2004 M3, battery light coming on at "high" revs?

Checked the alternator drive belt is tight yet?

I would if I knew how! ??

I know where this is, at the front coming off the erm, alternator, it seems "tight" but i don't know how tight it's supposed to be?

Huh! If I only knew how! Famous last words.

Why does everyone think that just because somebody wants to know what the problem might be they are a first class motor vehicle technician too?

The drive belt is what is commonly termed "fan-belt" and is a black rubber thing that runs around various pulleys on the front of your engine.

To test the tension you need a measuring stick and a strong thumb - ENGINE MUST BE STOPPED.

Find the longest run of the belt and press on it with your thumb - this is NOT a BMW approved method not scientific either - and if you can depress the belt vertically in it's plane of travel by more than 1/2" then it's too slack.

If you don't know how to adjust then find someone that can. I also suggest that you get a new belt fitted anyway.

However, it my not be the problem. Idiot gen light coming on can also mean that one of the diodes in the rectifier pack inside the alternator is going down - unlikely but possible. the alternator runs faster than the engine as it has a smaller pulley so there is also a possibility the slip-ring brushes may be worn and leaving carbon deposits on the slip rings. At high speed the brushes may not be making continuous contact with the slip-rings.

Just a though or two!

Steve

I HAD a 2002 M3 and one of the many reasons I got rid of it was because it went through 2 alternators in 3 years. Your indications are exactly what I had. Try to find an aftermarket rebuilt or a local rebuilder, as the price for OEM or Bosch is outrageous.

My theory is that the alternators were not designed for such a high-reving environment.

Bob

Actually, on modern cars (which includes all 2004's) the belt is a "serpentine" type belt and there is no adjustment. The belt has a spring loaded tensior which would have to be faulty for it to slip.

I think the alternator / regulator is a more plausible explanation. I would bring it in to be tested as the parts are quite expensive. It's definitely not normal.

Hasn't it got a serpentine belt like most modern BMWs - self tensioning via an idler pulley - and with a life of approx 70,000 miles?

Just an update for those interested.

Turned off the motorway today and battery light came on constant, took it to a friends garage and the battery showed at 11 volts, he said it should be 12.something without the engine running and 13 (or was it

Thanks for all the replies, much appreciated.

Daz

Daz - If your are in the UK - Find a LUCAS type auto electrical repair shop - usually listed under "Motor Factors - car & commercial"

The Commercial ones with their own repair shops will rebuild it for you.

Look for "Motor Factors" - NOT HALFORDS - and see if they stock CI (Commercial Ignition) spare parts for Bosch alternators.

The fairly large ones that the independent workshops use are the best bet here and get yourself a regulator diode pack and or slip ring brushes. Recon unit can be £100 DIY about £25 max.

Another idea is your local breakers yard -- usually about £40. with 7 day warranty

Or you are not going to have a thumb (or in my mates case the end of his little finger).

Might be the battery, although the symptoms suggest the alternator.