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I've got a 2007 Mercedes-Benz SLK (R171). My daytime running lights have died, and I'm a little puzzled as to how to go about fixing them.

The stock headlights have been replaced by aftermarket hardware, that contain LEDs for the DRLs. They worked fine for years, but last week they just died on both sides of the car. I've checked all of the car's fuses, and all of them seem to be fine.

The headlights on the car are otherwise identical in appearance to these sold on Amazon (brand: Sonar), only mine are not black.

Any and all ideas on how to get the LEDs to light up again would be greatly appreciated.

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    I'm not familiar with the exact wiring of your car, but I would look into the possibility of a failed relay or bad ground.
    – user288719
    Jul 5, 2016 at 15:08
  • And they did die at the same time, as far as you know? (As in you're pretty sure the actual bulbs aren't out?)
    – Cullub
    Jul 5, 2016 at 16:56
  • Do the high beams work? AFAIK, DRLs just run the high beams at half current. I understand yours may not, but I imagine the power still comes over the high beam circuit.
    – TMN
    Jul 5, 2016 at 19:16
  • They died together, as far as I knew. Also, I figured it out. I'll write an answer momentarily.
    – Kallja
    Jul 5, 2016 at 19:19
  • Glad you're fixing your daytime running lights. I ride a bike a lot and the DRLs make it a lot easier to spot a car coming up behind me in my mirror. Thanks!
    – dlu
    Jul 6, 2016 at 8:15

1 Answer 1

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After hours of tracing cables, removing panels and taking apart more of the car than I'd like to admit, I figured the issue out. Apparently there's a cable for control current (or something like that). The control current had gone out. The car still kept thinking the lights were on, as the LED control circuitry was apparently using the power the car was feeding it to illuminate the LEDs it wasn't illuminating. That's why there was not indication on the dash about the failed lights (the indication came on once I disconnected the power cable to the lights).

I traced what I believe to be the control cable back to close to the battery from a mysterious small black box next to the right headlight, where I found a casing for a glass fuse on the wire! To no one's surprise, the fuse had blown. Having now replaced the fuse, the lights are back to working as they should.

For the record, I do feel like an idiot. :D

In my defence, the additional fuse casing was sort of hidden behind something else in the engine compartment. Obviously it is related to the aftermarket headlights installed on the car by a previous owner.

So in summary:

  1. check all the fuses
  2. make sure you checked all the fuses
  3. verify there are no fuses you haven't checked
  4. locate the extra fuses you still haven't checked added with aftermarket components that are not in one of the car's fuse boxes and replace as necessary
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