I have a 4L80E trans that keeps going into limp mode. It will do it any time. Today I had the car trailer on and it happened alot. If I'm driving all I do is put it in nutral shut off and restart then it will shift. Sometime when it down shifts it will limp.Theres no pattern to when it will happen. Trans anly has about 4000 miles on it.My last trans did the same thing before it want out. I stoped at the dealer and got the run around.I check the codes and got a 68. All I found code 68 was cruse control cirrit bad. Any ideas?
1993 Chevy Silverdo Crew Cab
1 ton Dually
6.5 TD Allison 4L80E

The chevy dealer told me its a Allison, Allison told me its a chevy trans,Now I know its a chevy trans Still need ideas please

It isn't a Allison that is for sure. If your dealer isn't sure,apparently they are clueless,then you should take your truck elsewhere and have it diagnosed properly.


First thing to have done else where is have it scanned for DTC's.








Mac Edited by: Mackin

It isn't a Allison that is for sure. If your dealer isn't sure,apparently they are clueless,then you should take your truck elsewhere and have it diagnosed properly.


Yes, the Aly was not even out until about 7 years after that truck was built

I did find out is like a modified TH400 I guess my chevy dealer didnt have a clue. I'm going today to get the codes check but not at my chevy dealer. I'm hoping in 6mo to get a newer truck. Just dont know what kind yet.
Thanks

Had the codes pulled also had the machine on while driving.I did learn that if I put the shifter in drive it will shift 3 gears. Code 68 is overdrive ratio problem. Book says if engine rpm reading and actual rpm is off more then 200 rpm this problem will happen. The scan tool says the reported rpm and actual rpm does not match.Also the scaner rpm reading and my tack readingis off, my tack reading is 500rmp higher. At least I have an idea now I hope. The oil pump drive has a sender on it, I thought was the tack sender, The plastic cap was broke so I replaced it Didnt help. On my way home I unhocked the wires from the oil pump drive and now it will shift in to 4 gear but the converter will not lock in. Thanks