1968 DeVille transmission fluid leak

[Lennon_68]

First off thanks again for all your help so far!  I have one that I think is easy but that I don't know what to look for / order.  I have a transmission fluid leak right where the driveshaft goes into the transmission.   I believe this is some sort of easy to replace seal but it's difficult for me to find specific info on this.  Specific questions:

1.  Where do I buy the replacement seal?  Thinking maybe it's the Transmission output shaft seal?  Here's one on ROck Auto... is this something I can get locally in stock or usually special order?  https://www.rockauto.com/en/moreinfo.php?pk=11707105&cc=1025356&pt=8636&jsn=1368
2.  What am I taking apart?   I have a vague understanding that I need to remove the driveshaft.   No clue what's involved there.  Just googled and found a video that I think includes what I need to do maybe?   4 bolts on the driveshaft by the rear wheels then it slides out of the transmission.  GUessing then I can easily access the seal to remove the old one and put the new one in
3.  Do I need to drain the transmission fluid out first or just refill when done?  Any other steps I need to be aware of or just pull the 4 bolts, slide the driveshaft out, remove old seal, put in new seal, slide draveshaft in, replace 4 bolts, top off transmission fluid?


Update...  I forgot to rtfm...  id still take advice but im going to check the service manual tomorrow as thats probably going to be the first recommendation (and rightly so lol).

[savemy67]

Hello Lennon_68,

If the extension housing/output shaft seal is leaking, it can be replaced with the transmission in the car.  Please read the entire post, as the seal may or may not be the cause of the leak.

With the car on the ground, or level on jack stands, the transmission normally declines toward the rear of the car.  If the driveshaft is removed with the car in this position, some fluid will drain out of the transmission.  To avoid this, you can raise the rear of the car to be higher than the front so the transmission declines toward the front of the car.

When removing the driveshaft, match mark the driveshaft and the differential pinion flange, so you can reinstall the driveshaft in the same position relative to the differential pinion flange.

The inside of the driveshaft yoke (the part that slides into the transmission) is splined.  The splines engage matching splines on the outer circumference of the output shaft.  When you reinstall the driveshaft, and try to align the match marks, you may be off one spline when you insert the yoke into the transmission.  If the transmission is in Neutral, you may be able to rotate the driveshaft to align the match marks.

The attached photos show what a seal looks like adjacent to where it gets installed in the extension housing of the transmission.  The first seal I got (in a rebuild kit) was not the correct size, so I measured for what I needed, and the kit vendor sent the correct seal.  Be sure yours is correct before trying to install it.

There are special tools to remove and install the seal.  See the shop manual.  You may be able to rent these for nothing at your local parts store.  You can remove and install the seal without special tools, but it will be more difficult.  One popular method of removing the seal is to use a screwdriver or cold chisel to diametrically deform the seal flange, then pry out the seal.  Behind the rubber portion of the seal is a spring that backs up the rubber like a garter.  Be sure you have the spring when you remove the seal.

You can install the seal using any item that will make contact with the entire circumferential surface of the seal flange, and a hammer.  You can start installing the seal with a few gentle taps around the circumference of the flange to get the seal square to the extension housing,  and to hold it in place.  Some folks like to put a sealer around the portion of the seal that makes contact with the counterbore of the extension housing.  Be sure to check that the spring is still in place after you have driven the seal into place (the seal should bottom against the bottom of the counterbore).

All the foregoing assumes your car is one that does not have the vented yoke and O-ring on the output shaft.  If your leak is from a vented yoke, the O-ring may need to be replaced, which involves removing the extension housing.  This can be done with the transmission in the car, but it is a lot more work.

When you remove the driveshaft, look at the end of the yoke near the Cardan (double universal) joint.  Or look down the inside of the yoke.  If you see a hole, this is a vented yoke.  This may or may not be leaking, so you will have to determine if the leak is from the yoke, the seal, or both.

Respectfully submitted,

Christopher Winter

[Lennon_68]

Thanks Christopher!  I found the sections in the shop manual relating to this and think I have what I need to get the job done.  You provided some great info and context that the manual did not though, especially for the uninitiated like myself!!

I'm going to order the part now and will be back in a few days (?) with an update.  For anyone coming across this in the future the shop manual refers to the driveshaft as the propeller shift and I believe the seal im replacing is called the transmission extension housing oil seal in the shop manual.  Page 7-36 gives the steps for the seal but it refers to page 4-22 for info on removing the driveshaft. 

[bcroe]

I have never aligned any rotation position at either end of the drive shaft,
long as it goes in, is that in the manual? 

Be aware a seal failure may be caused by a bearing failure just inside,
allowing yoke movement. 

Bruce Roe

[DeVille68]

Hi, I had the same problem for years, and I solve it by tweaking the spring inside.

I adjusted the tension of the garter spring of the slip yoke seal on the transmission extension housing. Oil was leaking out just a little bit. So I removed the garter spring, opened it up and cut out a piece of about 3mm (about 1/8''). Reattach the spring and the leak is gone! Yeah 

Best regards,
Nicolas

[Cape Cod Fleetwood]

The Ark had a tranny leak.
Brought it to Bob, complete rebuild/restoration with a promised 24 hour turn around.
Life is gooder with great mechanics who care about the car and me, and an AmEx card of course.
This might be a pic of the seal Christopher showed in his pic, this is The Ark's tranny...

[Lennon_68]

Thanks for the additional info guys!  I haven't dug into the tranny leak yet as I've had the front end on jack stands while I fuss with the bottom side of the engine.  I did buy that seal from RockAuto hoping it's the right one (and what's wrong).  Eventually I'll get back to the tranny and will provide some updates on what I found

[Lennon_68]

Friday I got the oil pan back on so Saturday I jacked up the rear end and messed with the transmission.  First off thanks so much to Christopher for the detailed instructions - they were invaluable (I read them a couple times before digging in then as I got stuck different places went back to them and found the answers I needed were there and I'd just glossed over them).

I'm happy to report that the seal I ordered off RockAuto was the right one and that the seal replacement stopped the leak at that location.  The transmission pan is still leaking - it's obvious now and I'm sure it was always leaking there I just hadn't noticed as it's leaking more slowly there.

I'm going to consult the manual and come back here if I have questions.  I assume it's going to be easy as long as I have a spot to drain the fluid out (other than just dumping it on the floor).  After that I assume it's just ordering a gasket, removing some bolts, cleaning up the surfaces, and putting the new gasket with some RTV...

[The Tassie Devil(le)]

Whatever you do, do not use any old RVT, as this stuff is not compatible with Auto Transmission Fluid.

The only one I have found, and used successfully is Loctite 5900.   I also used this on the chain drive cover on my Eldorado, as there was no replacement gasket available down here.

Talk to any transmission shop, and they will advise plain old Vaseline to hold the gasket in place.

Bruce.

[Lennon_68]

Thanks Bruce!  I would not have known that.  For draining the fluid I dug through the service manual but I couldn't find anything about draining the fluid.  On the page about removing the bottom pan it just says remove screws, remove gasket. And drain oil from pan if transmission is still in the car.  Isn't there going to be a lot of fluid coming out when the pan is dropped? 

[bcroe]

I hope you fix those leaks.  There are so many places a TH400 can leak.  The
input and output shafts are more obvious, as is the pan gasket.  The old  style
dip stick tubes can be a problem, I have changed over to the late 70s style
with funnel shaped gasket.  At this age my shift shafts rust outside, the rust
gets far enough inside to destroy the seal.  A new shaft and seal fixes it. 

Another problem is the O ring sealed speedo gear support.  The plastic gear
itself becomes so worn on high miles, that it does not touch the seal.  And there
is the electric connector for kickdown and other things. 

The biggie on Eldo TH425s is the big chain cover in the rear, which has less
than adequate gasket gripping surface at the bottom.  I copy the GMCers
that skip that gasket and use a tube of VALCO to glue the cover on.

Keeping my drip pan, Bruce Roe

[Daryl Chesterman]

Yes there will be a lot of fluid coming out when you lower the pan.  I use the bottom 8 inches of a 55 gallon drum to put under the pan, then remove the pan bolts, working from the back, on each side of the pan up toward the front.  Leave the second one back from the front on each side, then remove the front bolts.  Slowly, alternately loosen the two bolts on the sides, that you left in.  This will allow the rear of the pan to slowly drop down and drain fluid out of the pan.  If the pan is stuck to the gasket and does not start dropping down when you start loosening the two bolts, you might have to gently pry on the rear of the pan to get it unstuck.  The bolts only have about 5/8" of threads.  If you loosen the two bolts too much before the pan starts to drop down in the rear, when you break loose the stuck pan, it will suddenly drop down, causing a flood of transmission fluid to come out.  This is a MESSY job, and it is best to have a large drain receptacle like I use to drain out the fluid.  Once you have drained out a lot of the fluid, you can support the pan with one hand and remove the two remaining bolts to completely remove the pan.  Do not upend the pan into the drain receptacle, as this would keep you from examining the bottom of the pan for any metal pieces—hopefully you find none!  After the pan is removed, if you can leave the drain pan under the transmission overnight, it will let most of the oil drip off of the inside of the transmission.  Once the pan is removed and cleaned with solvent, you will want to make certain that all of the bolt holes are flat.  Many times people try to stop the pan leaks by tightening the pan bolts, and they overtighten them and dimple the holes.  Then, the new gasket won't seal.  Be sure the transmission surface that the pan bolts to is clean of any gasket residue or sealer.  I have always used NAPA's ATP transmission gasket kits that come in a red box, that have the black gasket, not the cork gasket.  The black gasket has bolt holes that are a bit undersized, and retain the bolts in the gasket.  Just place the gasket in position on the pan and start all of the bolts through the gasket, with one on each side of the pan sticking through farther than the others.  Place the pan up against the transmission and start the two extended bolts first, to hold up the pan ,then run the others up.  Incrementally tighten the bolts, doing half of the final torque the first time, then the final torque the second time, then going over them again with the final torque.  I have never had a pan gasket leak by being sure the gasket surface of the  transmission is clean and smooth, and being sure that the holes on the pan are flat and smooth.

Daryl Chesterman 

[The Tassie Devil(le)]

I agree with method Daryl uses, and this way, the possible mess can be kept to a minimum.

Another way would be to insert a long 1/2" flexible hose down the filler pipe, and then with a squeeze ball, like those found in outboard motor fuel lines for priming, suck the fluid out, and into another container.

Bruce.

[bcroe]

As good as the TH400 is, they omitted the drain.  You can avoid
the mess by adding your own, here 1/4 in steel is brazed in, drilled
and taped for a drain plug.  Bruce Roe

[Cape Cod Fleetwood]

Thanks Bruce!  I would not have known that.  For draining the fluid I dug through the service manual but I couldn't find anything about draining the fluid.  On the page about removing the bottom pan it just says remove screws, remove gasket. And drain oil from pan if transmission is still in the car.  Isn't there going to be a lot of fluid coming out when the pan is dropped?

You may not have noticed but I changed The Ark's tranny pan. It holds several more quarts of fluid, has a drain plug for next time, and the bottom is fluted which allegedly aids in cooling, purchased from Summit Racing. The Ark uses *FORD* transmission fluid, recommended by Hyannis Vintage Auto, its all they use. Its 'greasier', 'slipperier' for those old gears, you'll feel the smoothness in shifting immediately. Even with the rebuild they used the Ford (NAPA) fluid I supplied. Use a PURE CORK seal and the ones tranny shops get are significantly thicker than the ones available to us little people. Just a dab (finger touch) of gasket sealer is used on the pan in a few spots to hold the gasket in place as you bolt on the pan. Don't over tighten. I only use WIX filters. And this will start a fur ball (again) on tranny fluid and filters, but I'm sticking with the people who make 7 figures annually restoring cars and their vital organs. YMMV