lamp failure monitoring unit
 

I am troubleshooting some lights that aren't working in my 90 300d.
All interior lights that are connected to the dimmer (probably the dimmer switch)
and the two front parking lights. I tested the voltage to the left side and I am getting 2 volts. The lamp failure light is on.
I just removed the lamp failure monitoring unit and I see that some of the metal strips are warped melted etc.
Also noticed a wire to the connector below has a little insulation missing-I might have just done this when I wiggled the connector off with a screwdriver. (I will fix this with electrical tape.
Should I replace with a used unit or do some closer inspection of the wires below? Most of the wires down there look pretty "new" without looking too closely.
Might the new used unit just burn up as well if there is an underlying issue...or would that just blow the fuse?

If the lamp failure unit has burned strips in it, you pretty much have to replace the unit. A good used one is probably the best choice. I had to do that with one of my cars some years ago.

great, I'll look for one on eBay.

Does the lamp failure feature stay on at all times, or just when you turn lights on?

It stays on until the fault has been rectified.

The reason why I ask is because on my fairly pristine 91 300d, the warning light turns on for bulb check, but then only comes on when I turn on headlights, goes off when I turn headlights off. If it’s staying on all the time I have to wonder if something else is going on….

With mine the light stays on only when the lights are on.

Ok that’s the same as mine.

Check the connections in the rear taillights. They seem to be the most prone to oxidation and corrosion, even if not exposed to moisture.

There was a leak in the trunk lid which I mostly fixed, so it was humid back there for a while. I will have time to check all of that out soon.

strangely enough all of the lights in the back appear to work fine.

My understanding is that the contacts can be corroded and have just enough impedance to trip the sensor but still work.

My 240D doesn’t have the smarts, but the blinkers get that way sometimes. I’ll get fast blink even though both lights are illuminating; I guess one doesn’t quite take enough because of slightly dirty contacts. Moving the bulbs resolves it for another year or so.

My plan is to go through the tail lamps and turn signals and clean all of the contacts and add some dielectric grease to it to protect it for a while.

I am also going to check the wiring from the relay to the sockets before I plug the new relay in.

Or it's the bulbs in the rear window a ton of people forget about.

yes it looks like the 3rd brake light has been glued back together at some point so it might be a little harder to get into.