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Hi, I recently encountered the problem that if my car stands for more than 3 days the battery is dead
It is a pre-facelift W221, but interestingly it doesn't have the 2nd battery in the trunk but has the small Li-Ion battery under the dashboard.
I replaced the main battery and the aux battery 1 year ago and also tested the main battery with a battery tester and it checked out OK
OK, so now to the standard troubleshooting for things like these:
Put an amp-meter between car and battery, lock the car and put the key far away (so the proximity sensor doesn't trigger).
Now pull fuses one by one and see if the amps drop.
No luck here, but then I discovered the following:
Normally around 0.3 Amp are flowing, but every half a minute you hear a relais firing and the amperage shoots up to 3.2 Amp, this last for maybe 15 seconds and then drops back down to 0.3 Amp, rinse and repeat
OK, now I pulled fuses AND relais and found the following:
(see picture): If I pull either the marked relais or the marked fuse the amps drawn drop from 3.2 A to 2.2 A
That's as a far as I got
I checked and found that both the fuse (#20) and relais (C) belong to "engine electronics"
12-09-2018, 10:56 AM
