M42 Stalling Itself

[RedTeam]

Hi guys. Sorry for the long post.

My '91 318i has been sitting for about four months while I was at school. I tried to start it up yesterday, but the battery was dead. While trying to jump it, I wasn't paying attention and I switched the leads (ground cable was orange, positive cable was black, I know it's a rookie mistake).
Anyway, I realized the mistake after ten minutes of "charging" and trying to get the engine to crank. When I flipped the leads, it cranked but didn't turn over because the battery was almost fully discharged. So I charged the battery (1 year old) overnight and it started up fine today.
Despite being fully charged, my battery warning light is on when the car is not running (turns off once the engine starts). After about an hour/20 miles of driving, the ABS light and Check Engine light came on. I was pulling into a shop to get new tires when I noticed my blinkers stopped working, and the engine nearly stalled itself as I was parking. It's now able to start itself with a jump (battery's down to <8 volts) and run for 15 seconds before stalling.
I'm thinking I fried either the alternator or the ECU when I reverse jumped the car. It's stuck on a lift and they won't let me get in there with a multimeter.
Has anyone seen a problem like this before? I'd like to order parts to fix it as soon as I can. I appreciate any input you guys have.

[roguetoaster]

Alternator regulator/brushes are going to need to be replaced, or so I would assume. Fortunately this is a simple enough operation and doesn't even necessitate an intake manifold removal.

Or if you feel more comfortable you can just swap the whole alternator. You should also plan to replace the alternator ground while you're there.

[shameson]

Or if you feel more comfortable you can just swap the whole alternator. You should also plan to replace the alternator ground while you're there.

X2, sounds like alternator. Changing the whole alternator might not be a bad idea if you don't know the age of the one in the car. Ground cable is inexpensive so like he said definitely do that too. When I changed my alternator I was glad I had a new ground because the old one was in bad shape.

[RedTeam]

Sounds good. I'll test the voltages when I get the car back this afternoon just to make sure. The PO claimed the alternator was recently rebuilt, so is this a result of switching the jumper leads?

[roguetoaster]

Sounds good. I'll test the voltages when I get the car back this afternoon just to make sure. The PO claimed the alternator was recently rebuilt, so is this a result of switching the jumper leads?

Probably, if you're lucky it will only be the voltage regulator that bought it and not some sort of almost fried connection that works sometimes.

[RedTeam]

Tested the battery today. 13.5 volts sitting, 12.5 volts while the engine's running. Alternator will be off for repairs once I figure out how to remove it from the lower mount bushings.

[downforce22]

you could always try swapping just the voltage regulator. not sure if thats the issue, but for like $20 and 2 screws its an easy test. i wish i knew it on the first alternator i swapped

[roguetoaster]

Tested the battery today. 13.5 volts sitting, 12.5 volts while the engine's running. Alternator will be off for repairs once I figure out how to remove it from the lower mount bushings.

If the bolt is all the way out just pry on the housing and it will come free.

[RedTeam]

Loosened the bolts on the alternator to give it more wiggle room and it came right out. The unit wouldn't even register on the testers, so looks like I'll need a new one. Thanks for all the help guys.