My transmission (722.3) on my 1991 420SEL has gotten slow to engage in reverse, and when hot, it may not engage at all until the gas is blipped several times (and then it lurches into gear). Once in gear, though, it doesn't seem to slip. Forward gear shifts all seem fine.

Although I wasn't expecting this to necessarily help, I tried changing the vacuum modulator settings, to see if this would affect reverse engagement. Three CW turns of the adjuster resulted in somewhat firmer shifting in forward gears, and may have made reverse engagement somewhat better. (Now it takes 2-3 seconds when cold, but when hot it is still worse.)

My question is: When I first took the rubber dust cap off of the modulator, a small amount of fluid oozed out, presumably (and by look/feel) ATF. Is some long-term accumulation of a small amount of ATF under the dust cap normal? Or is this an indication of a leak in the modulator, or its diaghram? I have seen no evidence of any appreciable loss of fluid from the transmission.

If this does indicate a small leak in the modulator itself, what would the symptoms be? My guess is that this might result in higher vacuum levels getting "past" the modulator (i.e., less modulating of high vacuum), which in turn would seemingly cause softer shifts, and possibly even poorer reverse gear engagement.

Perhaps this is why three full turns of the adjuster didn't seem to make that much difference. I have not tried driving the car with the vacuum line disconnected, but did try simply engaging reverse and drive with the vacuum line pulled off the modulator. It seemed to engage gears better with no vacuum.

Any thoughts?

Bob Schleicher
1991 420SEL - 171,000 miles
bullswin5@aol.com

slight oil in the vac modulator is 'normal' as it seeps around the screw threads. However, the bad news is the tranny is due for a rebuild. The reverse clutch linings are probably gone, internal pressures low (leaking internal seals), and or the oil pump shot. I presume the mechanic did check the operating pressure and it was correct?

Anytime you have to adjust the modulator (after initial setup), it indicates significant wear and/or internal seal leaks--neither of which are good. Deteriorated shift quality is a serious sign of wear/impending failure.

Even small amounts of water are death to the clutch linings.
(I suspect some defective friction disc's were made since I found 1/5 in my reverse clutch flaked off, 4/5 in the disc brake clutch flaked off, and the back clutch was like new)