Tranny Question From A New Owner - PeachParts Mercedes-Benz Forum

mountain280 · #1 12-15-2006, 03:18 AM

Just brought home a 1992 300E from Texas. 56,000 verified miles with all the stamps in the service book. The car looks and feels new. Drove it home 2100 miles without a hitch except for a tranny that started to shift harder as the temperature dropped as I headed north.

Cold morning starts produce delayed and very abrupt shifts. This condition smooths out after warm up but there's still a pop in the shifts as compared with the silky smooth shifts while driving down south.

Trans fluid looks normal,the level is good and given the mileage I'm not suspecting mechanical wear. I've search the problem and am suffering info overload. Could the more experienced and technically advanced members please check my thinking on the problem.

1) Electronic warm up cycle induces long shift duration and in my case hard shifts. I understand it's function is to get the cat to operating temp as quickly as possible but it annoys the he11 out of me. Can I disable this feature by disconnecting the single pin connector at the tranny without damaging anything?

2)Before I try to fiddle with the modulator valve on the tranny I should first check the quality of the vacuum going to the valve. Do I disconnect the line just before the valve to test and what values are considered normal vacuum? What values do I look for under what conditions.

3) How do I test the modulator valve. What condition is considered faulty?

4) If proper values are found for the vacuum source and the modulator is good do I then attempt to adjust the modulator valve and, if so, how is this done?

Is this a sound plan of attack or am I over simplifying? Please help. Sorry for the long posting.

mbdoc · #2 12-15-2006, 08:02 AM

If the car is a "federal" version?? the system doesn't care if the up-shift delay is disconnected.
California cars will turn the check engine lamp on if that is done.

I would start with fresh ATF & filter in the trans. Then see if a modulator adjustment is needed.

Gilly · #3 12-15-2006, 08:19 AM

I would inspect the throttle valve cable to make sure it isn't adjusted too tight, and that all the linkage is freely working, sometimes these get so badly neglected that you need to take the linkage apart and clean and lube everything. If it's not adjusted correctly the warm-up mode will be even harder than normal. I'd discourage trying to disable that. It's only slightly harder, for just a few miles, barely noticeable normally.
Edit: The modulator is adjusted by removing the rubber cap and there is a small "T" key that is rotated. CCW is softer, CW is harder. Go for the linkage first, and if that doesn't fix it, then I'd try just small CCW adjustments (no more than 1/4 turn at a time) of the modulator.
Gilly

lee polowczuk · #4 12-15-2006, 10:24 PM

when was the last time the fluid was changed... filter could be clogged..

i would change the filter and put mobil one in it first...

dpetryk · #5 12-16-2006, 08:22 AM

Heat and age cause the O rings and other seals in the tranny to slowly get hard. They dont seal well and things begin to change. Your observation about the problem getting worse as the temps went down would support this idea. Even though you dont have a lot of miles on the car it is possible that the tranny needs to be overhauled. Fresh rubber pieces make a big difference in the shifting qualities of that tranny.

Educaid · #6 12-16-2006, 10:49 AM

I've got a similar problem. When cold, the 3-4 shift is hard and annoying. It goes away once the car warms up. The dealer adjusted the Bowden cable but it still does it. I'm tempted to put 6 oz. of Auto-Rx into it to see what happens. The car is a 93 300E with 92,000 miles.