E430 Strangest Brake light electrical problem
 

The strangest thing has happened; instrument cluster says light failure when I hit the brakes. Ok – usual brake light or contact failure, I cleaned everything up but still got the warning.

I happened to try to further trace the problem this evening as it got dark and found that with the LIGHTS OFF if I hit the brake pedal the brake lights come on and so does the FRONT driver’s side marker light and running light and the warning inside. Foot off the brake and they all go out again. With the lights ON all three brake lights are on constantly.

Any ideas where to turn first to trace this?

thanks
2002 E430 4Matic

Sounds like a single filament bulb in a double contact socket in the drivers side brake light.

Sounds more like a defective ground point to me. Look at the rear lamps carefully with the brake applied to see if one of them is slightly dimmer. Alternatively, measure the voltage from the bulb ground to the body at each side, with the lamps on.

Followed the first suggestion and checked bulbs. Looked ok so following Matt’s idea I started testing voltages. Both sides seem to be ok. For giggles I swapped the bulbs and holders from side to side and sure enough the marker light swapped sides too so it is something in the bulb or holder.

I will buy a new bulb in the morning and see what happens. Thank you so much for the great replies.

Best.

Always change brake lamps in Matched pairs..same lamp/same manufacturer/Brand.
The warning monitor module looks for resitance differentail and different makes can trigger it...even tho they both work.

thanks - replaced the pair and the problem has gone away.
Can someone explain why the front light would come as a result of brake light failure/low voltage?

If the bulb base is not properly grounded for some reason and power is applied to one of the terminals, the other terminal will get power through the bulb. The other terminal is attached to other bulbs, some of which are on the front of the car.

On a double filament lamp, a filament can short across brake circuit/ parking lamp circuit inside the bulb, resulting in cross circuit feedback.

thanks all!