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At slow speed, suddenly the power steering quit. Then I saw that the battery light was on, no alternator output. Then I smelled rubber burning. I shut off the motor right away.

It was the serpentine belt smoking a little. I looked it over where it was visible but saw no obvious problem. I tried starting it up again and the belt started smoking a lot before I had time to see where. I decided not to destroy the recently-replaced belt by troubleshooting it, shut the motor off again, and went to get our big trailer to tow the X5 home.

An hour later I jockeyed the X5 to put it on the trailer. The power steering worked for a few seconds, then quit again and the battery light came on. Once the trailer was in place, I started the X5 up to drive up the ramps, and the problem was gone. I had power steering and the battery light stayed off.

What could do this? Some component on the serpentine belt must be seizing up. But which? The power steering has not been making any untoward noises, and no leaks are evident. The alternator has been as strong as Thatcher. I am going to leave the X on the trailer and take him to the indy tomorrow, if he has time to take it.

This is not a problem I have ever had before with a BMW or any lesser car. Weird!  Has anybody BTDT? (been there, done that)

The idler/tensioner is new from January.  But on Xoutpost somebody mentioned that the M57 crank pulley can fail and seize.

I would say that one of the tensioners or even possibly the alternator/power steering pump is seizing up. Had exactly the same on a previous car and it turned out that the alternator bearings were failing

I dropped the X off at the indy and he showed me a crank damper. Once it's in hand, you can see how it would fail. Its large rubber inner component breaks down and de-laminates from the inner metal core and outer metal pulley. This produces 1) smoke and 2) little chunks of rubber visible below the area, whilst the serpentine belt is not fully driven. Hence the weak power steering if any, and the alternator warning light.

The little chunks are what we saw on my car. What I thought was smoke from the belt was in fact smoke from the damper. He says it's pretty rare, but obviously it does happen. He sees it more often on the E39.

Hope this post helps somebody else with a similar M57 problem some day!

I've just had this exact problem!

Do you mind me asking what it cost to fix? I'm going to try and get mine into my Indy a sap.

The pulley itself was nearly $400. The Indie says he's had bad results trying to use third-party pullies.

Thanks for replying!

My Indy just called and it's the crank damper (dampener??) on mine too.

He's given me rough quote as follows -

New crank damper - £295
New belts - £25
Labour - I think he said about £150