For about the 15th time in the last week, I have tried to work out why my 190D throws the aux belt off the pulleys every time the engine is started. Its driving me mad. 
At least one of the pulleys is out of line, or not running true. If it's a multi-rib belt, start with the tensioner. Then the water pump, as you do the easy stuff(both to diagnose and fix) first. After that, try a straight edge between as many pulley combinations as you can.
Today I stripped some stuff out to access the crank vibration damper pulley. It looks like new and has no play, which is good news.
I then carefully cleaned the old rubber out of the grooves in all the pulleys. Reassembled, started the car and it ran fine for a few minutes but then the belt jumped off again. I now had it so it would run for a few minutes, so reassembled again, started the engine again and took a close look at everything. I noticed the belt was sitting too far out on the tensioner pulley and followed the line of it up to the PAS pulley. I could then see that this pulley was at the wrong angle. As I was looking at it I saw the belt jump off the PAS pulley and then all the others.
Got hold of the PAS pump and gave it a good tug, and it was obvious that the pump is either loose or has a broken mounting. It isn't easy to see as its surrounded by stuff, but I'm now going out to find out why its moving around.
I should have probably diagnosed this a week ago, but that's probably why I'm a keen amateur and not a professional.
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Topic: So what have you done to your car today? (Read 3123085 times)