Service engine light

[katrinekid]

I have a 1994 GMC 6.5 turbo and the Service Engine Soon light comes on.
It only comes on when travelling at a constant speed in overdrive above 60MPH. When I let off the fuel or kick it down out of overdrive it will go out.
Any speed under 60MPH it will not come on. Any ideas would be appreciated.

[JohnC]

First thing you have to do is check the diagnostic codes. When the light comes on the PCM stores a code that tells what's going on.

If you don't know how to read them, look here (http://www.thedieselpage.com/members/trblcode.htm).

[katrinekid]

John,
I am unable to access your link!

[murphyslaw]

pick up a manual for this rig, it tells you in there as well as some other good info. tho I have to say this last chilton i got was the biggest waste of 22bucks I have ever spent

[JohnC]

John,
I am unable to access your link!

(And now a word from our sponsor...)

that link is to a member's only area of the sponsor of this site. You may want to consider joining. Look here (www.thedieselpage.com) for lots of free info. For free info on reading codes, look here (www.kennedydiesel.com) under "tech tips".

[Monan]

I have a 1994 GMC 6.5 turbo and the Service Engine Soon light comes on.
It only comes on when travelling at a constant speed in overdrive above 60MPH. When I let off the fuel or kick it down out of overdrive it will go out.
Any speed under 60MPH it will not come on. Any ideas would be appreciated.

My 95 Chevy 2500 extcab longbox started doing the same between the 60-80kmh zone. It just started doing it last week.

[katrinekid]

When I jumper ground to the test terminal I don't get code 12 I get code 78.
Am I not supossed to get code 12 for three flashes and then any other locked in code. Also when I disconnect the battery it does not clear the codes.

[DmaxMaverick]

You will get EITHER a 12, or a valid code(s). 12 only flashes if all else is well, only to indicate you have entered diagnostic mode. Disconnecting the batteries will reset the codes, but it takes a long time, and that time varies. Could be 30 minutes. Could be 24 hours. Use the pedals to clear the codes.

[katrinekid]

DMAX
How do you clear the codes with the pedals??

[DmaxMaverick]

While in diagnostic mode, after the code(s) have finished flashing, apply full brake and APP for a few seconds (I count to 10). The SES should flash a couple times when it takes effect, but not always (I don't know why). Once you cycle power, it should display only new codes. If it continues to flash the same code, then the code is still being generated by the reason it was caused in the first place. In your case, the 78 (Turbo Wastegate Solenoid Fault) should clear, and not come back until you run it under load if it isn't fixed. If this code will not clear, you probably have either a completely open solenoid, or a poor connection in the WG circuit somewhere. You are lucky enough to know where to look for your problem, and it's pretty simple and straight forward.

[Mark Rinker]

The code 78 is an overboost code. No big alarm, common if you are running extra boost with a Heath boost controller.

If you boost control is stock, you might want to search the many posts on this topic to determine why. Your wastegate might be stuck shut, that would not be a good thing to ignore.