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76 Deville Rear Transmission Seal Replacement Tips

Started by JLanteigne75, May 20, 2021, 10:25:44 AM

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JLanteigne75

Hello Gents,

I have transmission fluid leaking from where the driveshaft meets the transmission.  The drip is slow (about a quarter of a quart/week) and does not appear to drip when running.

Research has lead me to believe that the extension housing seal is the culprit.  The shop manual lays the procedure out decently and I've watched numerous YouTube  videos on this job (on other cars), but I always like to be prepared as I can for jobs I haven't done before.  Therefore, does anyone have tips or things to be cautious of before I begin?

One question I do have is the actual name of the seal.  The shop manual calls it the "oil seal", and Rock Auto has an "Extension Housing Seal" AND "Output Shaft Seal" listed, although they appear to be the same.  I ended up buying it locally from Napa (I live in Canada) and they list it as "Automatic Transmission Seal - Metal Clad". They had a few different options that all allegedly "fits my car", but I ended up going with the ATP 11424 since that one also showed up on RockAuto.

I also read that another culprit for the leak could be an O-Ring that slides on the yoke, but I couldn't find that in the shop manual or somewhere to purchase online.

Any advice is appreciated!
Jonny Lanteigne
1976 Sedan Deville Hardtop

bcroe

Check your yoke, if it has no center vent, it is the basic version
that will work on any TH400 (they all have the same splines). 
In that case a leak will be from the seal in the housing.  But a
failed seal may mean wear of the housing bushing, allowing the
yoke to move around. 

The other version TH400 has an O ring just forward of the splines. 
The yoke to utilize this, has a long outer sleeve extending forward
from the splines, and sealing on the O ring.  This style yoke has
an air vent in the center.  This style yoke will leak slowly from the
vent if used in a non O ring trans, or the O ring fails. 

Most likely no drain has been added to your TH400, so raising the
rear wheels will help minimize fluid loss when removing the drive
shaft and extension housing.  Mine all have a drain added to help
keep thing neater. 

Likely the trans will have the support removed so the rear can be
dropped down a couple inches.  I use these supports to hold things
without a jack on the floor.  Just a couple angle irons bolted together
with spacers, the hooked bolts grab the frame and a nut adjusts
height.  I put a big wood block between the angle iron and the trans
pan to spread the strain.  good luck, Bruce Roe

Phil Weber

The part number for the Oring is 8616504 . You will need to remove the extension housing to access it .
You can buy small parts kits like this one on Ebay which has everything you need .https://www.ebay.com/itm/141635900617?hash=item20fa2854c9:g:-7AAAOSwPhdVKxYq.
While your there it's a good time to replace the speedo drive seals as well.

JLanteigne75

Here is a pic of the seal. It appears, although I can't be sure, that the fluid is coming from the hole in the center.  Is that hole what you mean by "center vent"?  I'll go ahead and replace the seal anyways, but would a leak from that hole indicate a different failure such as the o-ring?

Also, if it helps for diagnosis, when I had the front wheels up on ramps the dripping increased in frequency, and having it now with the back wheels up on ramps, the dripping has nearly stopped.

Jonny Lanteigne
1976 Sedan Deville Hardtop

Phil Weber


I have the same model as you and had the same problem .
To stop the leak from the breather hole you will have to replace the o-ring on the shaft. Just replacing the extension housing seal won't fix it.


The Tassie Devil(le)

The rear extension needs to be removed to access the "O" Ring seal, and it would be prudent to check the condition of the bush at the end in case this is loose, and causing the yoke to be loose.

And at the same time, you will need to replace the seal as it has to be removed to remove and insert the replacement bush.

Plus, when reinstalling the tailshaft yoke, put some grease on the splines, as the "O" ring is there to stop the transmission oil getting past it. (and out the hole)

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Daryl Chesterman

QuoteHere is a pic of the seal. It appears, although I can't be sure, that the fluid is coming from the hole in the center.  Is that hole what you mean by "center vent"?  I'll go ahead and replace the seal anyways, but would a leak from that hole indicate a different failure such as the o-ring?

Are you sure that is a hole in your picture?  From the angle of the picture it is hard to tell.  To me, it looks more like a lathe center.  To be sure, take a small wire and see if there is a bottom to the "dimple", or is there a hole?  The answer to these questions will tell you whether you need the o-ring or not.  As Bruce said, it would probably be best to replace the tailpiece (extension housing) bushing while you are going to the trouble of replacing the seal.

If it is leaking as much as you are saying, you can wipe everything down, and it won't take long to ascertain where the leak is coming from!  A half pint ("about a quarter of a quart/week") is a substantial leak  :D

Daryl Chesterman


wheikkila

Good Afternoon
This is a easy repair. To start remove the drive shaft. Be sure to check the ujoints for stiffness or roughness. Remove the two bolts from the mount. Jack the transmission up. Remove four bolts for the cross member and remove. Let the transmission down. remove six bolts for the extension housing. For parts you will need the extension housing gasket, bushing, seal and the o ring for the shaft. extra transmission fluid. Be sure to put a pan under the transmission to catch the fluid or you will have a mess. You will need basic tools for the removal. But to change the seal and bushing you will need the drivers to remove and install. Check your parts store to see if they have them for rent or some have them as free loaners. Please let us know on your progress.
Thanks Wayne       

JLanteigne75

Gentlemen,

Thank you kindly for your replies.

I now have the required parts in hand.  Against better judgement, I didn't carefully read the differences between the bushing and seal sizes on RockAuto. Perhaps Phil could provide some info on this since you have the same model? I got the GM 8616504 O ring like you suggested (thank you very much for the part number by the way). My seal is ATP 11424 (2.5" outer diameter), but the other option was a 2.381" outer diameter.  The bushing I purchased is Pioneer 555071 (1.995 O.D, 1.5" long), but the other option was 1" long.

Any info for the above would be appreciated since I would certainly prefer to be confident the parts will fit before I go tearing things apart.

Wayne,

Thanks for the advice! The brushing and seal drivers might be a short-term issue since auto part stores aren't currently renting where I live... oh how I wish I lived in the US.  Hopefully I can find a more "what I've got lying around the house" method from YouTube for the installation. I hope to do the repair this weekend.

I'm also starting to think now might be a good time to replace the transmission mount since I'm gonna be right there anyways.
Jonny Lanteigne
1976 Sedan Deville Hardtop

Phil Weber

The seal you need , I'm sure , should be 2.7" diameter . This is the size I fitted to my car . The OEM number is 8626009.

Attached is a page from the factory parts catalogue and also a page from a seal catalogue I have listing the size and also a good cross ref list.

Don't forget you will need a gasket for the housing.

Phil

JLanteigne75

Gentlemen, an update:

I'd like to document my experience in writing in case someone comes along looking for a solution for a similar problem.  I also want to say thank you for all the advice.

In short, I replaced the o-ring on the shaft and I have not had a leak since (going on two weeks).  I'm now confident to say that was the proper fix.

Because I was on a bit of a time crunch and didn't have the proper tools, I did NOT replace the output shaft seal and bushing.  A factor in this was that they seemed to be in OK condition.

A few unexpected "hurdles" I came across: 
     -  My transmission support crossmember bolted to the exhaust via a rubber mount.  That was just two screws to remove.
     -  To remove the crossmember, after unbolting, you need to push UP and over to one side and then slide the other end
         out.  It will not come out if you do not push up first (because the cavity its in slopes up and out).  Getting the crossmember
         out and back in was my biggest struggle, but with some wiggling and a whole lot of pushing it came out.
     -You cannot remove the extension housing without getting at least one of the ends of the crossmember out of the cavity.  No
       matter which way you move either the housing or the unbolted crossmember in the cavity, it will not be able to clear it.
     -It will take some effort to get the end of the driveshaft fully seated because of the new O-ring.  Never get emotional with it
       though and recklessly jam it in there - it would be a shame to do all that work and then break something.  (And ensure you lube
       the components as mentioned in the above posts)

All in all it was a job I think most somewhat mechanically inclined DIYers such as myself could do in a day's work if my hurdles happen to apply to you.  My $0.02.

Thanks again guys.  The girl is back on the road leak-free!
Jonny Lanteigne
1976 Sedan Deville Hardtop