Technical GM TH400 issues

If it is cavitating because of that filter, not being a screen type filter, it will cause all of your problems. So it can be fixed in the vehicle. I wouldn’t pull the trans out until I put a Allison type filter, make sure there’s two of the right size O-rings on the pipe. Those cloth type filters do not work! And make sure the vent tube is clear.

 

You don't have to pay you, and you don't have to mark up parts for yourself.

Get a few books, read through them, Grab a camera and get to it.

 

I screwed up the first one I did for a friend when I was 16, but after that they mostly all worked....

 

Here in Georgia, I just picked up my 400 that a great local shop rebuilt for me. Alto clutches, it already had a Transgo kit, 34 element sprag and nothing else too exotic for $1,200. $2,200 seems crazy for a regular 400.

 

If I had to charge $200 an hour, it doesn't look quite so crazy.

 

I have to charge >$200/hr., but I can rebuild one, in 4-hours, at a comfortable pace.

 

I paid about $800 with a new torque converter to rebuild my 400.

 

@1pickup, don't be afraid to tackle the rebuild yourself. Get one of the books that Gimpy pictured (I personally like the Cliff Ruggles one) and look it over. I think you will find it pretty straight forward. If you just do a "refresh" build with just seals, gaskets, and clutches it is even easier, less costly, and requires fewer DIY tools.

 

I'm not sure if this makes a difference or if someone has already mentioned it. There are two different filters for a T400. An early one and a late one. I would be sure I had the right one for the year of the trans. Gene.

 

If it went into gear once without slippage, it probably isn't a pump problem. Either the pump is being starved for fluid, or you have a massive internal leak. You could put a pressure gage on the test port, but your description already points to a pressure problem. Sounds like it either has to come out and be repaired or replaced.

 

I am thinking something similar. In park, convertor hasn't filled up yet after leak down. Trans will engage a bit until convertor fills up but now there isn't enough fluid left in pump and cavitation occurs.

 

Just as an additional tip, when/if you pull the pan, get 4 of those large donut magnets at Home Depot and epoxy them in the corners of the pan. That will grab most of any metallic debris before it gets to the filter.

 

Nothing, and I mean nothing, metallic can escape these:

https://www.magcraft.com/magcraft-magnets

 

With a 30 year old automatic transmission you are just wasting time and money trying to fix it. Seals dry out and crack, clutches fall apart and loose the lining. Old fluid turns to mud. Just bite the bullet and rebuild it. I was there and am glad I went to the trouble to pull it and rebuild it.