Transmission Mystery Leak
 

In the morning after starting and backing up my car (1993 600SEL w/ 720.45 auto trans) my transmission leaks oil about 1/4 cup. When put it in drive and subsequently in reverse it does not leak. The transmission shifts smoothly. If I start in the morning and go into drive there is no leak. The transmission fluid level is correct. The leak appears to be coming from the torque converter area. The drain plug on the converter is not leaking. Because of the quantity of fluid leaking it appears to be a purge. Is there an overfill protection circuit in the transmission that could be faulty?

John

Sounds like a servo cover leak, but I've never seen that transmission so cannot say for sure.

If so, it may be possible to change the cover seal with the tranny in the car.

Peter

Never heard of a "720" transmission, this should have a 722.3 in it.
If it's coming from the bellhousing it's more than likely the front pump needing resealing, which involves removing the tranny, after that it's an easy job.

Gilly

Gilly if the front seal is bad why does it leak only from a cold start and in reverse?

John

No good idea why it only does it then, but there is nothing else in the bellhousing to leak (that I am aware of, other than the torque converter drain plug)

Gilly

leaking tranny in the morning
 

The only thing to leak in the bell-housing is the oil pump--either around the outer O-ring (which it would always do, and a slow leak), or past the torque convertor shaft seal. This is probably the culprit and likely caused by a worn front bushing or the bushing is hanging up. It serves double duty--one is to keep the torque convertor centered, the other is to keep the oil pump mostly 'sealed' on the input shaft side. This bushing is a very close fit around the torque convertor shaft and interference fit inside the oil pump. The high pressure oil inside the pump is 'throttled' across this bushing so the elastomer lip seal around the torque convertor shaft 'sees' no pressure as the leakage past the throttling bushing is vented back to the tranny sump thjrough a drain passage. If too much oil leaks past the throttling bushing, the radial lip seal gets too much pressure on it and oil leaks out at a significant rate. Failure of this bushingis apparently very common. Dealers replace the entire pump ($350 part) even though the bushing is available ($17) because the bushing often works loose in the pump and a new bushing will not hold in the worn bore.

Tranny Leak
 

KEBOWERS, thanks for your thoughtful reply. If the problem is the torque convertor shaft seal would the heavy leaking only occur in reverse and only when the transmission is "cold". The thought that the seal is hanging up is intriguing since after warm up and driving forward the leak stops even in reverse. Another odd thing is that when I check the fluid with the engine off and transmission cold the reading is above the highest mark- overfull. Is there a possibility that I overfilled the transmission and the leak is from an overflow valve if any?
The problem actually started after I replaced the filter. I was not pleased with the way the filter connections fit. Is there a possibility that the filter was the wrong one and is not seated correctly? Thanks for your input.

John

Tranny will always read high cold with engine off, as oil drains out of the internal parts while it sits. With enough time, they usually drain the torque converter, too, and oil comes out the vent on top, making a mess.

A leak cold only and only in reverse is something else.

Peter