Battery drains while sitting - PeachParts Mercedes-Benz Forum

retx · #1 01-23-2007, 08:49 AM

What could cause this?
Its been fine since i bought it, but one night after work, it spun over slowly and wouldnt crank. Tried again, and the battery just died. Got a jump and it fired right up, and drove 25 miles home with headlights, heat, and wipers with no problem. Cranked up in the driveway after i got home too. This yesterday morning, it wouldnt crank. Just clicking from the starter. So, i took the battery to Autozone, where they did a deep cycle charge and confirmed the battery was fine. Put it back in the car and it had a charge of 12.54 on the meter. While running, charging system was kicking it up to 13.65+/-. Checked it a little while later with engine off and the battery was down to 12.26 then 12.19 and this morning, it was down to 11.90.
So, whats the first things i should look at, if there is such a common occurence? All accessories are off, by the way. The only thing i have done to it lately has been to remove the aux water pump(siezed) and by pass the leaking, broken, non-functioning acc servo.
thanx guys.

nhdoc · #2 01-23-2007, 08:59 AM

First check the obvious things like make sure your interior and exterior lights are all off. I remember a neighbor cursing his benz for the same reason and finding out he left the parking side light on by accident, but rarely being a night driver he never realized it was killing his battery.

Then, if they were all off I'd hook up an ammeter between the negative battery cable and the battery with the key out and doors all shut and see what the draw is with the car parked and sitting.

If it is anything more than a few milliamps then you probably have something running all the time and it sounds like you do and given the rate of discharge you describe it is probably a few amps. You can then isolate it by pulling fuses out one by one until the draw drops off. Then you'll at least know which circuit to start looking at for the defective part. Then you may need a wiring diagram to find all of the components on that circuit but at least you can pull the fuse out and continue to use the car until you fix the problem, assuming it isn't an essential circuit.

I have also heard a failing voltage regulator in the alternator can drain the battery. That would not stop by pulling fuses but rather would require disconnecting the alternator to see if that stops the drain.

Good luck.

retx · #3 01-23-2007, 10:09 AM

Quote:

Originally Posted by nhdoc

I have also heard a failing voltage regulator in the alternator can drain the battery. That would not stop by pulling fuses but rather would require disconnecting the alternator to see if that stops the drain.

Good luck.

thanx. like i said....all accesories are off. i triple checked it last night...knowing that it was dying again. i will try to isolate which circuit could be staying on today.
if its a failing voltage regulator...would it still be charging while running though, because i am getting average charging readings.

retx · #4 01-23-2007, 11:00 AM

I started with the fuses, pulling them one at a time and reading for any difference on the meter. Started with a reading of 300.3 milliamps. WOAH. nothing made a difference of more than 1 milliamps. ended with all fuses out. and it dropped to 288. then disconnected the altenator. no change. then i removed the acc servo harness, and bingo....only 1-2 milliamps reading now. wow, should that thing even pull that much? and not protected by any fuses. i removed it last week to bypass it so i could have full heat, and noticed it had a cracked body as well. i kinda knew this since i had a small leak over there, and when flushing the system, it was more pronounced. also found that my aux water pump was siezed. i reconnected the vaccum lines and power harness to the servo and placed it back, with no water lines running through it though. would it be safe just to remove the servo completely? and leave the vacuum lines/block and power harness laying over there?

nhdoc · #5 01-23-2007, 01:42 PM

I'd be very surprised if the circuit isn't fused...there are auxilliary fuses on the 4 corners of the fusebox and sometimes even outside of the box that handle these circuits. You really should find a wiring diagram and trace it out though.

On the bright side you used good solid diagnostic techniques to locate the problem and at least for now have made it so the car is driveable...that's good...now just figure out what you need to do to remedy it and you're set.

vwbuge · #6 01-23-2007, 01:54 PM

I had the same thing happen recently on my Subaru. Make sure your battery is fully charged. Before intalling take out all fuses. Hook up your multimeter to the installed battery and take a reading. It should be steady at a little over 12v. Install the fuses one by one carefully checking the voltage after installing each fuse. I had a draw on my power door lock fuse.

retx · #7 01-23-2007, 05:52 PM

Quote:

Originally Posted by nhdoc

I'd be very surprised if the circuit isn't fused...there are auxilliary fuses on the 4 corners of the fusebox and sometimes even outside of the box that handle these circuits. You really should find a wiring diagram and trace it out though.

just before i took the servo harness off, i had every last fuse out, and was still getting 300 mA. and then i plugged every last fuse in one at a time and checked the readings. unless it is fused somewhere else...other than the fusebox, i dont think its fused.

Quote:

Originally Posted by nhdoc

On the bright side you used good solid diagnostic techniques to locate the problem and at least for now have made it so the car is driveable...that's good...now just figure out what you need to do to remedy it and you're set.

and yes i already know what i need to do to remedy it. but right now i dont have 600+ for a new servo. im just hoping its not going to affect anything else (ccu controls, ie blower). then i can live with it for now.