PeachParts Mercedes-Benz Forum - View Single Post

jay_bob · #2 06-13-2013, 08:21 PM

I had this problem too.

It is the gear range recognition switch.

There are two sets of contacts in the switch. One is the switch and LED assembly that talks to the transmission control computer. The other is the set of contacts that controls the reverse lights and right outside mirror dip while in reverse function.

I was able to disassemble my gear selector housing and remove the range switch. Once you have the switch out you can either buy a new one ($150) or do what I did and open up the range switch housing. I used a business card to buff the contacts - very gently hold the contacts together and draw a business card out from between them. Never use sandpaper on any electrical contacts. I used a pair of fine tipped needle nosed pliers to very carefully tweak the bend on the contact leaves to make them touch better. Of course you don't want to do this too much or they will stay on all the time. While I was in there I also used a q-tip and alcohol to clean the circuit board for the transmission computer side of the switch.

Here is how to get the gear shift out of the car:
- Set the emergency brake, you are going to have to move out of park to get the shifter mechanism out.
- Turn on the key, foot on brake, shift to N, turn car back off. It won't let you remove the key but that is ok.
- Remove the console wood. Pull straight up and unplug the switches.
- On the back side of the shifter assembly you will see two electrical connectors. One is a multi pin (that is for the transmission computer) and the other is a two pin (that is for the reverse lights). Unplug the two pin connector. Turn the key back to the running position and put a jumper (a paper clip works) between the two contacts in the plug and see if your reverse lights come on. If so you know where the problem lies, time to pull the shifter. If not, then the problem is elsewhere. If you have a meter, check for continuity between the two pins on the shifter side when you are in reverse. Mine was intermittent and if I wiggled the shifter I could get continuity every once in a while.

Remove the bolts holding the shifter to the floor and lift it up. It will not come out because it is constrained by the shifter mechanism. You will see there is a joint in the mechanism secured by a pin and a flat clip. Tie a piece of string to this clip then pop it off. Use the string because if you drop the clip or it flies away, it will land on top of your transmission and you will probably have to lift the car to retrieve it. Now tie up the end of the linkage so that it does not fall away into the hole where you can't reach it. I used dental floss and tied it off to the rear view mirror.

Bring the shifter to a work bench and spread out covering. It is full of greasy lubricant. I don't remember the exact steps but you will need to remove the knob and then the top plate and then the range switch is like a cartridge that pops out. I know there are posts out here or on other forums for how to disassemble the shifter mechanism, that is how I figured mine out...

Either crack the switch cartridge casing open and fix the contacts, or install your new switch. Note that there is a white piece of plastic with a slot in it. This catches a pin on the shifter mechanism shaft. If you look at the side of the switch cartridge while rotating the white insert, you will see a white flag in a window when you are in reverse according to the switch. Put the shifter mechanism in the reverse position before inserting the switch cartridge so the pin lines up.

When you go to reinstall the shifter, join the linkage back to the shifter mechanism and reinstall the clip. Put the shifter back in N and lower it into the hole. Put the bolts back in and plug in the connectors. Turn the key on and test the shifter mechanism. Note the LEDs in the PRND4321 light up brighter when the handle is in each position. Those LEDs are driven by the range recognition contacts and transmission control logic, so that is how you know that part of the switch is correct.

Have a helper watch the reverse lights while you shift from P-R-N back and forth to make sure the reverse lights come and go as they are supposed to.