Idle drops slightly and interior lights dimming
 

I have a 2005 x5 4.4i with 109k miles.

My issue is that the car idles normally, then there is a slight drop in the RPMs - you can't really tell by the gauge but I can both hear it and feel a slight vibration change. Also, when this happens I can see the light in the vanity mirror dim, then brighten again (this also happens while I am driving). I recently had the Trans failsafe mode warning (2nd time) on but the battery was bad; I bought a new battery (holding charge well) and the alternator is also doing well.

The first time I had the Trans failsafe mode warning on I was warming up the car a in December (about 12 degress F) and I went inside. A few minutes later I could here the car beeping (not the horn) and the idle was extremely rough. I shut the car off, waited 1 minute and tried to start the car again but the Trans failsafe warning was on so I turned it off. I waited about 10 minutes and restarted and everything was fine.

Could my idle problem be some kind of leak or voltage regulator problem? Or could it be something bigger?

Do you have an error light on? perhaps there is a CEL you can have read.

voltage regulator is most likely... pull the cluster test and watch your voltage while driving, watch out for sudden drops and spikes... if you have software that can read the modules, read the EGS (tranny module) to see what else is listed along with the error - could be a voltage drop/spike... you might have a failing diode inside the voltage regulator.

100% voltage related. I had a similar issue, I ended up continuing to drive and fried my alternator and battery. Not the end of the world.

Thanks. Forgive my lack of knowledge, but what is the cluster test?

Took it to BMW for a diagnostics (only $50!).

Bad alternator. Any suggestions as to where to buy a new one?

If buying from a dealer, try here: BMW NJ dealer in Bloomfield New Jersey selling new BMWs, used BMWs in Essex county, north Jersey area

Replaced the alternator and it did fix my idle issue. But now when I accelerate, the interior lights all brighten and it's pretty obvious.

Does this indicate I got a bad Alt?

i just saw your question about the cluster test - look up posts by SlickGT and follow link in his signature - it will take you to the cluster test page...