4L80E Thoughts

[TerryD]

I found a 1992 K2500 GMC for a dang good price. 170k miles with 5.7L but not in bad shape at all. Only problem is that someone has been screwing with the transmission and there are two wires hanging down from the harness. A yellow one and a brown one. Being that it was raining and I was just pulled up on a hill crawling under it, I didn't look too closely to where they were coming from or going to but I know this:

Reverse is strong and solid

2nd gear position gets you a solid 2nd gear with over-run (engine braking)

In 1st, D or OD, the truck takes off in 3rd gear, will not hold back (engine braking), and no TCC operation.

Transmission seems to shift solidly (when it does shift) and the rest of the truck works great. Any advice on this issue from guys who have dealt with a 4L80E before? I'm going back on Thursday to have another, more indepth look at it with some diagrams if I can find some good ones.

Thanks guys!

 

[Slapperbar]

For 1991 Mitchell has a 410 Yellow wire in D16 on the PCM that does to the coolant temp sensor. There's a 413 Tan Wire in C13 that is Sensor Ground.

 

[kdoske]

If you look at the optishift manual I believe there is a section that shows each wires purpose.

 

[TerryD]

For 1991 Mitchell has a 410 Yellow wire in D16 on the PCM that does to the coolant temp sensor. There's a 413 Tan Wire in C13 that is Sensor Ground.

I saw one that listed the same, but I'm not sure about that.

If you look at the optishift manual I believe there is a section that shows each wires purpose.

According to their manual, Yellow is 2-3 Shift solenoid ground, but there is no brown at their controller unless it's their VSS output. But, if that yellow wire was grounded then it would force it into 3rd gear unless it was in 2nd (where it forces 2nd gear) or reverse. Sounds like the problem it's having. I'll have to check it out Thursday and see what's going on when I've got a little more time to dig. Don't want to fix it for her and have her sell it out from under me though...