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bimmaster, BMW Technician
Category: BMW
Satisfied Customers: 1882
Experience:  3 years dealer experience, master bmw tech
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I bought a 2002 325i bmw the battery was draining overnight.

Resolved Question:

I bought a 2002 325i bmw the battery was draining overnight. I notice the horn was not working so i got the horn fix, now everything in the car is going hay wire
Submitted: 5 years ago.
Category: BMW
Expert:  bimmaster replied 5 years ago.
Hi, can you explain exactly what's going on with your car?
What is the battery voltage at now? Is is fully charged?
Do you have a volt/amp meter to measure the drain on battery?
Customer: replied 5 years ago.
i bought the car and the battery was draining every three days, so i replace the battery, i thought this wud fix the problem two days later same thing happen. Took it two three place Goodyear, Firestone, and a local dealer they could not tell me y it was draining. So the horn wasn't working so I decided to get it fix, got it fix, about half way home all the lights started coming on, everything was malfunctioning in the car. I believe it electrical but where
Expert:  bimmaster replied 5 years ago.
Ok, based on your description alone, it sounds like the alternator may be the culprit.
These alternators tend to fail when the engine is hot. They can overcharge, or undercharge the electrical system,This will cause all kinds of warning lights.
The alternator can drain the battery when parked, or it can cause the battery to drain by not charging the battery intermittently when hot.
If the fault codes are pulled from the vehicle now, you may find overvoltage or under voltage faults. This would confirm the alternator is bad.
Or you'll need to connect a volt meter to monitor battery voltage when driving, it should stay between 13-14.2v. If it goes much higher, or lower with the engine running, replace alternator.
Customer: replied 5 years ago.
Ive had the Alternator tested two times all said alternator is reading good. At night I was taking the battery hooks off so it would not drain the battery, I would hook up in the morning and it would start. All this just happen when I got the horn fix, like 4 hours ago
Expert:  bimmaster replied 5 years ago.
The alternator needs to be checked when hot, when the warning lights and problem are occuring. The alternator will fail intermittently, so it may pass the test several times if it's not tested under the correct conditions.
Customer: replied 5 years ago.
OK, so to make sure im hearing u properly, I need them to test the alternator when its hot, nothing to do with electrical drain because it was draining at night this was the alternator
Expert:  bimmaster replied 5 years ago.
Sometimes when the alternator goes bad, it can drain the battery overnight, since its directly wired to the battery.
Also if the alternator stops charging, this will cause the battery to drain while driving too. Ask the mechanic to connect a volt meter and monitor voltage while driving with all power accessories on, like rear defroster, heated seats, lights etc. It will take 20-30 min to get the alternator hot enough. Stop and go driving is best to get it hot.

You may have more than one problem here, so some thorough diagnosis with a volt meter is needed to see if the alternator is bad, and also to check for the battery draw.

Here is some more info on how to test for battery draws. Keep in mind you'll need to wait 16 min for this vehicle to drop down to the lowest battery draw which should be around .040 amps.
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