I just had the same problem with my '91 420. This is a fairly easy fix with replacing two bushings that cost from $0.65 - $2.50 each - CHEAP FIX.

The instructions below are taking from the "Mercedes-Benz Discussion List" - http://hsb.baylor.edu/html/easley/mercedes/welcome.html

They were written by Richard Easley, and while I have seen several sets of instructions on how to do this, these instructions are the most complete I have seen. This is a fairly easy job, took me an hour, 45 minutes of which was spent trying to get the new bushings squeezed in. Also, follow Richards instuctions about taking off the linkage "arm" at the transmission (this is the one near the front of the car) it makes it easier. I used a pair of pliers and a closed face cresent wrench to help squeeze the bushing in place - it was not easy... But keep trying.

Anyway, here are the instructions from Richard Easley, taken straight from his web page:

The technical material for this FAQ (Frequently Asked Question) was provided by Richard Easley of Baylor University, and is provided as a service to the subscribers of the Mercedes-Benz Discussion List.

To receive similar quality tips as described below on a daily basis, consider subscribing to the Mercedes-Benz Discussion List, which is located at the following site:

http://hsb.baylor.edu/html/easley/mercedes/welcome.html

Introduction

The absolute first step in any diagnosis of automatic transmission problems in Mercedes-Benz automobiles is to ascertain that the transmission linkage bushings have not disintegrated. These bushings (there are two) are nylon and are subject to significant amounts of heat and dirt and sometimes oil from leaking components. As such, they are very susceptible to breakage and should be replaced at least every 60,000 miles and checked carefully during every major service. When the bushings are disintegrated, this can cause serious internal damage to the transmission because of improper positioning of valves in the valve body.

Automatic transmission diagnosis and adjustment on Mercedes-Benz automatics is one of the most frequently misunderstood procedures for do-it-yourselfers (DIYers). DIYers frequently assume the worst any time that they are experience automatic transmission problems and frequently, the problem is nothing more than a problem with shifter bushings or a vacuum problem, particularly in the case of diesel-engined MBs, where vacuum is "artificially manufactured."


Assumptions

1. Using the categories of mechanical ability from the Mercedes-Benz Discussion List <http://hsb.baylor.edu/html/easley/mercedes/subscribe.html>, you need to be at the level of "Medium Do-It-Yourselfer" at minimum, to replace the shifter linkage bushings on Mercedes-Benz automobiles. If you are below that level, you may want to provide these instructions for someone who is at the medium level or beyond.

2. Completely secured automobile on concrete floor

3. Safe equipment

4. Experienced (read: confident) technician

Tools needed:



1. 1/4" drive socket

2. Extension

3. 10mm socket

4. 10mm wrench

5. Long screwdriver

6. Long needle nose pliers (For example: http://www.harborfreight.com/cpi/taf/Displayitem.taf?itemnumber=38597 Also: http://www.harborfreight.com/cpi/taf/Displayitem.taf?itemnumber=33202





Shifter Linkage Bushing Replacement


1. Raise the front only as high as possible on concrete

2. Emergency brake activated

3. Car in park

4. Rear wheels chocked securely

5. Put securely on jackstands

6. Lower floor jack

7. Wiggle stands to ensure security

8. Put jack back under car as added security

9. Put shift lever in neutral.

10. With car in neutral -- but securely chocked and jack stands secure – find shifter linkage (Shifter near driveshaft, transmission linkage forward).

11. Carefully note arrangement of shift linkage and lever on transmission.

12. Do not adjust shifter linkage (assume correct, just bushings worn).

13. With 10mm stuff, completely remove bolt that goes through linkage at transmission.

14. With screwdriver, carefully pry linkage off of transmission (may have to expand the compressible "slit" on linkage just a bit).

15. With screwdriver, unsnap clip at shifter linkage and remove linkage from shifter (under car only).

16. Slide shifter linkage and transmission "lever" (which is still attached) forward. Remove from car. [Please note that the transmission lever has a plastic "tang" (round) that slides into it from the backside. This is the neutral safety switch and it remains on the car. Note that it is *not* forced on the transmission lever when it is reinstalled. It must slide on easily. Also, do not lose your orientation. In other words, do not move the lever's position vis-a-vis the transmission stud that remains.]

17. Once the linkage is removed from the car, replace the transmission lever bushing *only* on the bench. Again, make certain of proper positions of everything.

18. Replace shifter bushing *on car* (and underneath car only). [You can move lever around to get underside access, but make sure that the lever is back in the neutral position before reassembly of linkage.]

19. After installing both bushings, reassemble everything, making certain that neutral safety switch tang slides easily into transmission lever.

20. That's it. The first time is harder, gets much easier through repeated times . . .

Finally

Please let me know if you complete this procedure successfully; it took a while to type this, and I'd appreciate knowing when each person has completed the repair! Please e-mail me at richard_easley@baylor.edu

Note: To receive similar quality tips as described above on a daily basis, consider subscribing to the Mercedes-Benz Discussion List, which is located at the following site:

http://hsb.baylor.edu/html/easley/mercedes/welcome.html