89 c2500 TH400 rebuild or replace???

stutaeng

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Found out this evening that my uncle has the original TH400 that was in the truck. He had it swapped because it would not shift until high RPM. He didn't have time to fool with it at that time (no truck, no income) so he had another one built and put in. I have read that a bad vacuum modulator or bad vacuum source could cause that. Thinking I might check the function of the modulator and put that one in and see what it does. Worst case is it still doesn't shift right so I will pull and rebuild that one. Only issue it had was the high RPM upshifts. Thoughts?

I have done a fair amount of research from a number of vendors for a rebuilt unit and I am not impressed. Lots of TH400's but just a few are the correct HD units the truck came with & needs. Leaning real hard toward rebuilding it myself.

PS: truck is at my property in the country so I only get to go work on it 1-2 times a week for a few hours at a time. Damn HOA!

Thanks everyone for the advice so far. Stay tuned

I thought the regular pickup trucks got a standard TH400/3L80. Wasn't there also a 3L80HD(TH475)? I thought those were only installed on motor homes? I think they have a brake mechanism on the tailhousing. I don't know what the difference would be, maybe straight cut gears?

I was reading a few threads on the Jaguar forums, and some of those guys are very knowledgeable on the TH400/4L80e. Maybe do some reading over there. Performancetruck.net has a lot of information on the subject as well.
 

Schurkey

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my uncle has the original TH400 that was in the truck. He had it swapped because it would not shift until high RPM... ... I have read that a bad vacuum modulator or bad vacuum source could cause that.
Modulator
Governor
Plugged trans filter
Sticking valves in valve body
Kickdown solenoid/Kickdown switch
"something very weird"
 

tardisguy

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Got the original TH400 that was in the truck this weekend. Have not had a chance to do any teardowns or much else. Still waiting on my book and getting ready to go to Talladega this weekend for the NASCAR race. Scratch that off the bucket list!
 

stutaeng

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Awesome! I have acquired 2 spare, junkyard 4L80e trannies, a late and an early model. I'm thinking of taking the plunge and overhauling them myself. Mainly as a personal challenge (but also to save some $$$). I've got a lot of projects half-started, but this is how I roll, LOL.

Except for the foot press, seems like tools needed to overhaul them are just basic tools that most certainly I already have.

What book did you buy?
 
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