th400 slipping in 3rd?

[Skittlesquad]

I have an 85 k10 with a 6.2l and th400, the issue im haveing is when the trucks in third and i mash the pedal down or even just medium acceleration the engine will rev but wont go faster, could barely hold 60 pulling a trailer up hill, been doing this for about 1000 miles now, 1st and 2nd work great and no problems in reverse, fluid is still bright red and no burnt smell, what could be the problem?, the truck did sit for 3 years, only thing ive done is drain and change the fluid

 

[3500GMC]

The TH400 can down shift very seamlessly from 3-2. It could be that it shifts down and runs up against the governor in 2..

You can troubleshoot this by disconnecting the kick down wire (single wire) around the pan rail - driver side towards rear.

 

[ak diesel driver]

wouldn't that have a 700r4?

 

[Skittlesquad]

Its a th400, my truck was a gaser and i did the swap from a k20, prolly shoulda said that, it did sit for 2 years, i only drained the fluid and added new, had a drain plug on the pan, aftermarket,i dissconected the kickdown and still the same will cruze in third fine but any pressure such as a steep hill or load causeing me to throttle up the engine will rev until i let off and itll catch again,only in 3rd, i can whind out 1st 2nd and reverse

 

[turbovanman]

Sounds to me like 3rd gear is gone.