TH400 pressure issue
Thread Starter
Teching In
Joined: Feb 2013
Posts: 33
Likes: 0
From: London, KY
TH400 pressure issue I was driving my truck today when I looked down and noticed that at an idle in gear my trans pump pressure was falling down between 0 and 20psi. I shifted through the gears and it is staying that way in every gear. The truck has a TCI rmvb brake in it and it does the same thing in N or R with or without the button pressed. The trans has a brand new TCI pump in it with the TCI trans brake spring. Before it would get up to 200 to 220 psi on a hard pull. As of today the most I saw was about 120psi. I know every single clutch piston isnt bad. Trans was built about 100 miles ago. The pressure rises and falls with throttle. I previously had a Sonnax booster in the pump and the TCI spring but the pressure was way high around 320 to 350 psi. I changed the booster out last week to the factory TCI booster and now I have this issue. Will a bad booster valve cause this? What should I be looking for???
Thread Starter
Teching In
Joined: Feb 2013
Posts: 33
Likes: 0
From: London, KY
I know the spring seat is in there. Im wondering about the snap ring now but it could not have came out. If the spring and valve slid out then it should not build any pressure. The trans pressure needle moves up and down with the tachometer. It acts like its dumping somewhere. Pulling the pan this weekend. Im afraid the trans will be coming back out.
