I have a 1994 K-2500. The service engine light came on a couple of times last week. I took the truck to a mechanic friend and he checked the codes w/ a Snap-on code reader. He said I had a code 78 waste gate solenoid and code 86 low ratio. I replaced the waste gate solenoid and haven't had the service engine light since. My question is, does anybody know what code 86 low ratio means???

I have chilton's book and they give different #'s for dtc's(trouble codes) but the mention that you deal with the lowest # first, as the second # may be in response to the first #.

Low ratio is a trans code. It means the PCM saw an incorrect ratio in the trans. This DTC will set when trans commanded gear is 1 or 2 and the trans is mechanically in 3rd or 4th gear.

Thanks guys. I have a buddy that works at a GM dealer and he said Low Ratio code was the transmission. It acted up on me one day a couple of weeks ago. It wouldn't shift into overdrive. I thought I might have had the shifter in D instead of OD, but it wasn't. I stopped the truck and shut it off. I then restarted the truck and it shifted fine. It hasn't acted up since. Any ideas of what caused the tranny to not shift into overdrive that one time?????

I had a problem like that after off-roading it knocked the plug from the ecu to the trans out and wouldn't go into overdrive. Maybe it's loose?

Thanks guys. I have a buddy that works at a GM dealer and he said Low Ratio code was the transmission. It acted up on me one day a couple of weeks ago. It wouldn't shift into overdrive. I thought I might have had the shifter in D instead of OD, but it wasn't. I stopped the truck and shut it off. I then restarted the truck and it shifted fine. It hasn't acted up since. Any ideas of what caused the tranny to not shift into overdrive that one time?????





There is a valve in the Transmission that tends to stick on the older trucks (pre '97) when the transmission is cold. Mine won't shift out of second, to prevent it I warm the truck up about 4-5 minutes and it seems to prevent it. If I start and go and it sticks I just shut down and restart. Clear the code later. Changing transmission fluid help some but doesn't stop it. Aslo you don't need a scanner to check codes a paper clip will do the job.Edited by: 16gaSxS

I switched to synthetic tranny fluid this year, and I've had to change it again prematurely due to all the resins and burnt old fluid residues that have tainted my fluid. I Flushed it, filled it with cheap pertofluid, flushed it again untill the fluid ran clean, then dumped it again and filled it with royal purple $42 for 5 quarts. That wasn't long ago and it smells burnt again. My thoery is that 280,000 mls of residues are becoming solvent in the royal purple, similiar to how biodiesel breaks down petrodiesel resins in your fuel system, clogging your fuel filter with black. personal experience