Fluctuating battery drain

Faisalgondal

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Jun 8, 2014
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Mercedes e250 A207 Convertible
Hi

I've been having battery drain issues with my w204 c220 CDI (2007). It's been going on for 7 months. I had a dead battery one morning (July 18) not thinking much about any electrical fault I just replaced it as it was 11 years old.
It died on me again in November 18, which was strange for a brand new battery. I took the car in to a so called auto electrician who said that there was no drain on my battery but the cells in the battery were dead so I needed to get a new one, so I ended up buying another new one.

This new battery only lasted 2 months, so I took it back to electrician who said that there was now a drain but he couldn't find the problem in 2 weeks he had my car!

So I brought it home, did a battery drain test myself with my multimeter.

Checked battery voltage, showed up as good at 12.5v. Started the car, the volts went up to 14.2v (so assuming alternator is fine)

Stopped the car, removed negative terminal and placed one end of multimeter on 10amp setting on the negative terminal of battery and the other on the negative cable that I removed. Waited for amps to settle, reading was 0.25 (250 milliamps) drain. At this point no light was on, the doors were shut and the hood was in lock position.

I then removed one fuse and relay at time to see if I get a lower reading with no luck. The amperage kept fluctuating between 100ma to 250ma.

Next I removed the seat modules one by one from driver side and passenger side, again no luck.

I then removed each bulb (head unit, glove compartment, boot), still the same result.

After a while, the reading went down to 0.02 (20miliamps) but still kept fluctuating.

What I noticed is that the once the car reaches the desired 0.01 to 0.03 (10 to 30miliamps) it stays there for a minute or so until I hear a click in the rear SAM module and then the reading jumps up to between 0.09 to 0.2) a minute later it drops back down to 0.01/0.02. I tested it like this for half an hour and the same thing kept happening. Every minute or so, the reading would jump up and then back down again.

Any help will be appreciated as my car has been off road now for a month.

Thanks
Faisal
 

AMGeed

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Have you tried pulling the fuse for the Comand unit?
That is one well known cause of battery drain.
You should be looking for a maximum of 65ma drain with everything "asleep"
 
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Faisalgondal

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Have you tried pulling the fuse for the Comand unit?
That is one well known cause of battery drain.
You should be looking for a maximum of 65ma drain with everything "asleep"
Hi

Thanks for your response. Yes, the command fuse had been removed, but no luck there either. Even with all fuses in or out the car does settle down on 0.01 to 0.03 reading but this only lasts a minute or so. The click in SAM module is what spikes it. At this point, there is no movement in car, it's random.
 

AMGeed

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You may have to check further into the front and rear sams and the door and seat control modules.

Battery drains on this brand seem notoriously difficult to pinpoint at times. I've been through it on my car.
 
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Faisalgondal

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Notoriously difficult indeed. Not sure what else to check to be honest.

Just tried another drain test, same result. Timed it this time. It took 2mins to go to 30ma, then sudden jump to 120ma, fluctuated between 90 and 120ma for 40 seconds, then back down to 20ma, then 10ma for 3 mins, spiked up again to 120ma, slowly started dropping again for 1 minute, then settled at 20ma for a further 2 mins, before another click in SAM module took it to 150ma for 1min 30secs, dropped to 30ma.
 
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Faisalgondal

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Update:
I've managed to track down a faulty relay (circuit 50 Starter relay) 5 PIN green relay. Tested using a relay tester and this fails.
Could this be a cause of my battery drain? Will be replacing this relay. Found one on eBay but second hand and will take days to arrive. Was wondering if anyone knows of a replacement relay for this from Halfords or similar I can pick up tomorrow. Can't find how many amps this relay has.
 
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Faisalgondal

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2007—2014 Mercedes-Benz W204 (C-Class) fuse diagram » Fuse Diagram
Could be ???? Relay M A0025422319 is a general purpose relay made by Siemens its green 5pin
you may be able to get the part no from the original relay casing to check see photos here
104-Mercedes-Benz E M S R Class 5-Pin Green Relay 0025422319 V23134-B52-X309 | eBay

you must get the correct part no
Thanks Grober. That's the exact one I found on eBay. I will get that and wait a few days for delivery. I just thought maybe if I get the same amperage from an aftermarket relay that I could pick up today.
 

grober

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I believe this is possibly your relay but as you will see from the data sheet comes in several different formats so difficult to figure which configuration you have . Noticeable is an internal diode across some of the contacts- if this has gone faulty that might explain erratic behaviour? Have you tried pricing a new one from your MB DEALER?
http://www.farnell.com/datasheets/13801.pdf
 
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Faisalgondal

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Update:
I had the relay delivered yesterday and I installed it.
The rate of battery drain I had before has gone away, however I left the drain test running for 20mins after the car had been off for half an hour. The first 7-8 mins after stabilising the amperage remained at 10-20miliamps which is better than I've had before. Then it jumped up to 90 milliamps but only for 5seconds or so, which again is better than its been before as it was staying there for longer, it then dropped again to below 30miliamps and was fine for another 5 mins or so, then again jumped to 80miliamps but again for only a few seconds.

My battery will probably stay active for longer with short hikes like this but is this behaviour normal for a battery drain test?
 
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Faisalgondal

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Looks like that relay solved the problem. Thanks everyone for your help.
 

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