Mercedes C230 1997 160K: New Battery drains 1.3A while sitting

Chaud

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#1
Car keeps going if driven daily however after 3 days of sitting it would not start or even have any inside or dashboard lights come on when ignition is turned on. Battery was so much discharged that it was 4.2V when measured.Thinking the battery may be bad I returned it and got a new one. Before installing the new Battery I measured the current drain when this battery is connected normally. I was horrified to see it is drawing 1.3A. I know there will be some drain but not that much. Looked all the fuses they are good. Disconnected wires from Alternator still no difference. There is a short somewhere but I don't know where to look. I will appreciate your help for any guide, tip or circuit diagram. Thanks lot... Chaud
 

nickb2

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#2
Pull one fuse at a time while monitoring amp draw. Once you see it go down, fuse in hand, you now which circuits to focus on which are powered by that fuse. Make sure the glove box light is out when the box is closed and ashtray lights and such.

KAM (keep alive memory) devices like a clock, radio or other devices that are live even with ignition off should pull no more than 30milliamps. Anything over 30ma is suspect.

1.3a is huge. Could be a trunk light, a gps, several things like that. Also, wait a few minutes and see if amp draw goes down, sometimes a module may be alive for up to 20 minutes, but never with a 1.3amp draw though.

A small light bulb can pull 500milliamps, others up to 1 amp. Depends on what number your light is. On some of these mercedes, you have a pull down ski bag or something you may pull down with trunk closed,check to see if light is on even with trunk closed, open the glove box and touch the bulb, is it hot or can you see light seeping through in a dark environment.

Look for simple things first. ;)
 

nickb2

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#3
Forgot to mention, some of these high end cars, if not most of them, have a amplifier in trunk or hidden somewhere, pull that fuse also, it may be in stand by mode. Or if you have an aftermarket one, pull that fuse also.

Any cell chargers or gizmos plugged in need to be removed from the 12v receptacle. Jam the door switches when checking amp draw, because just turning off the dome light switch is not enough. The door switch may activate other circuits as well. So if checking amp draw and you need to have the door or doors open, do that.

Once you have pulled fuses (one at a time, and put them back in proper place) and found which fuse reduces amp draw, post back which fuse # or fuse description, I will provide wiring schematics proper to those circuits. You know I am good for it. :D
 

Chaud

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#5
Thanks nickb2. I appreciate it very well. Tomorrow I will get started as you suggested and update.
 

Chaud

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Results:
I started the measurements in the trunk fuse box first. Initially current spiked to 2.25A and then settled down to 0.75A. Took all the fuses one by one and current did not drop from 0.75A. I notice the trunk light was on so I depressed the switch and light went off as well as the current reading dropped to 0.03A. Big improvement.

2. I closed the trunk and started the current measurements in the engine compartment fuse box. Before pulling any fuse the current draw was still 0.03A which is good and nothing changed. Only one fuse as indicated by red arrow in the picture (light Blue 15A) when pulled current drops to 0.01A or 10ma. If I am not wrong this fuse supplies current to radio and radio does need some active power to keep the memory and functions remembered.In other words this current draw is normal. Please confirm if this is indeed Radio fuse.
Conclusion: The actual current drawn by vehicle when sitting is 0.03A (30ma) and noy 1.3A as I stated in my post. This happened because I did not let the current stabilize. Now if all is normal then why my battery died? Was it a bad battery or I am going to see the same outcome with the new battery I am putting in? Please comment if you see something wrong with my assessment or any new idea you may have to do additional testing. Thanks again and appreciate your help very much. Chaud
 

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#8
Easy test for bad battery...(assuming your alt is working fine)...after driving your car far enough to ensure that the battery is fully charged, disconnect battery cable. Wait 3 days. Reconnect battery cable. If battery is dead, you have a bad battery. The car can't drain the battery if a cable is disconnected.
 

Chaud

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#9
Dan, great suggestion but it is kind of late to run the test you suggested because I have returned the battery and got the new one. But will keep in mind for future use.
Thanks- Chaud
 

nickb2

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#10
Well the good news is that there no amp draw. Bad news is you can't cold crank the battery cuz you returned it.

Some parasitic draws can be intermittent. Those are the worst to track. Just make sure you have no bullsh$t plugged into the 12v receptacles, have your meter handy is you suspect something. If your brave and paranoid enough, take out all lamps that you don't need, in example, trunk light, glove box light, ashtray light etc.

On these models, when you present the key in door, it activates many things. I could post a wiring of this, but useless at this time since you state you have 3miliamps which is normal. 1.3 was way to big. No battery can hold more than 8hrs at that draw.