Brake Light Ground - PeachParts Mercedes-Benz Forum

pilot53 · #1 08-10-2019, 08:21 PM

Im troubleshooting electrical issues after some body work was done to the rear of my 86 SDL. The bulb burn out warning light on the dash stays on once I hit the brake pedal. All of the actual lights work fine. I think the brake lights are grounding through the dash light as it flickers slightly when i hit the pedal when the light it steady on. Where are the grounds for the rear lights?

SD Blue · #2 08-11-2019, 01:50 PM

The ground should be on the LH side, on the trunk well seam, near the antenna.

BWhitmore · #3 08-12-2019, 10:10 AM

Ground wires on Mercedes cars are usually brown in color.

pilot53 · #4 08-12-2019, 02:08 PM

Thanks, I located the grounds for the front and rear lights, all are clean and nice and tight. I also noticed the parking lights dont work when the headlight switch is set to Left or Right, they do work when all of the headlights are on. Any other ideas on what I can check?

Dubyagee · #5 08-12-2019, 02:51 PM

Check and see if the correct bulbs were installed if they had to be replaced.

My 87 300D had 1157s in the front turn signals and they threw the check bulb warning.

BWhitmore · #6 08-12-2019, 03:19 PM

Mercedes vehicles are especially sensitive concerning the resistance of bulbs used. It is recommended that only genuine Mercedes bulbs be used. In some models, if the stop light bulbs are not of the proper resistance the cruise control will not function. When the head light switch is set to left the left front and rear parking lights should work. Same with the right side. This is a safety item when parking on the street at night.

Diseasel300 · #7 08-12-2019, 04:13 PM

Quote:

Originally Posted by pilot53

I also noticed the parking lights dont work when the headlight switch is set to Left or Right, they do work when all of the headlights are on. Any other ideas on what I can check?

Standing lights only work when the key is off or in accessory. If the key is in "run" position the standing lights are not powered.

Diseasel300 · #8 08-12-2019, 04:15 PM

Quote:

Originally Posted by BWhitmore

Mercedes vehicles are especially sensitive concerning the resistance of bulbs used. It is recommended that only genuine Mercedes bulbs be used.

Utter nonsense. Any bulb of the correct wattage is satisfactory. MB bulbs are nothing special.

pilot53 · #9 08-12-2019, 11:58 PM

Thanks for the clarification of the parking lights, it turns out they do work as they should. I replaced all of the bulbs with the correct aftermarket ones, I have never had a problem with aftermarket bulbs before the bodywork. Im kind of stumped on what to check next, its still a problem in the brake light circuit but everything seems fine.

Diseasel300 · #10 08-13-2019, 12:01 AM

Don't overlook the possibility of the bulb warning module being the problem. They can be pretty flaky.

BWhitmore · #11 08-13-2019, 12:03 AM

Diseasel 300 thanks for your kind comments. Check this forum and you will see ample evidence about light bulbs in Mercedes cars.

pilot53 · #12 08-13-2019, 12:34 AM

Ok! I finally found the problem, turns out it was the wrong bulb! Someone had put a 10 watt bulb in the 3rd brake light socket instead of the 21 watt bulb. I replaced it with an aftermarket osram bulb and the warning light is off. Thanks for all of y'alls assistance.

Diseasel300 · #13 08-13-2019, 09:04 AM

Quote:

Originally Posted by BWhitmore

Diseasel 300 thanks for your kind comments. Check this forum and you will see ample evidence about light bulbs in Mercedes cars.

So does Mercedes make it's own bulbs out of some sort of material that nobody has access to? Some wattage that doesn't exist in the aftermarket due to some sort of special patent? Or is it more likely they put their sticker on an Osram bulb like literally every other manufacturer?

It only matters that the wattage is correct, it doesn't matter who made the bulb. Watts are determined by the resistance of the filament applied to a given voltage, there's no magic here.

See the OP's comment on reply #12, seems a generic Osram fixed his issue.

BWhitmore · #14 08-13-2019, 09:27 AM

You are entitled to your opinion. Using the term utter nonsense is offensive. I have over 50 years experience working on Mercedes vehicles and I simply try to help people with my experience, plus years of technical education. No knowledge is “utter nonsense”.

Diseasel300 · #15 08-13-2019, 10:06 AM

Quote:

Originally Posted by BWhitmore

You are entitled to your opinion. Using the term utter nonsense is offensive. I have over 50 years experience working on Mercedes vehicles and I simply try to help people with my experience, plus years of technical education. No knowledge is “utter nonsense”.

Everybody here is trying to help other people. That is the purpose of a forum. A prime example of an "opinion" is claiming that people need to pay dealership prices for a simple incandescent light bulb (made by Osram or Sylvania most likely). Claiming that opinion is "utter nonsense" is my opinion (and I'm sure shared by others reading this thread). It wasn't intended to be offensive.