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Old 10-27-2012, 08:40 AM
KC 540iA KC 540iA is offline
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Location: Kansas City
 
Join Date: Oct 2012
Posts: 5
Mein Auto: E39 540iA
Before I started the car this morning, I tested the battery and it registered about 12.18V. Started her up and the alternator registered 13.9 with no accessories on. Turned on radio, both heated seats, rear defrost, max ac, all interior lights, HID lights and fog lights and it dropped to 12.45V. Turned all accessories off and it went back to 13.8V. Of note, once the car idles down from initial start up, I still get a slightly rough idle.

Next I disconnected the battery terminals while the car was running and put the multimeter directly on the terminals. Readings were all over the place from 11.5V to 13.8V. I have some serious flickering of both inside and outside lights when disconnected from the battery. The battery by itself now tests at 12.8V.

I'm guessing it has to be the alternator. I haven't heard any squealing to suspect a bad belt, but I'm going to start by inspecting the belt and checking the tension. I'll also check the power and ground wires from the alternator and make sure there's no loose connections. If everything checks out and I still have the issue, I guess i'll be replacing the alternator. It really sounds like either the voltage regulator or one of the brushes has gone bad inside the alternator. I'm leaning more towards a brush since I'd think a bad regulator would lead to abnormal spikes above 14V. With my voltage being at 13.8V, dropping down and bouncing back up, it sounds like one of the brushes is not maintaining contact.
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