mlonas

2012 MB E350 A207 Cabrio Bi-Xenon Headlights 70K miles VIN WDDKK5KF5CF161680

Problem: Intermittent driver’s side headlight

Symptoms: Driver's side headlight fails unpredictably, sometimes when the car is first started but usually after 5 to 10 minutes, followed by "Adaptive Headlamp Assist Inoperative" message. The problem clears after the car sits for a while.

I am observing two failure modes:

Failure mode 1: The headlight just goes dead. Nothing happens when I go from auto to manual or pull the multifunction switch for bright lights. The stepper motor does not energize and lamp does not come on.

Failure mode 2: The lamp goes out but when switching from auto to manual it will flash momentarily and the stepper motor energizes. Same thing when the multifunction switch is pulled for bright lights. It typically progresses from this failure mode to the dead mode.

Once when the headlight was in failure mode I walked around to the back of the car and observed a really bizarre configuration of the tail/reverse/turn lights. Some lights that should be on were out and some that shouldn't be on were on. I haven't observed this again, but I honestly had forgotten about it and haven't been looking at the rear lights until today. Nothing out of the ordinary today. This is sounding like it could be a communication issue or a poor ground.

What I have done:

I swapped the driver’s side D1S headlamp with the passenger side. The problem did not move.

I inspected the wiring internal to the headlight as well as external. No damage or deterioration observed.

I replaced the driver’s side control unit with a new Hella unit and had the dealer program it. They said that the system ops-checked good and that the only fault code in memory was from my installation of the un-programmed unit. I’m puzzled that there were no other codes.

Any thoughts? I have an appointment to have the dealer take another look next week.
Thanks much.
Edit: I have noticed that when in failure mode if the headlight switch in in the Auto position the headlight does not flash when pulling on the multifunction switch (hi-beam). When it is in the “On” mode, the headlight flashes when calling for hi-beam.

CaliBenzDriver

Good job troubleshooting this issue 👏

Have you traced your headlight circuit to the GND point?
Usually located on the chassis beam behind the plastic wheelwell cover, you'll find couple problematic GND studs with 10mn.

Left/Right use two separate circuits with the same issue. I think the driver-side carries the 100Amp fan GND current....

mlonas

Painted, as in during production?
I haven't checked it because I did not know where it was located. I checked two 10mm ground points on the driver side of the engine compartment and they were clean and shiny. Gonna pull the plastic off now and see what I can find.

Thanks much!

CaliBenzDriver

Double-check if this issue is applicable to your model.


yes, indeed painted from factory!

> Poor GND paths:
There a fat lip of paint on the under side of the nut and the stack of eye-connectors is oxidized.



crowded connection with over 10 wires

> Shared posts:
Above pic shows heavy gauge power wires mixed with tinny reference wires.
Shared power & reference connections introduce noise into digital signals. Serious no-no!


> Delayed troubles:
Furthermore, L/R sides smart headlights work in tandem through a CAN connection.

Master/slave modules can bug each other crazy. This may explain how you noted a failure pattern after 10mn regardless of a new controller coded by dealer.

The headlights CAN signaling may be getting glitched by poor GND reference skewed under load.

mlonas

Mine had minor oxidation so I took a fine wire brush and burnished it until everything is clean and shiny. Crossed my fingers and put it back together and reseated the lamp for good measure... no joy. The symptom is the same. I still have the plastic liner off. Anything else I should check while I'm in there. Is there another ground point associated with the left circuit?

CaliBenzDriver

I have edited my post #4 with more info.

The module connector inside sometimes develops oxidation when enough rain moisture enters into the headlights. Also check the extermal connector with leaking wiring seals.

In the end it's one stupid detail that makes finding this time consuming.

mlonas

I added this to my Post #1: I have noticed that when in failure mode, if the headlight switch in in the Auto position the headlight does not flash when pulling on the multifunction switch (hi-beam). When it is in the “On” mode, the headlight flashes when calling for hi-beam.

CaliBenzDriver

If you can't find an electrical fault you'll need a Xentry/STAR to hopefully figure what's wrong with your CAN signaling.
As far as I know, aftermarket scanners have limited or no coverage for headlights data.