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thanito
Veteran
Date registered: Aug 2009
Location: Greece
Vehicle(s): MB G400CDI, MB 300E-24, Mini Cooper S
Posts: 253
| Brake lights inoperative
Hi everyone!
I need some help.. Today I got stopped by a random guy on the road who told me that I had no brake lights at all. I was surprised, since the truck is notorious of showing malfunction errors for bulbs, even if there is just a spontaneous low voltage error, or misplaced bulb. In this instance though I get no errors whatsoever in the bc.
Where should I start looking? I suppose the brake light switch should pop an error if it was malfunctioning, as I am reading that a faulty brake light switch may even put the car in limp mode.
I tried to look in the fuses, but in the diagram it just says 'electronic' for the brake lights and does not mention the location of the fuse. I take it that electronic translates to a relay, but which one?
Cheers
Thanos |
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AlanMcR
Expert
Date registered: Apr 2006
Location: US, CA, Los Altos
Vehicle(s): G300DT E300DT 230SL
Posts: 3500
| RE: Brake lights inoperative
The brake light switch has two internal switches. One opens as you first press on the pedal. The ECU is looking for this to manage cruise control and other automated functions. If that fails the ECU assumes that you have your foot on the brake and will do weird things like require a second press of the accelerator before revving, and it will prevent cruise control.
The other switch closes as you continue to press, and that is what drives the brake lights. I'm guessing that the second internal switch has failed, or perhaps a fuse along that pathway. The switch is a pain to reach up to, but pretty easy to test with a multi-meter.
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fernweh
Date registered: Apr 2006
Location: Calabasas, CA - Centenario, BCS - Luebeck, Germany
Vehicle(s): Few Mercedes-Benz, a Toyota Amphibious and a Vespa
| RE: Brake lights inoperative
The lights of the G-wagen models (2002+ for US vehicles) are controlled by the SAM unit and do not have separate fuses.
The brake lights are part of that, and the SAM computer should illuminate another lamp if the brake lights lamps fail. You might have to reset/erase any faults/errors with the Star system.
I do believe, the third brake light in the rear back door window, is not part of the SAM control unit system.
Some years ago, I did trace the brake light wiring all the way to the SAM unit in the center console, and could not find any wiring problem, but one of the brake lights would just work intermittently, or not at all.
The fault was in the SAM unit, and short of replacing that SAM, I did connected that faulty brake light to the wire lead of the third brake light, and had all three working nicely again.
Edited by fernweh 8/6/2019 4:54 PM
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