C280 battery drain - Electrical problems on 95(N) C280

[Andy Langman]

Having recently bought a 95(N) C280 from a Mercedes dealer, I was very excited with my first Mercedes, it drives like a dream..........Then the problems started........Only three weeks later I have found four different electrical problems, one still persists.  Can anybody help restore my faith in "the best engineered car in the World"?  
  My problems are:
1)  CD changer not recognised by radio.  Now kind of fixed but sometimes a power off and on recycle is required to make it work.
2)  Alarm siren not working.  Now fixed by new siren.
3)  Heated rear window going to standby after 5 secs.  This was fixed with a new relay.
4)  The remaining problem.  The battery has a permanent drain of 0.2amps, following two flat batteries I found the majority of this drain is going through the no.2 fuse in the boot, the "convenience feature control unit".  This was investigated by the dealer, a Mercedes franchise but no fault was found.  Next day, next flat battery the problem was indeed still there.
  Does this combination of faults or just this final one ring any bells with anyone, if so is there any advice?  
 The battery charges at around 4 amps when the engine is running, is this about right for a run down battery?

(Edited by Andy Langman at 2:21 pm on April 21, 2001)

 

[thomas]

C280 battery drain

Andy - It was nice of you to leave a message praising my electrical talents but I find Merc wiring so involved that I can only suggest the sort of things that I am sure you have tried.  Your 0.2 amps is 6 watts so you are may well be looking for something like the boot light which is 10 watts but I feel you have tried that sort of thing.  I would take the bulb out and see if it cures it.  Looking at the alternator I would expect it to charge at about ten amps on a flat battery, even more.  But I don't know about this on your particular car.  As I think you have tried most things I would plump for a faulty alternator but I don't want you to go rushing out spending your hard earned on my intuitive approach.  Can you check it for voltage and output?  Is there any current feeding back into it?  I saw another question that asked why do you have to disconnect the battery when charging - it is because there are diodes that keep the current going one way (out)  in the alternator and it is possible to wreck those diodes if you put a heavy charge on the battery with the alternator connected.  A small charge won't hurt them but modern batteries often won't charge from flat with a trickle charger.  If you do for the diodes, as far as I understand it, current can, when the vehicle is switched off, go on leaching out into the alternator.
Please don't think of me as an expert on electrics for I am a trifle out of date but I do hope you find the fault and that this is of some meagre help.

Don

 

[Andy Langman]

C280 battery drain

Thank you Dorian/Don.  It is good to find some advice.  I traced the 0.2 amps to the convenience feature fuse, which after 3 visits to my dealer to sort this problem out, they finally discovered that this does happen for 2 mins from switching the circuit on, ie from disconnecting fuse and connecting ammeter.  Despite being told I had a new battery when purchasing the car, and therefore not suspecting it, it turns out it was not new and was the problem.  So my flat battery only charged at 4 amps, well below that expected as you said.  Hopefully all works now, but thanks again.
 Must say I am not impressed with my dealer.

 

[Andy Langman]

C280 battery drain

Even less impressed, the above was wrong and the fault I pointed out to my dealer 3 visits ago caused all my problems.  Finally fixed with the help of Merc UK.  Thanks.