928USA

I have an ’86 928 - Mercedes automatic transmission. The seals between the transmission and the differential went bad so ATF fluid was leaking from the weep hole. I drained the ATF fluid, added AT205 (to rejuvenate the seals) and the proper ATF fluid and hoped for the best. The leak lessened and then……

Today I checked the gear oil in the differential. I opened the top plug and ATF fluid came out of the differential. I assume that means the seals between the transmission and differential are bad.

Question #1: Can I simply drop the differential, replace the plates and seals, and reinstall the differential (aka a cheap fix) or must I do a full transmission fix?

Question #2: Can I still drive the car with a differential full of ATF fluid?
Thank you in advance!

Schocki

Not a very common failure. There are two seals in the intermediate plate, if there is ATF leaking out of the weep hole, both have to be replaced. Access is only possible after intermediate plate removal and this means the pinion has to come off too. Bottom line: the transmission has to come out.
Pinion Removal requires some special tools and a hydraulic press.

Driving with ATF in the differential is not good, completely different fluids. You wouldn't run your motor on ATF either, or?


Pinion still in place

Intermediate plate removed

Intermediate plate

GregBBRD

I've never had to do this, as the mixing of the two fluids is amazingly rare and I've yet to see your particular issue occurring. It's actually rare for either seal to leak and push fluid out of the weep hole. The idea of the weep hole is that mixing of the fluids should not occur, at all, as in your case. My initial reaction is that the weep hole might be clogged or restricted for ATF to reach the differential.
(The above makes me wonder why the seals have failed; the reason for this could certainly lead to a complete rebuild.)
However, after thinking about your question, it may be feasible to move the differential section rearward enough to get it off of its' studs. (Granted, the two uppermost bolts will be a bit difficult to reach.)
Once the differential section is removed, access to the separation plate, with the two seals is fairly straight forward. The toughest part will be getting the pinion nut off and properly re-tightened.

I don't believe that ATF in the differential section would be instantly terminal, as Porsche used ATF in some of the early 5 speeds, which were difficult to shift.
That being said, the fact that you have ATF in the differential automatically tells you that both of the seals have failed (or the weep hole is restricted)....and I certainly would not think that the automatic part of the transmission would respond well to gear oil, if the exchange of fluids goes in that direction, also.

There's a couple of tricks to installing new seals, but if you pay attention to how the old ones are installed, this will not be an issue.
There's a few other pieces that you will need: O-rings and gaskets.

Mrmerlin

Any chance your looking at red line gear oil in your trans and not ATF,
whats the viscosity of the gear oil in the diff section does it compare to ATF?

928USA

Mr Mrmerlin;
Great thought on your part. But, when the fluid came out of the differential, the fluid level in ATF reservoir lowered.

TY!

GregBBRD

As I mentioned, the ATF should be pouring out of the weep hole, long before it gets to the differential.

Mrmerlin

Today I checked the gear oil in the differential. I opened the top plug and ATF fluid came out of the differential.
I assume that means the seals between the transmission and differential are bad.

This cant happen no way that the oils are mixing,
thats exactly why they put the weep hole in this plate to prevent a transfer of the oils.

maybe your differential was overfilled with redline red gear oil ,
IE overfilled like it was filled on ramps and now your checking it on a level surface or VicaVersa
and its coming out of the weep hole.

drain the diff down to the bottom of the fill hole.
NOTE make sure the car is level.

Start the engine and then fill the trans tank with Dextron 3,4,or 5,
once you check the ATF tank and its properly filled then go for a drive.

ZEUS+

It looks like you have had this problem for a while. https://rennlist.com/forums/928-foru...weep-hole.html If the seals between trans and differential are leaking now, the cause needs to be addressed. Fluid level, excessive heat, aeration, restricted vent. Fixing the leak doesn't solve the problem.

Dundertaker

Any updates on this situation? Was it the seals? Weep Hole? Enquiring minds want to know.

928USA

HI All,
I asked the site to delete this thread as I discovered it is was wrong, but it has no yet been deleted.

I put the car in the air and drained the differential; I did not see any gear oil. I drained the transmission and did not see any gear oil. What seems to have happened was the mechanic put ATF fluid in the differential and not gear oil. Problem solved.

Dundertaker

I wouldn't delete it....there is utility in stupidity. I'm glad you found it. I'm getting ready to pull my TT and Trans and noticed "weeping" from the bottom of the Tans/inter/diff. I'm pulling it tomorrow and will update. Drivers side corner....but there is fluid ALL OVER this thing so I don't know if I have a problem at this point or not, but this thread was helpful.