My friend drives an '89 635 and has started having electrical problems.
About a week ago his check engine light came on and his subwoofers(wired to the battery directly) stopped working. When he stopped the check engine light turned off and the subs came on. It was during the day so his lights were off.
When he drove it that night the lights would dim when he took his foot off the gas, but his check engine light would turn off and the subs would come back on.

If anyone has any advice it would be much appreciated...i told him it was probably a bad alternator(he has a used one in now) or a bad battery, and worst case a combination of the two.

I told him if anyone knew it would be the guys on bimmerforums so dont let me down...thanks in advance for all the help

My friend drives an '89 635 and has started having electrical problems.
About a week ago his check engine light came on and his subwoofers(wired to the battery directly) stopped working. When he stopped the check engine light turned off and the subs came on. It was during the day so his lights were off.
When he drove it that night the lights would dim when he took his foot off the gas, but his check engine light would turn off and the subs would come back on.

If anyone has any advice it would be much appreciated...i told him it was probably a bad alternator(he has a used one in now) or a bad battery, and worst case a combination of the two.

I told him if anyone knew it would be the guys on bimmerforums so dont let me down...thanks in advance for all the help


never connect a subwoofer directly to the battery. It should be connected to an amplifier :)
if the intensity of the light changes with the engine revs (that's what I understand from your post) the alternator seems to be ok.
How is the starting behavior? if the battery is dead it should not crank. Does it crank normally or very slow. Does the battery charge?
Disconnect the subwoofer and see if the other issues are still present or disappear.
Strange electrical issues are quite often due to some improper ground connection. have your friend check this as well.Good luck.
Wolf

+1 on bad grounds. That was the issue with mine, and some of the old issues have popped back up again, so I'm wondering if I need to go back and check them. It may not be the best answer, but it's the cheapest solution.

hmmm thanks for the advice i will be sure to check into that, oh and the sub runs to an amp then battery

The thing I experienced with subs was the vibration had loosened connections in the 94 325. Taillights, the sub power supply, license plate lamps were all casualties of excessive boom boom. (my daughter) Check all electrical connections, not just the grounds. Good Luck!

The first battery died, so I immediately replaced the original alternator.
The second battery died - where do I start trouble shooting?
The only visible modification is an after market stereo.

You certainly did not give us much to go on. If you think the stereo install has something to do with the issues, then you need to retrace the steps of the install. Check grounds and cable connections. How long did it take for the replacement battery to give up. Many auto parts stores will do a free system check expecially if you bought a battery and alternator.

Make sure the power wire or wiring in general to the stereo system isn't ran behind or has dropped into the pedal assembly. Something may have worn through the insulation causing something to ground out which in turn is causing all the other problems.

To the best of my knowledge, the first battery to die was a few years old, and a low voltage one from walmart. I assumed it was old and needed to be replaced. When my replacement died, I automatically thought 'old alternator' and replaced it along with the brand new battery which showed no charge. I drove the car for ~15 minutes 3 times before placing it back in the garage for some more work. Now, about a month later, the work is finished, but the battery is dead once again.
I'm charging the battery now and would appreciate some advice on electrical trouble shooting.

Many things can drain the battery over a month period. A few things to check would be: glove box light, trunk light, door lock heaters, faulty stereo ant, stereo memory, and faulty SI board. When I had several cars that stayed in the garage, if they were not driven ones a month, I risked a dead battery. I now use solar chargers placed by the window to keep the battery operating properly.

There have also been a few other threads here about battery draws and drains.

..... When my replacement died, I automatically thought 'old alternator' and replaced it along with the brand new battery which showed no charge. .......

Are you saying that you replaced the battery and the alternator and the alternator showed no charge?

Does the alternator light in the instrument cluster come on when you first turn the key on and then go out after the engine starts, or does it stay on after the engine starts or perhaps it never come on?

What year is your car?

Power from the ignition switch routes through the alternator charge indicator bulb in the instrument cluster and then too the alternator to provide the power to excite the field windings of the alternator for it to work. If the bulb is burned out, it will prevent voltage from reaching the alternator causing it to fail. Or if the wiring or connections are bad, it can cause low voltage to the alternator causing it to have a low output.

In the later cars, '87 or newer, there is a resistor wired in parallel with the charge indicator bulb so the the alternator will keep working if the bulb burns out.

Do you have an ETM (Electrical Troubleshooting Manual). i.e. wiring diagram? You can download a free PDF file of one here. (http://wedophones.com/BMWManualsLead.htm)

(If Image doesn't load: http://www.stormpages.com/countchocula1/bmw/etm114.htm )
http://www.stormpages.com/countchocula1/bmw/bmwetm114.jpg

Was the voltage regulator changed with the alternator? Maybe the amp is pulling power. If the problem came with the new installation, it's time to check every wire and ground connection that was added. Maybe even dissconnect all new power leads and see if the check engine light/electrical system returns to normal. Sometimes we create our problems, I've done it and it really gets frustrating when you assume the replacement part was good when it to, was bad.