Tail lamps on my 300D 2.5 are acting up, and I can't figure it out. On the driver's side, all lamps are lit normally except one of the bulbs is out. On the passenger side, all bulbs light, but dimly. When I depress the brake, all the passenger side lights go out. There is a bulb out light on the dash, of course.

Bulbs are new. Fuses are new. Checked the ground from the passenger side tail and cleaned the contacts. Bulb holders are not original, but I know I was at least getting the bulb out on the driver's side with the old ones.

Any ideas where to start looking? I guess it must be a problem with the wiring before the bulb holders - but that doesn't narrow things down much. Maybe the pin connectors, from the car to the bulb holder? I don't know - I'm clueless on electrical stuff. I do have a multimeter though - is there a systematic way to track this down?

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I had a problem similar to this.

Take the rear tail light assembly out from the inside of the car. The lights are in a metal holder. Take the entire metal holder off of the assembly.

You'll see white calcium type stuff where the bulbs go in. Use something to scrape that stuff off. It prevents a good contact. There's also a bar that goes from an upper light to a lower light. That bar gets the calcium stuff on it too. Scrape that off.

When putting the bulbs back in, you might want to use just a bit of dielectric grease on the bulb to make sure it makes good contact. It's possible the metal tab at the very bottom where a bulb makes contact in is too low now over the years. Use something to bend that upward to make sure the bottom of the light bulb is making solid contact with that.

You also might want to replace all of your fuses. The original aluminum ones might look fine (though pitted which increases resistance I think), but the new brass ones are supposed to be better, causing lower resistance or something.

I had a problem similar to this.

Take the rear tail light assembly out from the inside of the car. The lights are in a metal holder. Take the entire metal holder off of the assembly. It's held in with sort of some plastic clips. Push them down or in, to get the metal part out. It comes out pretty easy.

You'll see white calcium type stuff where the bulbs go in. Use something to scrape that stuff off. It prevents a good contact. There's also a bar that goes from an upper light to a lower light. That bar gets the calcium stuff on it too. Scrape that off.

When putting the bulbs back in, you might want to use just a bit of dielectric grease on the bulb to make sure it makes good contact. It's possible the metal tab at the very bottom where a bulb makes contact in is too low now over the years. Use something to bend that upward to make sure the bottom of the light bulb is making solid contact with that.

You also might want to replace all of your fuses. The original aluminum ones might look fine (though pitted which increases resistance I think), but the new brass ones are supposed to be better, causing lower resistance or something.

CRC electronic parts cleaner and a bunch of Q-tips are recommended for the buildup of acid. I keep a spray bottle handy and spray it into every electronic connection in the engine bay as I go, to keep all contacts clean

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Thanks for the tips, guys. I did use some dielectric grease on the bulbs when I changed them all out a few months back, but I guess I'll take a closer look at the bulb holders for any build up or anything - I didn't realize they came apart.

I've thought about just biting the bullet and buying brand new bulb holders, but my concern is that the problem is somewhere else, and new bulb holders won't help...

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Dont just replace the rear light assembly .Do use some silicone grease on the seal thats on the rear of the light .To stop ingress of water in to the boot area ..

If you are having problems with ALL of your lights in an assembly, it isn' the bulb holder or anything else in the assembly. You have a bad ground. The dim lighting and the lights going out instead of continuing to function are two big indicators. Something simply isn't making the trip.

Haul out ye olde service manual and find the ground points for each light assembly. Go find those ground points, take the nut off, and apply dielectric grease to each ring terminal, then reattach the nut securely. If you have any corrosion on anything, clean it off with sandpaper or steel wool.

Another "gotcha" I've been bitten by several times is loose terminals in the plug. The pins are pretty crappy in a lot of these plugs and can be just loose enough to "work" but not. Any time I take off a plug in my car, I use a really small screwdriver and pry the female pins inwards towards the center just a bit. Every plug gets a thin smear of dielectric grease before it goes back together. Been doing this for close to 20 years with the SL, and every joint I've redone hasn't ever done me wrong again. The SDL has been the same way. Gotten rid of a LOT of electrical infidelity just by tightening plugs and applying dielectric grease.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Ugh - this doesn't make sense to me. On second look (and with the help of my son to depress the brake pedal), the driver side lamps are working perfectly. On the passenger side, three of the bulbs are dimly lit - including the brake bulb (!) - and all three go out when the brake pedal is depressed.

The passenger assembly has a connection to ground right next to it - I've removed the bolt, scrubbed the connectors, and applied dielectric grease a couple of times now - no change. I'va also pulled apart the pin connector housing, made cure the female ends are fully seated, cleaned and scrubbed everything I could find - nothing. The back sides of the female connectors look pretty crappy, but the side that mates to the male connectors looks fine.

I even swapped a couple of the pins around (the colors didn't exactly match their respective connections) in the remote chance that someone had been in the connector and screwed things up, but I couldn't get anything working any better than when I started.

I haven't touched the ground on the driver side, it looks a little harder to get to - but would a bad ground on the driver side leave the driver side good and mess with the passenger side?

I feel like this is something simple. It really seems like a bad ground, but I don't know how many more times I can clean that thing.

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Success!

Turns out it was a bad ground after all, of course. But not the ground from the lamp assembly to the body - the grounding (vertical) strip in the bulb holder wasn't making contact to the lower bulb (horizontal) strip. I knew it was going to end up being something remarkably simple.

Anyway, it turns out that a new bulb holder would've fixed the problem - but at much greater expense, and without me learning what the problem really was.

Thanks everyone for your helpful input.

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A bad common ground connection will cause applied voltage to seek an alternative form of ground elsewhere through alternative paths. Since you have a common ground point between several bulbs, if the main ground opens up you can have several bulbs light, but at reduced brilliance. Some behavior can get really crazy depending on what's open and where.

Glad you found it! Tracking down something like that can get a bit maddening. Even worse when it's something flexible such as in a door and you get an intermittent fault!

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I had the same problem and ended up removing the vertical metal strip and soldering a ground wire to the horizontal strips as a replacement. You have to work carefully, first removing the strips so you won't damage the plastic with the soldering iron or gun.

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

I demand FULL CREDIT!!

Hahahahaaa.

Glad I was able to help.