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Discussion Starter #1
Well, I'm scratching my head on this one. Symptoms; intermittent loss of right brake light. I can go long periods where it works fine, then the 'light out ' illuminates on the cluster. It's been going on since last yr, but like I say, I can go for long periods and it's fine.

I've assumed all along it's that big module in the screw down box adjacent to the fuses. Found two red fuses in there that were still the aluminum and awful. I figured I had it knocked out after that. Nope, no joy.

So I took that large module out and brought it into the house. Re-soldered every single trace hole on both sides. Reinstalled, no joy.

Then I went over to my other TE and replaced every relay and module one by one in that screw down box. . Joy! It braked at idle without triggering the 'light out' warning. Feeling victorious, I went to the store....no joy.

Not sure now it's the module. And before anyone suggests it, the light strip is squeaky clean and everything works but the brake light on that side.

So then I got a wild hair to replace the brake switch. Pulled that off the other TE as it was replaced and OE not that many yrs ago. No joy.

I don't want to just buy that big module because given my other one had no effect substituting, I'm thinking that's a rabbit hole.....and they're probably expensive if even available.

Always the same; turn the car off, restart, no 'light out' warning. Depress the brake and the 'light out' comes on and stays on with no right rear brake. Drive around for awhile and the warning light goes out and I have a brake light.

Any ideas?

Kevin
 

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I have had this happen also. Light goes on in the cluster and then off. Interesting thing is now and then a bulb blows. I drive a 93 400E so here is an uneducated guess, the bulbs are so old they are on the verge of failing but before they do, we get a phantom lights out indication? This may have been going on now for a couple of years..............
 

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Sounds like a ground problem BUT as mentioned the bulbs are critical on these cars. Only use OSRAM (Sylvania) and/or Philips of the correct type.
 

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Discussion Starter #5
I swapped out bulbs with no effect. I thought it too might be a ground problem , but wow...where to start?

It would be a cheap effort to try better quality bulbs.....I'll report back.

Kevin
 

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Have you tried pulling back the bulb holders/fingers at the base of each bulb to ensure solid contact? Sometimes that's all it needs
 

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And make sure you have two identical bulbs. So if you have a new Osram in the trouble location, make sure you have an equivalent new bulb in the left stop light location
 

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Discussion Starter #9 (Edited)
Yes.....bent them all 'up' after I scrubbed the sockets. As I said, the whole strip is almost spotless sans corrosion. One of the strips had broken tabs and was zip-tied in place when I bought the car. So I replaced with one from the old TE. In this climate, the my old strips look like brand new.

I'm voting for a ground somewhere....just don't know where to start. It can't be the ground to the strip because every single other bulbs works as it should. Has to be related to the brake light circuit alone, I would think.

One oddity I found was that in my original TE, the brake switch had this long piggyback tower plug on it along with a singular small one. In this car it has just a simple large plug and a smaller one. I looked everywhere under there for some sort of jury riggin' or factory deletion....I found nothing. Both cars the same yr and 4Matic. That brake switch is an integral part of the 4Matic system. Seems odd to me the factory switched some wiring configurations same yr, but entirely possible I guess.

Kevin
 

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...Both cars the same yr and 4Matic. That brake switch is an integral part of the 4Matic system. Seems odd to me the factory switched some wiring configurations same yr, but entirely possible I guess.

Kevin
Well, it is a W125 errr . . . I mean 4Matic after all. Remember, they're all customer funded R&D projects. ;)
 

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Check the metal grounding buss bars in the tail light assembly. There is a cheesy mechanical connection between them that comes loose when the little plastic tabs that hold the buss bars in place warp or break. I was having the same sort of problem (intermittent loss of brake light) so I soldered a jumper between the two buss bars and problem fixed.
 

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Discussion Starter #13
Thanks flooby! I'll take a closer look at that. Yeah, it is really cheesy....I remember that when I took one apart and decided to use another from the donor car.

Kevin
 

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Check the metal grounding buss bars in the tail light assembly. There is a cheesy mechanical connection between them that comes loose when the little plastic tabs that hold the buss bars in place warp or break. I was having the same sort of problem (intermittent loss of brake light) so I soldered a jumper between the two buss bars and problem fixed.
Flooby to the rescue(again!)
This is EXACTLY what I was gonna add, Flooby's solid advice on that buss bar. I had wackadoo rear lights issues when I had just purchased my E320 back in '13. Had the bulb assy removed, resoldered, $40, and it worked like magic.I posted pics of the buss bar connections at the time.
Flooby, If ya get down to Fla, that case of Yuengling is still cold.:cool:
Good Luck, Kevin!
 

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One more suggestion, have an assistant press the brake pedal while the bulb out indicator is on while you wiggle the bulb out relay around. If the dash light goes out. You might have two bad relays or a bad contact where the relay plugs in.
 

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Discussion Starter #17
The Osram bulbs came today....no joy. I'm thinking this is going to be more exotic than I first imagined. So....here's a pic of my light strip pulled apart. You can see the brown wire carrying the ground. The other wires go to their perspective bulbs. I'm not sure what flooby means; soldering in an extra ground wire? Like from that spade ground connector back to metal bar tab for the brake light?

Light strip.jpg





Kevin
 

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Discussion Starter #18 (Edited)
*Update: Maybe some joy!!! I decided to solder that grounding spade to the bar. Just as I started I gave a little tug to that brown wire.....came right out of its connector....poor crimp. Soldered all of it and put back. Started the car, no joy. Turned off the car, restarted....joy! Did this half a dozen times and no 'light out' and the brake light works! Getting ready to use the car, so I'll report back. Had this success Sunday, but went to the store and lost the brake light again.

If it was a loose ground from that brown wire, why did it not affect the other lights on the same strip?

Keep your fingers crossed...no dude, that was the bird, play nice.....

Kevin
 

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Discussion Starter #19
Joy!!! I guess it had to be that brown ground wire. I originally checked the spade connection, but didn't pull on the wire.

In the words of the great AvE on YouTube...I lifted the skirt to check for a proper chooch, but not enough skookum.

Thanks All for the many suggestions!

Kevin
 
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