TH 400 Torque Converter Drain Back Solutions?

[72step]

I have a torque converter drainback issue on my TH 400. The tranny was rebuilt about 8 years ago and the torque converter is a B&M I got from Summit about 12 years ago. I know the simple fix is to drive the truck every week, but I don't think it's normal for my garage floor to be covered in tranny fluid if I don't use the truck in 2-3 weeks. When I go to check the dipstick it marks very high. The tranny is top notch other than that it runs like hell-it's a TCI racing tranny. Would a new torque converter solve the issue? I replaced all seals on the tranny last year that I could get to without taking it off, but the problem is with the tranny getting over full by the fluid contents of the converter. Any info is greatly appreciated. Thanks.

[jeffspower]

Could you possibly have the wrong dipstick and/ or tube, causing it to read too high, and creating an overfill condition to start with? Air bubbles mixed in the fluid showing up on the dipstick after driving would indicate this.

They all drain back to some extent. At work, when we put a PTO on a 3000/ 4000 series Allison, we get to it right after shutting the engine off. If we don't get one done by day's end, the next morning there is an ocean of ATF on the floor from converter drainback raising the fluid level over the PTO opening.

[72step]

How would I know if it's the wrong dipstick for a th 400??? I do remember buying the dipstick and tube at a GM dealer at the time I installed the tranny back in 97 because of course TCI didn't include a dipstick and tube. It is the type of dipstick with a red lock on top that reads "trans fluid." It's very possible the dealer sold me the wrong dipstick. I have very little faith in that Chevy dealer's parts department.

[jeffspower]

Good question. The only way I would know of is to set it beside a stock stick & tube & compare where the full mark lands in relationship to the point of entry on the transmission case. Checking for air bubbles is where I would start.

Maybe someone has a tranny on the floor they can check... I have several, but they are 140 miles form here!

[1968SWBBigBlock]

I would bet good money you have a bad o ring on your dipstick - I went thru this same issue with my T350 - the cure was a Milodon milled AL x SST flexbraid dipstick tube
The can keep the tolerences so much tighter on the milled al vs the stamped steel

[jeffspower]

Hmm... I guess I assumed you meant it was leaking out the vent tube from overfill. Exactly where does the fluid leak from?

[1968SWBBigBlock]

Drain back from the TC overfills the cranckcase - the lowest point seal after the tranny pan is the dipstick tube.
Most dipstick tubes are mild steel with a crimp in the tube and an oring for a seal
These orings dry rot over time and allow the tranny to leak after the truck sits.
It may not do it all of the time depending upon location of the pump and torque converter

[72step]

I replaced that O-ring last year when I replaced every single seal I could get to without removing the tranny and that was one of them. As far as where it's leaking from I can't tell; I guess because tranny fluid is so thin, it doesn't just cling to anything, so it's very hard to tell. It was even hard to tell what general area it's coming from because there was a huge puddle of tranny fluid underneath the truck which I had to clean up before I got under there. It was almost impossible to tell where it's leaking from.

[jeffspower]

Clean it up, put some UV dye in the fluid, run it then park & wait for it to leak. Shine a UV light around it, then you will find it.

[72step]

Jeffspower: Where do I buy UV dye and a UV light?

[72step]

So it's normal for the torque converter to drain back fluid into the tranny to where it completely fills it up and finds the weakest seal on the top of the tranny???

I'm not an auto tranny expert but you would think it would make sense for there to be some sort of a one way valve preventing the torque converter to lose it's contents into the tranny.

I can tell you that this wasn't a problem in the past.

Tranny shops won't answer my question because they just say to bring it in for a rebuild.

What would a new converter do???

[jeffspower]

Here's a cheap leak detection kit:

http://www.sjdiscounttools.com/mas53351.html

You can get them at parts stores too. Yes, it is normal for a converter to drain back. That's why you can't check the fluid level with the engine off... it will read high. A new converter won't change a thing.

[Lugnut64052]

Those tubes with the O-ring seals were always problematical leakers. The tubes that use the sleeve type seal work much better. Maybe all you need to do is change to the better style tube and seal.

[72step]

Okay, so summing everything up I really need to check that dipstick to see if it's a proper one. My tube does take the sleeve type seal instead of the o-ring. I have a feeling that the dipstick tube and stick might be the wrong ones. They are genuine GM parts from the Chevy store, but those geniuses could've easily sold me the wrong tube because I couldn't really specify a year make and model for the 400 Tranny since I did get it from TPI racing. The way things are set up through a computer, they can't just go to the the shelf and find a tube and stick for a TH 400 tranny, they probably entered a year make and model of a GM vehicle that was too new and they sold me the stick and tube for a 700r4 or something like that.

Does anyone have a th 400 dipstick and tube off the vehicle as where you could give me accurate length measurements? Otherwise I'll have to figure something else out. Thanks in advance.

[Hilandr451]

I had the same problem with the TH400 in my '66 C20 for years. In 1995 or '96, I had installed a re-built TH400 I got for $200.00 from a tranny guy who happened to have one he built for a customer on the side and the customer couldn't come up with the cash to pick it up. About a month or two later it would leak out 2 or 3 quarts of tranny fluid if it sat for a week without driving. I talked to two other people who had the same symptoms and one of them, a '70 C20 owner, told me that there was a seal inside the tranny that either doesn't get replaced or gets installed wrong by the build shop. Of course, being as I couldn't exactly take it back for a warranty and being the cheap bastard backyard mechanic that I had become, I opted to just not let it sit that long and to carry 3-4 quarts of tranny fluid behind the seat!

I drove it for 12 or13 years like that until a month ago when I pulled the TH400 and installed the 700R4.

I hope this helps to open up the question again - I seriously doubt replacing the dipstick tube o-ring will help - IMO this is a drain-back issue and while a little converter drain-back is normal, it shouldn't overflow the pan. I would see if you can find an ally at a tranny shop who can actually give you some advice rather than just telling you to bring it in for an overhaul. Even though you might just need to tear down the case to fix it - at least you'd know what the problem is.

I have overhauled exactly 3 transmissions in my life, a 1943 Willy's transfer case back in 1983, a 1979 Subaru 4WD transaxle in 1985, and a Honda 5spd transaxle out of a 1977 civic way back in 1986. None of them were automatics and none of them GM. So when I ran into tranny troubles with my so-called "new re-build" back in '95, I wasn't exactly enthusiastic about tearing into it to fix it.

[ltitsnw]

I had the same issues with a turbo 350. I was sure it was leaking from the dipstick tube, got that fixed and sure enough started leaking again. Talked with a buddy(former GM tech) told me look at the seal on the shifter shaft. Small seal,very small, designed like an axle seal. Cost about $2-3 dollors. Put that in and I haven't had anymore leaks. Even if it is draining back, it's along way up to the vent tube. Trans should hold all the fluid, only a few places it could get out. If it's not the tube, I'll bet on the shifter shaft seal....Troy

[Eddie H.]

I had a similar problem with a 700r4 and it turned out to be the selector shaft seal as well. I also found out that sometimes, even a new seal will leak due to a groove being worn into the shaft. I had enough room to put in two seals back to back, and this fixed the leak.

[70 net440]

Does the leak stop after sitting a while? If it does, then you might have an overfill problem. How much fluid did it take to reach the full mark when you first installed it? A stock depth pan and stock diameter convertor should take about ten quarts to reach the full mark. Fluid should only come out of the vent tube if there is a preassure problem while running. Not while sitting.