My 1996 S500 Coupe has frustrated 2 expert indy Benz mechanics and I am at my wits end. My beautiful, perfect running Coupe has a battery drain, and its been in the shop over 60 days between two places. The first guy replaced the alternator and its has a new battery, both of which have been tested and tested the entire car, and replaced a harness to the ignition switch. The battery charges and holds a charge fine, its just during the day at the train parking lot or overnight it goes dead.

The last guy posits that it is a short in the main harness, removing fuses does not seem to stop the drain, and at the end of the day he just said 'I don't know.'

Thoughts on possible cause or testing/troubleshooting methodology? How huge a job would it be to just swap out the main wiring harness - jeez I am afraid of the answer...

All thoughts greatly appreciated!

 

well dosent sound like your indy are very good. first of all i hope he didnt charge you to much for that altenator that it more than likley didnt need. in all my years working at a deal i have never seen a altenator cause battery draw. how you diagnos this problem is hook a fluke meter between the negative terminal and the negative battery cable. turn the meter to amps and check how many amps are drawing it shouldnt be that much mb says that it can have up to 50 milliamps draw however i have seen some s classes with about 30- 35 milliamps draw. after you figure out how many amps you have drawing you start pulling fuses till you find the draw if you cant find a draw you start pulling control units and everthing else that is unfused power going to it.

honestly i would say tke your car to a dealer because many of the independant shops tecs dont have enough training or experience with this sorta problem. its to indepth they dont know where all the contol units are at that need to be unpluged. nor would they know the proper way to program them if they need to be replaced. nor do they have the propper computers to program them. is what mb has done is made it way to hard to indys to work on these cars. they can do all your services though.

 

Thanks, wound up finding this myself. Traced it to unit on top of the heater blower. Replaced that and problem solved. No fancy electrical trouble shooting involved, once I decided to actually look at this myself (I lack confidence in my ability to solve electrical problems). I just heard, fainter than a whisper, the fan running after I shut the car off. It was really hard to hear, but I've thought of shooting the mechanics involved...