Bout a week ago my bride tells me her battery light came on in her 05 mini non S. Electrical test showed 11.4 volts at the battery and dropping. Bad alternator. Though, when I gave the alternator a light tap with a wrench,
it changed its sound, idled down the engine, and showed 14.5 volts at the battery. Good alternator. I am able to drive the car without incident until I shut it off. Upon start up the battery light glows, voltage drops, and a tap to the alternator fixes everything until I shut it off again. Does Mini sell a stick long enough so I can tap the alternator from the front seat or do I need to get out every time. (Kidding). What is up with this. Also, where can I get a spanner for lifting the idler pully in the Chicago area. Seems like a unique tool. My method of a bottle jack and a pry bar from underneath seems dangerous, though effective.

 

Tapping the alt, sounds like sticking brushes (not bushes) rather than a dead unit. Get it out and clean it's works and you might have the cheapest fix on the planet. Worn brushes can usually be replaced if you go to the right shop (either DIY or let them). Good luck - Hope this little prob goes away fast! PS: Don't leave fixing it as the weird 'now you see it - now you don't' connections can screw up the unit.

 

Have you checked to ensure the engine and battery ground wires are tightened down?

 

Also check the jump stud connection in the little black plastic box mounted on the drivers side firewall. This connection has been known to corrode and/or loosen up causing the same problem that you mention.

 

But tapping the alt fixes it temporarily - not slamming the door.

 

Check the other things first. They're easier to check than pulling the alternator. If those things don't make a difference, then pull the alternator. It could also be worn brushes that simply need replacing. You didn't say how many miles are on the car.

 

Brushes problem. Isn't this where I came in?

 

I'm starting to have the same issue but Haven't tried tapping it or anything yet. I have a radar detector that will tell me the voltage. Since yesterday it started going low. I had the battery tested and everything is fine. I figured I'd come check here before anything else to see if this was common. So whats the checklist I should run through?