Freshly Rebuilt TH400 issues.

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86Peter454

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So i just rebuilt a turbo 400.

i did the internal dual feed mod.

i eliminated the governor.

i modified the valve body to make it full manual.

i did the boost valve modification on the pump.

mostly followed the info in this. http://www.crankshaftcoalition.com/wiki/TH400_rebuild_tech


so i got the transmission in with a brand new converter. the stall on the converter is supposed to be 1k over stock so like 3000 max id assume. it is behind a 454.

the transmission is slipping really bad in first gear. a little bit in second third seems to be doing okay and reverse works just fine.

i dont have a pressure gauge on it to get the line pressure of each gear. but my first guess is that line pressure is not good causing it to slip.

the one part i didnt mess with was the pump. i left it alone. its the one that was on the transmission when i took it apart and i did not take the pump apart and check the clearances in it. so my first thought it that the pump is not good but the pump did work in the trans when the truck was parked like 8 years ago.

so just looking for some insight and what people might think my issue is and if they have anything for me to try to troubleshoot.

TL;DR newly rebuild th400 slips in first and second really bad. need help diagnosing.

any help is much appreciated.
 

1987 GMC Jimmy

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I’m no guru, and I hate to be this guy, but the first thing I’d be doing is checking line pressure in each gear, especially if the pump could be the culprit.
 

86Peter454

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lol yea i didnt want to fill anyones head with the fact that it is the pump but i didnt want leave any possible important details out.

ill see what i can do about checking line pressure.
 

86Peter454

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also not sure if it matters or not.

idid the boost valve mod in the pump. when you do this you can replace the vacuum modulator with a aluminum plug. well i just installed the modulator without connecting vacuum to it. not sure if thats wrong and i have to actually replace the modulator or if its okay to just leave it how i have it.

just wondering if it could be related to my issue.
 

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Need to take a look at the modulator but that might be a key. The goal with a plug is probanly to block fluid flow, and disconnected modulator might not
also not sure if it matters or not.

idid the boost valve mod in the pump. when you do this you can replace the vacuum modulator with a aluminum plug. well i just installed the modulator without connecting vacuum to it. not sure if thats wrong and i have to actually replace the modulator or if its okay to just leave it how i have it.

just wondering if it could be related to my issue.
 

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Again, I have to disclaim that I’m not a transmission guy, but the modulator’s a vacuum component that’s exposed to fluid so I’d worry about the innards degrading over time and leaks down the road.
 

86Peter454

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Again, I have to disclaim that I’m not a transmission guy, but the modulator’s a vacuum component that’s exposed to fluid so I’d worry about the innards degrading over time and leaks down the road.

i could put a vacuum plug on the end of it to block anything from getting into the vacuum port to prevent that from happening i think. seems like a valid concern to me.



Need to take a look at the modulator but that might be a key. The goal with a plug is probanly to block fluid flow, and disconnected modulator might not

it is an adjustable modulator should i adjust it all the way closed or open or something?

i have a pressure gauge on its way amazon prime should be here tuesday so i can do some actual diagnostics.
 

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i could put a vacuum plug on the end of it to block anything from getting into the vacuum port to prevent that from happening i think. seems like a valid concern to me.





it is an adjustable modulator should i adjust it all the way closed or open or something?

i have a pressure gauge on its way amazon prime should be here tuesday so i can do some actual diagnostics.
I am not sure on the way it is working with the span of adjustment, but if you have a pressure gauge hooked up, try one way then another to get good pressure
 

86Peter454

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okay ill give it a shot once i get the gauge in.
 

86Peter454

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Okay so I got a gauge in.

Testing sitting still. Brake applied at 1000rpm.
P 150
R 155
N 150
3 95
2 160
1 160

Then took for a test drive and reved up to 2000 in each gear.

3 150
2 190
1 190

It slipped bad in 1st not so much in 2nd and even less in 3rd.

Is it possible too much pressure would cause it to slip I don't see how that's possible.

Any help is much appreciated. I didn't change anything since the first post before doing this test.
 

crazy4offroad

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Possibly a folded or cut lip on the direct apply piston.
 

86Peter454

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Wouldn't that cause pressure to be lower though?
 

crazy4offroad

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I looked at the article you posted in the 1st post, they have a similar article for the TH350 I went by for my rebuild. In that article, they had a procedure to check the pistons by assembling the clutch packs in their drums and setting them on the pump, and applying air pressure to certain ports in the pump to verify the pistons were applying pressure to the clutches. I assume there is no similar procedure for the TH400 since it wasn't mentioned in the article? It would have been nice if it were possible. When I did mine I had a serious air leak during that test, and it turned out to be a folded piston lip. I know they are a struggle to get right, did you have any trouble with any of them during your assembly?
 

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