No power 3rd brake light W210 1997 E320

[gmercoleza]

Like the title of the thread says, no power to the third brake light. Where do I start tracking this down? I checked under-hood fuse and there wasn't anything for a third brake light, same with the door-jamb fuse box (that one did have some sort of brake light fuse, but it wasn't blown). The regular tail brake lights work.

I removed the fixture and tested for voltage at the terminals that the wires plug into; reading was zero.

[mbdoc]

On the fuse box on the side of the headlight switch.

[gmercoleza]

Originally Posted by M.B.DOC

On the fuse box on the side of the headlight switch.

Thanks for the reply. Are you talking about the one to the left of the headlight switch - in the doorjamb, on the side of the dash? I checked fuse #2 in there and it's good. Is that the fuse you mean?

[Bill Adkins]

Is this car equipped with a bad bulb warning system, if so is the third brake lamp bulb monitored? The 210 had tremendous issues with defective electrical attachment at the hat shelf. The contacts overheat allowing the plastic to melt and loose contact.
Bill

[gmercoleza]

Yes, it has the bulb warning system. Interestingly, the 3rd brake doesn't seem to be monitored. I guess I'm trying to determine where the contacts and wiring go beneath the hat shelf, since I have tested the exposed parts with my meter and there is no voltage there. As mentioned before, I checked fuse #2 in the cockpit fuse panel and it is OK.

[Bill Adkins]

I would be surprised if the third brake light mounting isn't the problem. The light housing slides rearward into a pair of guides that locate it on two electrical contacts. Remove the light rear cover, in this case it will be the front as you look back at it from the seat. With the cover removed a attachment clip should be located and releasing it should allow the lamp housing to be removed. The contacts should then be visible, prior to testing with your meter use it to test battery voltage, I have seen many failed voltage tests due to equipment. With that being said you will test acrossed the two contacts for system voltage with brakes applied.
Bill

[gmercoleza]

What you describe is exactly what I did - like I said, I removed the fixture and tested at the contacts, and there was no voltage there with my wife stepping on the brake pedal. My VOM is functioning properly as tested at one of the tail bulbs.

The purpose of my post was to find out why there is no voltage at the terminals.

[dhjenkins]

Do you have voltage @ the #2 fuse?

Have you traced any wiring that goes to the 3rd light and tested for voltage there?

Could simply be a defective wire, broken inside the insulation. It happens more than you think, and ususally occurs near the termination points (like the contacts). I don't suppose you have any insulation piercing probes...?

[gmercoleza]

Originally Posted by dhjenkins

Do you have voltage @ the #2 fuse?

Have you traced any wiring that goes to the 3rd light and tested for voltage there?

Could simply be a defective wire, broken inside the insulation. It happens more than you think, and ususally occurs near the termination points (like the contacts). I don't suppose you have any insulation piercing probes...?

Good ideas - I'll give these a try. It's been a while since I've had to troubleshoot an electrical issue; I need to get back into logic mode...

[Bill Adkins]

OK, there has to someone watching that still has access to the MB net. Look at the diagram and see where the light monitor function is done. No single light circuit is fused seperatly. Newer cars used the head lamp switch and older used a multifunction relay in the fuse and relay center, I just don't remember. Someone give a hand here, I don't want to move half my garage to find a book. The light wiring doesn't go through a hinge as in the trunk or door hinges so wire failure isn't as likely.
Bill

[mbdoc]

Headlight switch IS bulb-out monitor IS fuse box on side of dash IS most likely the problem.

Power comes from that unit to all 3 of the brake lamps.

[gmercoleza]

Originally Posted by M.B.DOC

Headlight switch IS bulb-out monitor IS fuse box on side of dash IS most likely the problem.

Power comes from that unit to all 3 of the brake lamps.

Sorry MB Doc, and thanks for your help, but I'm not understanding what you typed. Are you saying fuse #2in the dash fuse box is most likely the problem? I checked it and it's fine??? I'm puzzled...

[mbdoc]

I just said that the headlight switch is most likely the problem.

As long as fuses aren't blown.

[gmercoleza]

So the 3rd brake light is wired through the headlight switch?

[Bill Adkins]

Thanks Doc, being as you have the diagram you can check the light wire color and pin assignment so he can verify a good circuit to the third brake lamp. Diagnosis complete
Bill