept adjusting it and it kept going back....

good luck

 

The computer will just keep on correcting the changes you make.

 

This is right. Disconnect the 6 plug harness (8 plug? can't remember) to the distributor and set your initial timing. Mine ran best around 12 degrees BTDC. Too much more initial and I always had serious pinging.

That doesn't eliminate the real problem though. Once you set the initial timing and plug the harness back in, you should immediately see more advance. If you don't, there is definately an issue.

 

 

thanks for the input. I tried unhooking the plug and setting the initial timing back at 12 once i plugged it back in i checked the timing and it was still at 12. as i slowly rev the motor the timing actually goes back to 9 and as you keep reving it, it jumps back to 12 but no advance still. I have another question is there a place to hook a code reader up? I asked one of the local shops today and they said that there isn't a place to hook a code reader up on this model. please let me know thanks

 

There is a place to hook the reader up but:

A) Their reader won't and

B) You don't need their code reader

There is a simple diagnostic using a paperclip and th port under the ashtray. Let me spend 5 minutes with the service manual and I'll write it up for you.

PS. My guess is that your computer is not working based on what you've said so far. I doubt the diagnostic will tell you anything. Does the "Check Engine" light come on before you start it?

 

Take out your ashtray and make sure that you're in a quiet place. Under the ashtray, you'll find the ALDL port. It looks like modern diagnostic ports but was actually for the assembly line to use. You should see 5 separate terminals. Two on one end don't have the same divider that the others have. These are the ones that you'll want to jump. An unbent paper clip, staple, jumper line or even an appropriate sized screwdriver will do. All you need is the two terminals to touch and stay connected while you run the diagnostic. Also, your "check engine" light needs to be working as that is what you'll use to read any stored error codes.

Put the ignition in the "run" position, but don't start the engine

Jump the two terminals described above (D and E if you want to be technical)

You should immediately hear some clicking going on under the hood. That is your carburetor solenoid cycling and is good. Silence is bad.

Now watch your "Check Engine" light. It should begin flashing. Each code is two digits and is separated by a pause. The code 12 should be the first one you see (one flash, pause, two flashes). As I recall, code 12 will come on 3 times before the next code. Simply record the error codes and report back.

If your light doesn't work or you get no codes, the diagnostic procedure is very different. Again, report back and we'll go from there.

 

thanks for the tip. when i did it the first time i got 3 12's and a 21 i think. but when i tried a second time the light just stayed on what does this mean?

 

Code 21 is the throttle position sensor curcuit. The TPS is on the carb. Make sure the connector is on it. Otherwise, could be a short in the wire or the sensor wasn't installed properly. This may well account for how it's running. When there is a fault in the system, it'll default to full rich.

When the "Check engine" light is on steady, is it full bright or kind of dim? If dim, make sure that the ground to the computer is tight and clean.

 

It is bright after i jumped the wires it sometimes goes back off. Is there a simple way to check the throttle position sensor is there a place where i can check continuity of the wires in other words do you know where the wires lead? thank you very much for the information i really appreciate it.

 

If you find anything out please post as I am having the same problem. I in the process installing new interior and when finished the timing advance problem will be next.

 

Do you guys think a new throttle position sensor will correct the problem? thanks

 

One step at a time Karl.

First, check the fuse to the CCC. That could account for the light staying on.

If that's ok, take the lead off the TPS. With the ignition on and the computer jumped but the engine not running, check the voltage from the black to the dark blue then the black with the grey on the TPS lead. Both should be more that 2 Volts. If they are. Start the car again and recheck them.

 

The fuse is fine. the voltage from black to blue running or not is 1.8, and from black to gray 1.1 when its running and 1.2 when it isn't.

 

You need to check the Voltage at the ECM between terminals 21 and 22 and 2 and 22. I think the wire colors should be the same as the ones on the TPS harness.

If the Voltage is below 2 than the problem is with the ECM harness. If over 2 then there is a fault in the TPS harness.

 

between 2 and 22 it was 1.7 with out tps hooked up and .1 with up to 1.7 with throttle to the floor.
between 21 and 22 it was 1.1 with or with out tps hooked up.

 

Sounds like it's the ECM harness then. I'll check the manual again when I get home but would guess that the easiest thing to do would be replace the harness.

That would probably account for the light being on steady as well.