1992 Gmc Extended Cab Service Engine Light

Larry767 · June 10, 2009

I have a 1992 GMC Extended cab pick-up with 5.7L engine and about 230K. I bought this truck 1.5 years ago and it seems to run fine. I typically only use the truck to get to work and back which is only about a 6 minute drive so I really don't drive it hard. The handful of times I have had it on the highway I have noticed that after about 5 minutes at highway speed the Service Engine light comes on and stays on until I stop and turn the engine off. Tonight I checked the codes and there are none but it has been several ignition cycles since I last saw that light. I will take it for a burn down the highway tomorrow and check the codes immediately after the light goes on but if no code is stored after that (which I suspect) then does anyone have any idea where to start troubleshooting this?

bill190 · June 10, 2009

In the ECM (engine computer) there are integrated circuit chips which "output" to various things. And it is possible for another connection to be "shorted" on one of these chips and cause other things on that chip to "always be on". These chips are called QDR.

For your vehicle/engine, the QDR which drives the service engine soon light is #2 in the ECM computer. And things connected to that chip are...

A4 - EGR or EVRV control.

A5 - Service Engine Soon Light.

A7 - TCC solenoid or shift light.

B4 - Not used.

So it is possible that one of those other things above is "shorting" when the vehicle is warm or has an intermittent short. Or possible the ECM is bad. But I would check all those other circuits/devices first before replacing the ECM.

And things like this can be a royal pain because it is not happening all the time! You would need to "catch it in the act"!

What you could do is very closely examine all the wiring/connectors to the above devices including the service engine soon light and look for anything which does not look right, bare wires, grease, poor connections, etc.

The test procedure for this is on page 3-161 of the GM 1992 GM Light Duty Truck Fuel and Emissions Service Manual. Where it says this could be the problem, is on page 3-19, lower left corner, ECM/PCM Quad Drive (QDR) Check.

Larry767 · June 10, 2009

In the ECM (engine computer) there are integrated circuit chips which "output" to various things. And it is possible for another connection to be "shorted" on one of these chips and cause other things on that chip to "always be on". These chips are called QDR.

...things like this can be a royal pain because it is not happening all the time! You would need to "catch it in the act"!

Bill,

Great tips. I did take it for a spin down the highway until the light came on. Am I correct in assuming that you can't check the codes without first shutting the ignition off, shorting the pins on the service connector, then turning the key on? I tried starting the engine with the jumper in place but the service engine light just flashes. Anyway, as expected, only code 12 showed so whatever is causing the light to come on isn't actually storing a code.

As for it being heat related, the evidence doesn't support that. I believe I can drive around town for 1/2 an hour and the light will not come on. Hot or cold though after 3-5 minutes at highway speed the light comes on and stays on until I cycle the key. Therefore, it seems to be speed related but I'll confirm that with some more testing. I'll also check the harness to the transmission as the shift points do seem to be off (high RPM before shifting to 2nd and it just doesn't feel right at the other shift points either). I have a friend who says he knows how to fix that but I haven't had it up on the hoist since I talked to him to have a look.

Thanks again,

Larry

dwhults · June 10, 2009

When the MIL goes on, take it to an Autozone, Advanced, Pep Boys, etc. They will check your codes for free but not erase them.

bill190 · June 10, 2009

Am I correct in assuming that you can't check the codes without first shutting the ignition off, shorting the pins on the service connector, then turning the key on?

I wouldn't imagine you could retrieve any codes with the engine running as the computer would need to use all its time to run the engine.

If you turn the engine off, then back on does the light go away? If yes, this would make troubleshooting even more fun!

Basically if the light is off or it is flashing, then the problem I described above would not be happening at that time.

bill190 · June 10, 2009

I looked in my book and for the TCC (Torque Converter Clutch) Solenoid, it says this solenoid should not measure less than 20 ohms. Says if it does, it could cause failure of the ECM driver (QDR) as I described above.

So because you are having transmission problems and the factory manual points to this as well, I would closely examine the TCC solenoid and its associated wiring.

This is the type of problem which could damage the ECM computer (that output chip), then someone replaces the computer with a new one and that computer also gets damaged. So the thing to do would be to repair the TCC problem if that is the cause, THEN replace the ECM if needed. (In that order.)

Larry767 · November 30, 2011

I looked in my book and for the TCC (Torque Converter Clutch) Solenoid, it says this solenoid should not measure less than 20 ohms. Says if it does, it could cause failure of the ECM driver (QDR) as I described above.

So because you are having transmission problems and the factory manual points to this as well, I would closely examine the TCC solenoid and its associated wiring.

This is the type of problem which could damage the ECM computer (that output chip), then someone replaces the computer with a new one and that computer also gets damaged. So the thing to do would be to repair the TCC problem if that is the cause, THEN replace the ECM if needed. (In that order.)

Bill - I had forgotten about this thread. I am just in the process of cleaning this truck up to sell it and wanted to take a stab at this again. I never did look into the solution above. Can you tell me where the TCC solenoid is located so I can check it? Do you know what the GM part number is for it so if I need one I can look for one?

Thanks,

Larry