ljschalm

Hello.

I have a 91 R129 w about 50K miles. When cold I get a fairly severe clunk when shifting from 1st to 2nd and again 2nd to 3rd. After driving 25 or more miles the noise becomes intermittent...in some case, goes away all together. The noise seems to be at the rear or the car. The universal joints seem to be very good. I cannot see any excess wear in the rear end or suspension.

Any ideas?

salabellera

I would consider trans fluid and filter change, if you have not done so.

ljschalm

Thanks, Already done....

BJonesFL

This symptom is usually attributable to a vacuum leak either at the manifold (right rear of the engine near the firewall, and/or a leak at the transmission end at the vacuum modulator located on the passenger side of the tranny. If the vacuum is good on both ends of the line, then it's almost for sure it's the modulator. Your indy tech or a good and honest trans shop can adjust the modulator for you for next to nothing. If it turns out the modulator is bad, a replacement is under $50 installed. Or you could get lucky and only need the insert the goes into the valve for a few bucks. Link to part description...

Click here: Your Parts Search Returned 2 Part(s)
The link apparently didn't copy in. The parts can be viewed at autohausaz.com

Once installed, it will still have to be adjusted to your liking.

And finally, the clunk does sound like it's coming from the rear of the car, but in the vast majority of cases, the clunk is simply the result of the harmonics of the car, i.e., fault starts out in transmission and travels via drive shaft to rear, then clunks.

Please report back what you find out from your tech/trans shop.

Bob

ljschalm

Thanks Bob,

Very helpful advice. I failed to mention that when cold, I get a very long shift from 1st to second...usually it is the first shift of the day. If I let the car warm up for more than 2 minutes it is not nearly as bad. I live in Minnesota, but the car is garage kept in a heated garage @ 50 Degrees F. This is when the clunk is most pronounced.

I will check the entire system for vacuum challenges. Given the car's age, I am ordering both the modulator and the plastic cap.

I will replace early next week (I have a lift so this is not so bad) and let the forum members know the results...

Les

BJonesFL

The advice may be helpful only if it solves the problem

Re: Cold shifts- Our cars start out in 2nd, unless manually put into 1st, or you put the accelerator to the floor as in drag racing. The shifting is normal for MB. When cold, the shift timing is elongated to increase RPMs to warm the engine and make heat available quickly. If you always warm the engine for 2 minutes you won't hurt anything, but, waiting until the idle controller brings the idle down to normal RPMs (roughly 30 seconds) is plenty of warm up...get goin'

Re: Parts you order-A replacement modulator comes with a cap so you really don't need to order one unless you want a spare around. This is the specific component within the modulator that fails most often.

Good Luck and have fun-

Bob

ljschalm

Hello Bob,

New part is installed. Any suggestions regarding the adjustment procedure? Right now, I have it centered.

Can you explain the impact of rotating the adjustment knob?

Thanks, Les

One other comment. Star indicates that there are in fact two modulators. I verified with MB parts that the recommended part at Autohaus replaces the version originally installed before 7/92.

BJonesFL

Then you have the correct part.

BJonesFL

To adjust vacuum at the modulator, turning the adjustment counter clockwise increases vaccum to the trans and will make shifting smoother. Conversely, clockwise turn makes shifting 'firmer'. I pulled up a video on youtube that does a better job explaining the adjusting steps than I could write. When you open the link, you can skip over the ad for trans rebuild kits to get to the adjustment steps. BTW, this video was NOT made using your specific transmission, but the adjustment steps are the same. Remember to turn only one 'click' at a time, then test drive. If OK, then stop. If not OK, turn one more click and repeat test drive. Good Luck-

http://www.youtube.com/watch?v=mL6zZiYIn2E

Bob

ljschalm

Bob,

A BIG THANK YOU!!!! The vacuum issue was difficult to find...I had a melted hard plastic line above the transmission. Could not see it...but eventually found it. Shifts as smooth as glass.

I will leave the adjustment as it is. Thank you again!

BJonesFL

Les, I'm glad this all worked out for you. Now warm it & drive it...

Regards,
Bob