Perplexing TH400 leak

Check to see if there is a groove on the output shaft where an o-ring should be. Many times that o-ring gets forgotten or left off (or dislodged upon installation of the yolk). Make sure you know if the output shaft uses a "O" ring or not by removing the extension housing. If it has the O ring land, then find a yoke that was made for the O ring. The yoke will not have splines on the very end so it will slide over the O ring. Yokes that are designed to work with the O ring usually have a hole in the plug in the end. The O ring seals the transmission fluid from entering the splines, so you have to grease the splines, and the hole vents the air that is trapped in the yoke. It's pretty common to not have the O ring in place and get a nice big leak out that hole.

 

Matt,
Is a yoke like this available new or only used? I also have one of these Leaky T400 it only leaks after sitting and I think the yoke has a hole in the end of it . I have changed the dipstick o ring previously after a rebuild of the trans. Tony

 

You can get new counterbored yokes

 

My th350 has drain back leakage. Doesn't seem to be coming from speedo or fill tube or shift shaft or govenor cap etc.......What is case connector leakage? An Oring? Drop the pan to fix? I can't seem to find much information on repairing this type of leak.

My Inspection cover and pan is very wet after sitting for days when transmission fluid was topped off the week before. Picture shows pan is pretty full according to the dipstick level before starting.

 

Just an update on my fix. But for sure Larry and all are correct about the converter drain back. Anyway, checked all the areas mentioned. Now this was a couple years ago. What fixed mine was installing a Moroso TH400 (thick!) rubber pan gasket. It was simply leaking from the front area of the pan. Even after installing a new cork gasket. My race car started doing the same thing and this also fix it as well. Just an FYI .

 

Your " Mechanic " won't find the leak because he is looking at it right after you drive it to him . Clean the side of the trans case above the pan gasket . Then after a few days check for the leak .

As said the Shifter shaft seal is really a good shot . There are 2 different seals . One is thicker than the other and is the best to use . Speedo gear housing AND the speedo cable seal is another . Plus like Larry said the fill tube is tricky as it seems to flow around the case and look like its coming from somewhere else .

Converter drain back is the reason . My car leaked 2-3 quarts thru the shifter shaft seal over 6 months .

 

Moroso# 93103. It has the steel core as well.

 

Yes, totally agree. Mine wasn’t leaking from the shifter shaft. Just the pan itself is another option to check. My questions are for anyone that might know.
This drain back. From what? What creates it? The aftermarket converters used? Does the factory converter drain back? I realize it may not be the converter itself. It could just be the reservoir for the fluid.

 

Yes . As the converter drains back it will put fluid above the Gasket level in the pan . Then seep thru the gasket . I had another that did that .

 

When my trans was original it did that, I eliminated 80% of it by going to a cast aluminum pan, the other 20% was cured after the last rebuild by Janis, the shift shaft seal was replaced.

 

There is a decent amount dripping from my Inspection cover. Car is usually backed in with a normal 2% slight floor grade towards my garage door. Could this be "case connector leakage"?

 

Correct, I realize that. Is drain back normal, is my question.

That certainly can’t help. And whip’s the fluid all over. How I finally found where the leak was coming from in my deal was cleaning everything, then jacking up the front end while it sat. No leak when doing that, made me think. That I believe uncovers the shift shaft seal as well as the front pan gasket area. When I sat it back down and carefully watched over a couple days, it was very apparent it was coming from the pan gasket.
Also, someone mentioned cast aluminum pan. Think I changed to that (with the cooling tubes) at the same time if memory serves. So?…

 

I added a bit more fluid to check if it would start dripping again. Seems to be coming from the back of the pan onto cross member onto the floor. I put a level on the bottom of the pan and it is still tilting forward. I guess it could just be the pan gasket.

I'm thinking i should drop the pan, make sure it is true and use the rubber gasket.

 

Eliminate a source of leakage and get a cast aluminum deep pan.
The flange does not warp, and provides a good tight seal against the gasket.

 

Duh… I should have related that. Knew that it was higher on the stick while not running since my early teens! Where is that head slap emoji?

 

You should remove the inspection cover and see if its leaking down from the front pump .

But that seems like a classic dipstick tube that flows around an " LOOKS LIKE " it is coming from the Inspection cover area .

 

What is the case connector oring?

 

Ohhhh. Thanks!

My th350 uses a kickdown cable of course...... I do see a sending unit on the side with nothing connected.