Some time ago my W211 started to complain about disabled comfort functions after a few days of not being driven. This is a precautionary measure to reduce drain on a battery that has a low voltage, and includes disabling things like heated seats and rear window until the voltage has increased above a certain threshold. But when this happens after just a few days of standing still, something is wrong. Low voltage can be caused by four things:
- A bad battery
- A bad alternator
- An improperly working battery control module
- Current leaks, e.g. due to faulty electronics
Since the voltage was fine when the engine was running and the message went away after some time, I could practically rule out the alternator and the BCM. As the measured voltage was quite low (around 12.2V) even shortly after fully charging the battery, the battery itself was the most likely culprit. It had been replaced only two years ago (see production date of week 20 2021), but the guys on the Dutch Mercedes forum were quick to point out that the OEM Mercedes batteries aren’t that great.
I replaced it with a new Varta Silverline AGM battery, and chose the 90 Ah version (as recommended by Varta) to have a bit more capacity.
With this replaced, the next issue cropped up: on a cold start I would sometimes get a red display with the urgent message to visit the workshop because of battery issues. Using my iCarsoft diagnostics computer I could see that the cause was the auxiliary battery for the SBC pump. This is located below the right-hand air filter. The measured voltage was below 11V, so clearly another dead battery (production date was 2017). After replacing this battery as well (Varta again), everything is fine again, and the car will happily start without error messages even after not being used for two weeks.
After replacing the main battery, you’ll have to calibrate the windows by closing them fully, then pulling the buttons up for a couple of seconds. Early W211s may also require calibration of the steering angle sensor (by turning the wheel fully to the left and right), but this was not the case for my MY 2005 car.
