TH400 Leakage questions

[72step]

I have a TCI racing TH400 tranny which was rebuilt with TPI products back about 7 years ago, and it is a badass tranny and still is, but because of being parked more than driven throughout the years, it's leaking tranny fluid all over my garage floor. (no big suprise, as all older GM autos I have owned have done this at one time or another) Now, I usually have been just putting a quart when I want to drive it, and it runs great, but I need to fix this problem. My question is, do I need to get it completely rebuilt? Or what seals need to be replaced? It's got less than 2k miles since the rebuild. Of course the transmission guys I've talked to want to rebuild it completely, but it has very little miles. I'm just wondering if it's not just some seals I can replace. Thanks.

[QKENUF4U]

sounds like you need to crawl under the car and look to see what seals exactly are leaking. if its been sitting with very little use then the pan gasket could have dried out and thats a really easy fix.
good luck

[skokie]

My 400 had this same issue. I couldnt tell where it it was leaking from so I got under there with a socket and to my surprise about half of the bolts were loose. After I tightened them up no more leak.

[72step]

Thanks, That makes sense, and it could be, because if I drive it, say today, I won't see any oil on the floor till about a month, so once all that oil drains into the pan, it takes a while to get to where the gasket is. I'll give that a shot first. Thanks.

[JimKshortstep4x4]

Our experience with TH 400 leaks has been with primarily three items. Pan seals, (it is hard to get a good one that does not seep), shift lever seal, and the dipstick tube seal. While other areas can have leaks, this is where most of ours have occurred.

Since your seems to leak as the fluid drains back, I would look at the dip stick tube first. The TH 400's hold up really well, I cannot believe that a rebuild is in order.

Jim

[Green Machine]

Half of the contents of the torque convertor, say around 4 quarts can drain back into the pan after long periods of sitting. The pan gasket should hold that, but other seals like the linkage seal and dipstick tube were not designed to be submerged in tranny fluid.

Normal fluid level is around pan gasket height. Next time it sits a while pull the dipstick and I'll bet the tranny shows way over full, like 3 or 4 inches past the mark. New linkage and dipstick seals may help. Cheapest fix would be to start it every 2 weeks if possible.

[JustLiveIt]

Mine needed the pan gasket, shifter seal, dip stick tube o-ring, electrical kickdown connector, speedo gear cover o-ring, and a new speedo gear. The speedo gear had a groove in it so it would not seal against the new oring in the speedo gear cover. All of these are pretty easy to change. The shifter seal was probably the hardest because I could not get up there to make sure it was all cleaned out before I put the new seal in.

A place called "Whatever It Takes Tansmission" is a good place to get all these little seals and gaskets cheap. Go to: www.wittrans.com You have to call them but they are very friendly and don't charge you too much for the parts. Get some extras of the shifter seal because they are easy to tear.

[72step]

Thanks for all the input guys. What I think I'll do next weekend is jack it up where it sits, put it on jack stands, clean up all the tranny fluid from the floor with thinner like I usually do, and just crawl under there with a light and I might be able to pinpoint the leak. Any tips if it is indeed the pan gasket?-Especially being that the TH 400 pan gaskets are notorious for leaking? Thanks again.

[JustLiveIt]

After a couple tries with the pan gasket I ended up getting a cast aluminum pan and a thick cork gasket. I found that the machined surface on the cast pan sealed better than my sheet metal one. Thats just my experience. You should be able to get a good seal with a sheet metal pan but after 2 tries I just got the cast pan. I wanted the drain plug anyway.

The rubber gaskets from napa are nice but you have to be careful not to squish them too much.

Just my $0.02

[72step]

What did ya give for that pan?

[JustLiveIt]

About $125 I think. At first I got the "stock depth" pan and had to return it because it would not bolt up. The filter would bottom out on the inside of the pan. The sheet metal pan would fit but not the "stock depth" cast pan. Then I got the extra deep one but still had to mix-n-match filters to get it to fit. The filter that came with it hit the pan and made the pan hang about an 1/8" too low. Not sure if the truck T400's and car T400's were different but sure seems like it. Maybe someone can shed some light on this.

[71Dragtruck]

I know of 2 other TH400's that guy's had that if they sat for 2.5 to 3 months, the torque converter would drain and puke fluid from the rear seal or vent. Both of them were in good condition also, and otherwise did not leak, in fact my enclosed car hauler suffered ones rath after a buddy stored his truck in it for winter. Went to place the tray under but because the front of the trailer sat a bit higher it puked a month early out the rear seal.

Not sure on this one, but a old tranny rebuilder once told me that a TH400 will work just fine, and it won't harm it to run a bit low on fluid, said it would help prevent any oil foaming also. I've never had a TH400 so couldn't comment on that one, but I can testify he was not crazy in the head, and could rebuild a tranny with his eyes closed.