Fixing leaky TH400 - 1988 Burb V20

[Silverbullet555]

My Th400 is leaking from every hole it seems. Go figure.

I have the tool to replace the seal for the shift shaft.

I'll source the case connector from somewhere and vacuum modulator from Napa or from Amazon.

Question has to do with removing the pan. Do I need to remove the cross member or will the pan slide out without that. It is a B&M deep pan if that matters.

Any other tips for replacing all the attachments and seals?

1988 V20 Burb 4x4
185K miles

[68Timber]

You can drop the pan without moving the cross member. At least with a factory pan you can, but it shouldn't matter.

[Silverbullet555]

Originally Posted by BRUISER

You can drop the pan without moving the cross member. At least with a factory pan you can, but it shouldn't matter.

Thanks.

[Electricduane]

I just did this. On mine, the kickdown shift connector was dumping fluid. It is located on the driver side of the transmission. Not only had the oring worn out, the actual round plastic piece had shrunk over the years. I found the piece online for something like $10. Of course, I initially just replace the o-ring, then had to take the god damn thing apart again because the plastic piece had shrunk.

Other advice I wish I had known: my parts store had a basic gasket. It was cheap and flimsy. I asked for a premium gasket. They handed me a cork gasket. I used it along with Permatex aviation gasket sealant. I either used too much of the sealant or torqued the gasket too hard (144 inch/pounds is what I read online is correct), because it split in several places at the bolt holes. I went to lubelocker.com and ordered a nice metal / rubber gasket. I eventually will do this project yet again because it does have a slow leak. I also replaced the vacuum modulator, because I think it may have been leaking a little bit. That part is about $20 and involves one bolt, very easy fix.

Good luck.

[Silverbullet555]

I already have the lubelocker and will install that. Picked up a new modulator today at napa. Can get the case connector locally on Monday and will do the shift shift seals.

Thanks for the tips.

[Silverbullet555]

Originally Posted by Electricduane

I just did this. On mine, the kickdown shift connector was dumping fluid. It is located on the driver side of the transmission. Not only had the oring worn out, the actual round plastic piece had shrunk over the years. I found the piece online for something like $10. Of course, I initially just replace the o-ring, then had to take the god damn thing apart again because the plastic piece had shrunk.

Other advice I wish I had known: my parts store had a basic gasket. It was cheap and flimsy. I asked for a premium gasket. They handed me a cork gasket. I used it along with Permatex aviation gasket sealant. I either used too much of the sealant or torqued the gasket too hard (144 inch/pounds is what I read online is correct), because it split in several places at the bolt holes. I went to lubelocker.com and ordered a nice metal / rubber gasket. I eventually will do this project yet again because it does have a slow leak. I also replaced the vacuum modulator, because I think it may have been leaking a little bit. That part is about $20 and involves one bolt, very easy fix.

Good luck.

How is the case connector held in?

[Electricduane]

It is a round plastic piece, about 3/4" diameter, with a rubber o-ring and 3 plastic legs that click in to the hole in the transmission case. It just presses in, no tools, and the 3 plastic legs provide the tension to hold it in. I put a dab of grease on the o-ring to be safe. The kickdown wire plugs into the end (outside the transmission case) of it, and another blade-connected wire hook to it inside the trans case.

IMO, it's a ****ty design because over time the o-ring or the plastic piece itself (or both!) is guaranteed to fail, shrink, etc . . . there are (predictably) many internet forum posters who have all had this same problem. And WHEN this problem arises, you have to drop the #/@:$ pan to get to this piece of **** plastic gizmo to replace it . . .can't get it loose (far as I know) unless you drop the pan . . . covering you and your garage floor with transmission fluid . .

[Front Range]

Make sure the transmission vent on the top center of the housing is not plugged. Beyond what's been said a quality gasket will take care of most leak issues. If installed correctly both the following gaskets work well without any additional sealant added to the flange. Both have a steel core.

Moroso 93103 Perm-align is about $30.00 - Any aftermarket dealer

VICTOR REINZ W39341TC Tough Cork for $6.54 - Rock Auto

I prefer to use a thick flange aluminum pan with drain plug. Better sealing IMO and easier to drain before removal.

[Silverbullet555]

Thank you all for the tips. I will check the vent. I'm assuming a coat hanger or other small wire should suffice.

I do have a B&M deep cast pan so it has a thicker flange and a drain plug.

[Silverbullet555]

Just updating. Got the case connector and filter at a local transmission distributor. A whopping $7. Will get it done this weekend.

[Silverbullet555]

Got the pan off. Hope I can get it back on when it is time. The gasket came off nice and clean.

Got case connector out. It was RTV'd in. I'm guessing you're not supposed to use RTV on it. Not sure if it was a previous transmission shop or the previous owner. I would like to think it was the previous owner and not a previous transmission shop.

You don't use RTV on them, correct?

The insulation on the wire going to the plug on the outside of the transmission case connector is cracked. Hopefully I can get a new plug and patch it in.

Other question has to do with the filter.

First, is it supposed to loose on the retaining bolt. I assume it is.

Any value in shimming it with some O-rings to reduce the movement?

Second, I noticed my replacement filter has media (Dacron I think). The current one doesn't. I'm reading of some issues after switching to the Dacron filter.

Thoughts?

[Electricduane]

No RTV on the case connector or anywhere else, is what I was told. Also, a dab of grease on the case connector o ring protects it from getting boogered up when it clicks in.

The insulation on my kickdown wire is also cracked. It still works. I haven't bothered to trace the wire, but I think it goes to the gas pedal (mine is 6.2 diesel). I'm sure it would not be hard to locate a new kickdown wire & plug to replace the old one. Thank god it doesn't require dropping the pan!

Filter is loose on the bolt. Make sure the o ring on the uptake tube is not worn. It presses in the hole with a small amount of friction. As for the filter, I didn't think too much about it. Any new one will be much better than whatever was in it. They certainly laat 30k miles and no doubt people sometimes leave the same filter in there for 70k plus miles- not that I'd recommend it...

PS: re did mine with lubelocker gasket. Cork gaskets suck, very flimsy and weak. Now I know. One of my pan bolt holes stripped. Ironically it was the one by the kickdown switch case gizmo. Luckily I got a longer bolt and a nut on the far side of it. Lord willing I will never have to mess with that transmission pan again...

[Silverbullet555]

A couple more questions.

Pan is back on using a lubelocker seal. Case connector replaced. Vacuum modulator replaced. Filter replaced.

Question. How deep does the shift shaft seal sit. I put the new one and it didn't seem to go that deep. I have heard that some two seals can fit (as a possible fix to the leak) but I don't see how two could fit.

I'm wondering if someone had put two seals in at some point based on the RTV I found on the case connector.

Second question. If I were to be considering dropping the Tcase to reseal it, should I hold off on filling the transmission with fluid, or is that irrelevant (my thoughts).

PS, the reason it takes me so long to move to the next step is I have 10 projects going on at any given time, plus quail season, duck season and ice fishing season.

Thanks for all your help.

[Electricduane]

Your research has exceeded mine - I didn't mess with the shift shaft or the Tcase. But I'm sure the answer is to NOT fill the transmission unless/until you are done messing with the T case.

For what it's worth: Before, I had fluid dumping from the kickdown switch connector and leaking a good bit from the pan seal. Those 2 items are fixed. Now mine is leaking (slowly) from the vacuum modulator thing (which I replaced!) and from the shifter input seal.

All of which lead me to the conclusion that a wiser person told me about TH400s: they are old-tech, unsophisticated workhorses that just leak. Bright side, they run a long, long time. As for me, I have decided I can live with a slow leak. The kickdown shift thing was dumping like a pint a night. A few drips I can live with. Alternative is to get a nice, new, expensive 4L80e. Someday. . .

[Silverbullet555]

Originally Posted by Electricduane

Your research has exceeded mine - I didn't mess with the shift shaft or the Tcase. But I'm sure the answer is to NOT fill the transmission unless/until you are done messing with the T case.

For what it's worth: Before, I had fluid dumping from the kickdown switch connector and leaking a good bit from the pan seal. Those 2 items are fixed. Now mine is leaking (slowly) from the vacuum modulator thing (which I replaced!) and from the shifter input seal.

All of which lead me to the conclusion that a wiser person told me about TH400s: they are old-tech, unsophisticated workhorses that just leak. Bright side, they run a long, long time. As for me, I have decided I can live with a slow leak. The kickdown shift thing was dumping like a pint a night. A few drips I can live with. Alternative is to get a nice, new, expensive 4L80e. Someday. . .

At this point, there was no reason not to replace the easy stuff. I think the case connector was the main culprit though the vacuum valve as really wet. The engine drips a bit so it will never be totally dry unless I pull the engine and rebuild it.

I don't know if I will pull the tcase right now or wait. I think i also have a rear axle bearing going south as I can hear click when I spin the wheel by hand with it up on stand.