My daughter has a 1987 300SDL Mercedes-Benz W126, and had trouble starting the car in Park, so she was starting it in neutral. As the Neutral Safety Switch (NSS, Mercedes part number 0005454906) got more and more worn, it stopped starting in neutral as well. I had her tow the car to the shop here, and removed the NSS from the driver's side of the automatic transmission. It is held on with two 10mm head bolts, and mounts around the shift rod. You need to remove the shift lever, which is held to the shift rod with a different 10mm bolt, which is mounted forward/rearward. Once all three bolts are removed, the neutral safety switch will come right off the side of the transmission. DO NOT attempt to remove the 1" OD wire loom at the connector until you get the NSS off in your hands. There is a locking ring that turns counter-clockwise on the outside of the connector, and it's difficult to see (and easy to break) if you start pulling on the connector without being able to see what you're doing.
Once that was off, I got the part number of 0005454906 off of the side of the switch, and started calling around our hometown of Phoenix, AZ to locate one. Unfortuntely, not one parts house (even the German import specialists, BAP) didn't have one in stock, and it was already 4pm when I started calling around. To add insult to injury, the switch was going to be the better part of $100.
So, that left us with several possibilities: We considered temporarily running a jumper wire between the two neutral connections (the ones that are the closest to the locating pin, as you're looking at the connector, however, I was concerned about the car being started in gear accidentally, and causing a fender bender, even if this was ony a temporary work-around while we waited for a switch to arrive.
Another possibility was to leave the car apart up in the air until tomorrow, but that didn't seem logical either. Instead, we decided to drill out the pop rivets that hold the neutral safety switch together (inboard half and outboard half), and see if there was a contact inside we could bend or tweak to get them to touch again. And...there was!
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The two brass contacts you see on the far left in the blurry photo to the right (click to enlarge) are the two contacts that need to be touching in order for the car to start. The other two contacts (to the right in the pattern of four) are the ones for the reverse lights. Under the pie-shaped black piece of plastic you see at the top, are two small contacts that are supposed to touch when the yellow plastic "arm" rolls past the cam plate, which moves the flat yellow plastic piece into contact with the small brass contact arms, making the circuit. All I had to do was bend the contact over a little bit to get it to CLOSE the gap a bit between the two contacts, and we were once again getting continuity between the contactors when the cam was in either Park or in Neutral. Problem solved.
I greased the cam with a bit of white grease when I put it back together, then reassembled the cover plate and used new pop-rivets to secure the cover. You could use small bolts from Home Depot if you wished, however, make sure to use Red Locktite on it so that they don't rattle out.
After reassembling everything the way it came out, we were up and running...not 60 minutes later, nor $100 lighter!
Hope that helps you get your MBZ up and running if it's giving you trouble.
By Kevin Fell
KevinsOffroad.com