Problem with Rear Right Taillight – Possible LCM Issue?

[Spunbearing]

Originally Posted by wpoll

Full disclosure - I don't know how this circuit works - this is guess work.

But, given that the IC driving the lamps doesn't have a heat sink or look to be able to dissipate much excess heat, any dimming of the lamp circuit must be via PWM (yes, same PWM as the control of the aux. fan - although used in a different way in this case). The noise you hear when it's faulty supports this theory. I'm guessing the LCM varies the mark/space ratio of a square wave going to the tail lamp to modulate the brightness. The PWM used for lamp dimming is usually in the 1-2kHz range, right smack in the middle of our hearing range. When your tail lamp assembly is faulting (short or similar to ground etc.) the IC is delivering a high current at 1-2kHz and you can hear this, via the protesting components.

I should drag my oscilloscope out into the car park and check the tail lamp circuit- this would confirm the PWM operation. But it's cold and dark outside!

This fault just HAS to be an intermittent fault path to ground in the lamp assembly somewhere...

So the chip I installed died a horrible death as you can see from the picture. I agree wpoll and I think what was happening is that when the signal was being pulsed for the tail running lights it was causing repeated arcing beneath where the connector inserts into the tail light housing which would account for what I was hearing and why I could not see any shorts. This would also explain why when I tested the light for shorts I couldn't find one. I installed one of the replacement repair Mercedes light sockets along with another chip and everything seems to be working fine. I will update thread if it breaks again but it has been a few weeks so I think problem solved!

Thank you everyone for your input!

[wpoll]

Great follow-up - thanks.

And it's a warning to us all - not attending to ATLS (arcing tail lights syndrome) can result in more than just ruined tail lights - it seems it can actually damage the LCM too.

[RFaber]

if you post it, they will respond!
brilliant find wpoll! my soldering technique is a bit rusty, (especially for Surface mounted stuff!) hopefully the op gets his x sorted out!