4T80e Fix for leaky driver's CV shaft / output shaft bearing

I had some interesting transmission issues when I first got my Deville and I had to pull the transmission out twice in the middle of winter. So once my new transmission developed the dreaded driver’s side output shaft leak I decide I was going to come up with a fix that didn’t involve pulling the transmission again. Hopefully what I figured out can help out a few people.

Doing a bunch of internet searches gave me the root cause of this issue. The output shaft bearing inside the transmission gets worn down and as it does the output shaft starts moving. Once it moves enough the output shaft seal cannot hold the fluid. The output shaft pulls the seal out of round and you start to get a gap where fluid will drip out of.

So the solution is simple, just replace the output shaft bearing to keep the shaft from moving. Unfortunately this requires the transmission to be removed and a costly rebuild to take place. So we need to use a substitute bearing that will take the function of the output shaft bearing.

When you look at the output shaft seal it is actually recessed inside the transmission a good amount. There is a center plate that will actually pull out. Once this plate is out there is a nice recess for a bearing to sit in.

By the way, most of these pictures were taken on the old broken transmission that I have sitting on my garage floor.








Of course you need a properly sized bearing that will fit inside this hole, but be large enough to fit around the CV shaft. Such a bearing does not exist. To solve this I had to have something made. I chose a bearing that fits just inside the opening in the seal housing. I had the inside diameter of the bearing machined out by a guy that I know so that it will slip over the CV shaft. He had to come up with an interesting method to put the bearing in the lathe since you can’t grip the outside if you are working on the inside because it will just spin. I can give you my guy’s contact info if you want him to make you up a bearing. By the way, the bearing that I chose is a wheel bearing for a motorcycle, so it should be able to take the side loading of a CV shaft and have no issues with the RPM. The over all size was a 40x62x12mm bearing.









With the proper diameter on the inside I was able to work on the outside. The overall diameter is just a tad too small for the opening. It is maybe a half mm shy of a good snug fit. So I took the center plate from the seal and fit it around the bearing. The thickness of the center plate is just enough to make the bearing fit snug in the hole. I also cut out the center of the plate so that it would not rub on the bearing part that does move.










Now comes the fun part, since the bearing is 12mm deep it will stick out of the side of the transmission case(as seen above) and interfere with the CV shaft from locking into place. I measured the CV shaft end and there is plenty of metal to do some clearancing without having to worry about weakening the CV shaft. That being said I am not drag racing my Cadillac, if you are then you may want to rethink modifying the CV shaft. The key here is to try and keep the CV shaft balanced. Ideally you would want to do this on a heavy duty lathe, but I was stuck with my angle grinder. I took my time and was careful and it turned out well. It needs to be ground until the curved transition between the shaft cover and the larger diameter piece is gone. When in doubt try putting it on and see if the CV shaft snaps into place. I would try this without the outer shim on the bearing so it is easier to get things on and off. You will also have to clean up the shaft itself if there is some kind of build up(rust).

Here is the modified shaft next to a stock one. If you are good you won't have to pull the CV shaft apart, but I found it easier to do the grinding this way.











Once the CV shaft is to the correct dimensions it is just a matter of putting it all together. I used some locktite red on the shim/center plate on both the bearing side and the seal holder side. That way it should stay in there pretty well. Once the CV shaft is locked into place there really is no way that it can come out though. It may wiggle out a bit, but not enough to go anywhere. There is too much of the bearing inside the seal holder to go anywhere.







Oh and always remember to start this by replacing the output shaft seal, the old one is most likely damaged from the output shaft movement. Not all auto parts stores seem to carry them, I have had good luck getting the seals from Autozone.


I have a few thousand miles on my car since I put this fix in and I haven’t had any issues with it. I do a lot of highway driving at 70-80mph. So if you are having this problem it may be worth trying this out, especially if your alternative is a transmission rebuild.