Classic/Antique Car Repair: Transmission Leak, cadillac coupe deville, torque convertor

Question
Hi Dick,

I have a 1967 Cadillac Coupe deVille, with a 429.  The car is all original and I'm getting around to fixing her up.

After driving the car I store it for sometimes weeks before driving it again.  After a few days, or weeks, suddenly it seems the entire transmission dumps all its fluid?

I finally got around to getting under there to find where it was coming from.

It's dripping out of the center of the shaft coming out of the rear of the tranny?

Do you have an idea?

Thanks in Advance!
Bob Langell  

Answer
Yes, this problem is common with older cars. What you are having is called "converter drainback".  

You see, the oil pan on the transmission only holds about 1/3 the total oil fill of the transmission - the balance of 2/3 of the oil is pumped into and normally kept in the torque convertor which is at the front of the transmission; half of which is physically HIGHER than the rear shaft seal!

When the transmission gets old, the seals that prevent the oil from draining back out of the convertor begin to leak, and thus the oil pan gets way overfull - to the extent that the overage has to find another place to go.  If there is a vent on the top of the main transmission case, it often will come out of that, and that could be what you are having - it comes out the vent and then runs over the top of the case, following the shortest path to the ground, which could make it look like it is coming from the rear seal.

OR, it is possible that your rear seal is leaking - they do wear out - but normally that seal isn't submerged in oil, so it makes little difference if it is worn, so long as you drive the car every day.  

It is the practice of letting the car sit that is allowing the case to be overfull. You can investigate this yourself, by noting the oil level on the transmission dipstick at various time. If the transmission fluid is topped up properly (warm engine, select each gear, then shift to park, fluid to full mark) and then the car is parked for a couple of weeks, check the dipstick BEFORE you start the car - I'm betting it will show way over full!

If this is the case, next time you have the transmission worked on, be sure to tell the shop that you are experienceing "convertor drainback" and they will know what to do.  In the meantime, drive the car at least once a week, and you probably will prevent much of the leakage.

It wouldn't hurt to make sure exactly where the oil is coming out - if it is the rear seal leaking it out, that is a much cheaper repair than the drainback seal - as the rear seal repair can be done without removing the transmission from the car.  Just fixing that may stop the problem, or at least greatly reduce it, as then it won't leak until the oil lever gets all the way up to the vent!

Dick