I absolutely love how rebuildable these old Merc's are.
When I got mine a year ago the backup lights didn't work. I never really bothered messing with them as it's not a requirement for safety inspection in our state, but since they mentioned it again this year when I inspected it, I figured I'd get to the bottom of it.
Looked through some threads on here about diagnosing the problem (wiring diagrams, light bulb, continuity checks, safety neutral switch), diagnosed it to the safety neutral switch, pulled it off & tested it & yep, it was bad.
The switch mechanism is mounted to the housing with rivets, so it's not exactly meant to be "fixed" if it's broken. But, at $40 a pop + shipping for a new one, out came the drill. I drilled the rivets out, popped the mechanism out, & saw the problem (metal switch wasn't making contact). Bent it back into shape (they look like the old ignition points if you're wondering), tested it w/ the volt meter, put it back together, put little nuts & bolts through the rivet holes, tightened it up, mounted it back on the car & whoala....I've now got back up lights again!
No money out of my pocket & just a few hours of TLC on my end fixed it up great.
It just goes to show how rebuildable practically every part on these cars really can be. Built like tanks, easy to rebuild, and fun to work on. Gotta love em!
Here's some threads that helped me in case you run into the same issue:
Wiring layout:
Neutral Safety Switch Troubleshooting 84 300D
- This one gave me the info I needed to find the closest wiring harness to the switch without climbing under the car. It also let me jump the switch & prove the wiring and bulbs to the back up lights worked.
- This is also a great thread for seeing how to by-pass the neutral switch if you need to start the car w/ a dead neutral switch.
Wiring Diagram:
Electrical diagram for -79 240d - Mercedes-Benz Forum
(the one on Autozone is actually a bit easier to read)
Once again, the forum helped me fix my old 240....
-Graydon