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THM 400


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scott67
Novice

May 14, 2010, 11:46 AM

Post #1 of 7 (1335 views)
THM 400 Sign In

Greetings,
I recently rebuilt the THM 400 transmission in my 87' chevy truck.
After installation all was well until a line (rubber coupling) blew and dumped fluid, burning up many a clutch plate.
I went back through it with another rebuild, put it back in the truck and cannot get any forward motion. Reverse works great.
Pulled it back apart last night and don't see anything amiss. I have not diassembled clutches yet though.
Any transmision wisdom as to what I'm missing...
Thanks!
Scott


Sidom
Veteran / Moderator
Sidom profile image

May 14, 2010, 6:49 PM

Post #2 of 7 (1327 views)
Re: THM 400 Sign In

It's been ages since I've had one of those apart but it might not be a bad idea to put a new set of frictions & clutch pack seals in it....

I know you can air test the clutch pack to make sure it engages after you have all the frictions & steels in them. Just find the pressure port on the clutch pack and hit it with a little air using an air nozzle with a rubber tip and see if it applies. You may want to do this to the old ones before you tear them down to see if you can find a bad one. Take a good look at the forward clutch pack assembly...........

Whoops....looks like I miss where you said put a kit in.......when you get it apart air check, maybe you nicked a seal going back together.......






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(This post was edited by Sidom on May 14, 2010, 6:52 PM)


scott67
Novice

May 14, 2010, 7:00 PM

Post #3 of 7 (1321 views)
Re: THM 400 Sign In

Thank you will do.
Just found a piece of seal in the very port you were referring to.
Also found only five ball bearings in the valve body. I distinctly remember putting all six back because I had one roll away and needed to purchase more.
So, two things - make 100% sure all six balls go back in the proper locations and check all clutch assembly seal quality, as well as overall condition of clutch packs.
This transmission got very hot and a seal may have been affected enough to break down.

*Just pulled the intermediate clutch apart and want to make sure seals are facing the right direction (on piston). My Haynes manual says to make sure they "face the pressure" but would like to verify this -with the piston on the bench and depressions facing up like you would have while placing in the cluch assembly...
Thank you


(This post was edited by scott67 on May 15, 2010, 5:58 AM)


Sidom
Veteran / Moderator
Sidom profile image

May 14, 2010, 8:23 PM

Post #4 of 7 (1318 views)
Re: THM 400 Sign In

Well those 2 problems could definitely do it..........

Not sure how you would lose a check ball out of the valve body unless it fell out when you were splitting it and didn't notice it.

Valve bodies can be a little tricky so go slow....One trick you can use, if you aren't sure where the check balls go, look at the separator plate very carefully. You will see little indents on the passageways that had check balls, the ones that don't won't have the dimples......

It's a good idea to get the right ones..........At one time I did use the balls out an air fitting in pinch once when a tech split one by accident & 2 balls went down the oil drain and the car had to go. It shifted and when thru all the gears.....talk about getting lucky.






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scott67
Novice

May 15, 2010, 7:22 AM

Post #5 of 7 (1308 views)
Re: THM 400 Sign In

*Just pulled the intermediate clutch apart and want to make sure seals are facing the right direction (on piston). My Haynes manual says to make sure they "face the pressure" but would like to verify this -with the piston on the bench and depressions facing up like you would have while placing in the cluch assembly...
Thank you


Sidom
Veteran / Moderator
Sidom profile image

May 15, 2010, 8:11 PM

Post #6 of 7 (1284 views)
Re: THM 400 Sign In

It's been a while since I've had one torn down but that sounds about right the lip should face the pressure. You should air check it once it's assembled to make sure it engages.........

You're probably already doing this but I'll mention it anyway just in case. When you're reassembling the trans if you take a 5 gal plastic bucket & cut a small round hole in the center, you can set the trans up right on the bucket & the output shaft will go thru the hole. Makes it a lot easier to put back together.

I would've mentioned it before but, well, it just plain sux having a bad memory.............






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Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

May 15, 2010, 10:52 PM

Post #7 of 7 (1279 views)
Re: THM 400 Sign In

">>it just plain sux having a bad memory<<"

Sidom - you are incredible! I used to have a bad memory............ I think??

T
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Long retired now






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