Early Mercedes automatics from 1968 up to 1981 (including diesel automatics) have an external adjustment for the reverse band. It is not uncommon for this band to wear which can lead to intermittent reverse transmission slip, especially when you are going up a steep incline backwards. Numerous times I have also seen the lock nut for the adjusting bolt loosen up. This allows the bolt to back out resulting in the complete failure of the transmission to engage in reverse. I know first hand that more than ten transmissions were replaced due to this condition. I can estimate that hundreds have been replaced over the years at the cost of thousands of dollars to unknowing customers. Kind of hard to swallow when you figure many of them only needed to have the adjustment bolt turned in and the lock nut retightened!
Problem & Solution
Common among these chassis:
The adjustment bolt (stud) is located on the forward right (passenger) side of the transmission (see center of picture). It is above and slightly forward of the kick-down solenoid. it is either squared off at the end or has a slot in it for a screwdriver. It may be hard to locate if the transmission has caked on grease and grime packed into that area. Before you inspect it I strongly recommend you take your car to a car wash and spray clean the area with degreaser and high pressure spray.
The adjusting stud goes straight into the side of the transmission housing with a large locking nut that secures it from rotating. First check to see if the big lock nut is loose. That is a sure sign the adjustment bolt has backed out. Even if you find the nut is tight it is still a good idea to adjust the bolt. Loosen the locking nut first by turning it counter clockwise. While holding the nut from spinning turn the stud in (clockwise) until it stops without excessive pressure. Then back the adjusting stud out 3/4 of a turn. Hold the adjusting stud in position and tighten the locking nut securely. I highly recommend you use thread locking compound on these threads!
NOTE: If you are having a hard time turning the stud use a grinding tool or cutter to cut a deeper slot in the end of the stud. This will allow you to turn it or hold it with a straight blade screw driver.
If your transmission goes into reverse and does not slip - you have solved the problem - and maybe saved big bucks. If your transmission will still not go into reverse then you have a possible problem with your valve body or broken parts inside the transmission. If you decide to take it to a transmission shop make certain they have had experience with Mercedes automatics. Not all transmission repair shops are created equal! As an alternative you can begin looking for a rebuilt or good used transmission.
If you have changed your fluid and filter, and adjustment does not help then you will need to replace your transmission. Kent has written a manual on this procedure. If you are having other shifting problems in your forward gears then refer to Kent's manual on Automatic Transmission Tuning and Adjustment.