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Old 04-20-2009, 08:50 PM   #1
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Join Date: Mar 2009
Location: Milwaukee, WI, USA
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My Ride: 2000 BMW 328i
Radia/amp draining battery when car is asleep - please help

I am a new owner of a 2000 model 328i. This is my first BMW ever, and I simply love it. I have a little problem with the vehicle and I am trying to troubleshoot; I would love your help with that.

Whenever I turn on the radio or the cassette player I hear a momentary noise as if a capacitor is charging/discharging or something. After the noise the radio plays just fine until I turn off the radio. I hear the same momentary noise when the radio turned off. The noise is heard in all the speakers, so I suspected something wrong with the radio/amplifier system.

On top of that when the car is asleep (i.e. after 15 min of turning off everything and locking the car) there should be only 20mA current draw from the battery. But I measured that it takes almost 400mA from the battery in sleep mode. As a result if I donot use the vehicle even for a day the battery goes dead and I have to jump start it. So I pulled the fuses and determined that fuse #41 (radio amplifier circuit) is the culprit. Then I pulled the amplifier from the trunk to check whether there is anything wrong with the amplifier. My father-in-law who is an electrical engineer found that there is nothing wrong with the amplifier. He also found that there is full voltage (about 10v) at the amplifier even when the radio is turned off. Ideally there should not be any voltage at the amplifier when the radio is off. So he is thinking that there is something upstream that is causing this.
1. did anyone of you had any problem like this?
2. Personally, I am suspecting the radio head unit. I am suspecting that it is providing the power (or the signal to turn on the power) to the amplifier when the radio is off. I am planning to buy a cheap replacement unit and try it.
3. Are you aware of any thing other than the radio head unit that might provide power to the amplifier.. I am imagining some kind of control circuit outside of the head unit here...
4. Do you have any other suggestions on how I can troubleshoot the problem?

thank you very much for your help,
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Old 04-20-2009, 10:34 PM   #2
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Location: Arkansas
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hmmm. I'm no expert in this field, but I would suspect a couple of things. Are you using the stock speakers and amp? I would start by checking the alternator. second I'd see if I could go to autozone or the dealership to see if it has stored any codes.
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Old 04-21-2009, 06:15 AM   #3
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Location: Milwaukee, WI, USA
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My Ride: 2000 BMW 328i
Thank you Albarran9. Yes I have stock speakers and amp. I have an OBDII reader; I will check for any stored codes.
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Old 05-10-2009, 10:43 AM   #4
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Location: Milwaukee, WI, USA
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My Ride: 2000 BMW 328i
Issue resolved

Figured out the problem.. It was the radio head unit. I got a used one for $35 in ebay, installed it and the battery drain issue is gone. thanks
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fuse 41 battery drain

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