It could be corrosion - but where
 

The light out indicator light is on.

The rear right tail light is out (all other lights work fine). But the bulb is fine.

For a test, I took the panel of lights from the right side, and plugged in the left side - all lights lit properly.

I have a wire diagram. It shows the wire goes from the tail lamp unit to the External Lamp Failure Monitoring Unit (in fuse/relay box).

Where is the External Lamp Failure Unit (ELFU)?

Do the wires actually go from the tail light unit to the ELFU? Or is there a join box some where in between.

Just trying to trace the wire to test all the connectors.

Many thanks

Hi Bill, what model car is having that problem?

On 75% of MB cars with lamp monitor unit..the wires go directly to the bulbs from that unit.

300E 2.6 with Lamp Unit - the locks in
 

W124

The lamp unit (my phrase) holds all the lights for the tail light assembly - reverse, turn signal, parking lights ....

The wire, by the lights, has one of the connectors with 6 plugs.

The other end of the wire is what I'm looking for - so I may test the electrical connection.

2nd question please
When I do find the connectors - what should I cover the wires and bulbs with to prevent further corrosion?

Summary
1 - If one end of the wire is at the tail light - where is the other end?
2 - What should I coat the light bulb with for the electrical connection - grease - any grease or special grease. Should I also use the grease on the Mercedes connectors?

Thanks

Two things to try - first, re-bend the copper contact on the offending light bulb socket to provide more pressure on the bulb, plus clean it and the sheet metal areas where the bulb screws in.

If no dice, then pull out the lamp monitor module (in the fusebox on my 126) and open it up, re-solder all the connections from the PCB to the pins. All modules of this style tend to have solder joint issues on the pins. If yours is similar to the W126, it's a rectangular black box located in the main fusebox. It's roughly the size of a 3.5" hard drive. YMMV.

No grease should be necessary on the bulb (it will ooze everywhere from the heat) or on the Mercedes connectors (they are silvered and even silver oxide is VERY conductive).

I believe that on most MB of this age, the 'other end' of the wire is indeed the lamp monitor module.

Oh - which one is the Lamp Module?
 

The hunt for the other end of the wire continues.

On the w124 (1990 300E 2.6) I dismantled the fuse box, and found many goodies, none of which I wanted to break in error.

Under the fuses seems to be all the wires, but I'm guessing the the wire from the lamp (green/red) is headed to the lamp module.

Is the Lamp module in these pictures?

I took the fuse box apart and attached the pictures. Please suggest which picture has the best view of the Lamp Module. Is it one of those square things in the back? How do I remove them, so I may access the wires underneath?

I have an account with AllData and some DVD's - but they just show the logi=cal wires, not the physical (where it really is).

Thanks

Well, AllParts had the answer
 

The big black thing in picture 6 is the light module ...

I did a search on AllParts site .

Manufacturer: HELLA
Part Number: MBZ030112
Description: Bulb Check Relay
Packing Size:PIECE; Per Car Qty:0;
Bulb Check Relay

So - no need to buy a new one - but how do I remove the module to inspect the wire underneath it.

Yup, that's the sucker all right. Same as in the 126 (notice the 126 part number).


Anyway, pull that out (straight up and wiggle), then open it up. It's easy to open those style housings by inserting credit cards or similar around the edges to disengage the latches.

Once open, re-touch all the solder joints from the PCB to the pins. If you want, inspect closely while wiggling the pins, you'll probably see the cracks in the joints appear.


PS - Get those leaves out of there!

Pictures of Light Module - but what next
 

1990 300E 2.6

So I found the Light Module, and took it apart. Yes - credit catrds help take the module apart.

But there are many electric components - none that I want to mess with.

So - is there a way to fix the Light Module?

Seems the tail light has black wire with red line and white dots. This goes to the post number 3 - ok - but what do I do?

Buy a new Light Module?

How do I determine the light module is bad - At $240 - I'd like to be right first.

I could use the light module from my 1995 e420.

Here are some pictures ...

The question is -

1. How do I confirm the light module is broken.
2. Isthere a way to fix one (seems tight)

Don't pay any money for a light module. I'll send you one for the cost of postage if you want.