Trunk light mysterious stopped working

[j-galap]

Hi folks,

The trunk light on my 1974 240D W115 recently stopped working. The bulb itself is definitely fine and I can't see anything obvious that's caused this. Anyone have any ideas? Is there a specific fuse for it? I checked the fuse diagram and there doesn't seem to be one for it.

One thing to note: my reverse lights have never worked. Related?

Thanks!

[gmog220d]

Check the trunk light switch. It's part of the bulb holder, to the left of the latch piece. The switch grounds the bulb circuit when the trunk lid is open. Check for 12V at the red wire, and check for continuity to ground at the brown wire.

As for the the backup lights, in a manual shift car the switch is part of the shifter assembly inside the passenger compartment. I'd guess that for your automatic transmission equipped car the switch is still in the shifter, so you might start there.

[rumb]

On later cars the wire would break near the trunk hinge.

[Idle]

Originally Posted by gmog220d

Check the trunk light switch. It's part of the bulb holder, to the left of the latch piece. The switch grounds the bulb circuit when the trunk lid is open. Check for 12V at the red wire, and check for continuity to ground at the brown wire.

As for the the backup lights, in a manual shift car the switch is part of the shifter assembly inside the passenger compartment. I'd guess that for your automatic transmission equipped car the switch is still in the shifter, so you might start there.

If you have a manual shift car pull back the shift boot and look for a small round switch with wires coming out of one end and a button coming out of the other. This screws in and out of the frame work of the shifter. When you move the lever to reverse the shift lever pushes against the switch button. Yeah, it's just that simple.

Shop for a reverse light switch and you will usually find a picture of what it looks like. There things are usually less than $10. And you can test the switch by pulling the wires off, touching them together, and seeing if your back up lights come on.

[j-galap]

Thanks folks!

It's an automatic transmission model.

I've inspected the wiring and unit on the trunk side and all looks good. Definitely seems like the problem is at the shifter end!

[ejboyd5]

Some Mercedes models require both the ignition and headlights to be turned on before the backup lamps will illuminate. Check for this feature before diving too far into other diagnostic efforts.

[Sugar Bear]

Check for loose starter bolts... wouldn't that be great!!!

[280EZRider]

Originally Posted by j-galap

Thanks folks!

It's an automatic transmission model.

I've inspected the wiring and unit on the trunk side and all looks good. Definitely seems like the problem is at the shifter end!

A very common problem with MB reverse lights not working is found at the neutral safety switch mounted on the left side of the transmission - automatic. It has an adjustment tab that can be moved to make contact for current going to the reverse lights as well as the starter motor while in park / neutral. While the manual tells us to use a small allen wrench to align a tab slot with a hole on the switch, it's much easier - and without all the trial and error - to simply move the tab until the reverse lights come on. At this point, the starter will most likely engage in both park and neutral positions. Note that the NSS has nothing to do with the trunk light.