The tensioner pulley on mt 2014 q7 3.0 broken while I was dropping my kids off school. As soon as I got into the parking lot, I see a alternator light on dash, couldn't longer turn the steering wheel, it's it feels like it's locked. I think it's all related due to the serpentIle belf being off.

I purchased a new pulley, but want to know what could the cause be? Bad pulley, bad belt?

 

cause? age, wear

you loose power steering and alternator, cause thats what that belt is powering

replace it and be happy it didnt damage anything else. You will need to tow the car.

 

A lack of serpentine belt maintenance, which includes the entire belt system (belt, idlers, tensioner), as needed, and not just replacing the worn belt. You should take the opportunity to examine/test the other idler pulleys for bearing damage/looseness, as well as the other drive pulleys like water pump, A/C, power steering pump and alternator to ensure all are tight...no shaft/pulley play/wobbles, etc. Especially check the water pump for play and bearings issues. Rotate all by hand first to feel for any tight points, move around to see if it wobbles/has play in it, and give it a spin and listen for bearing issues. If they sound like the old metal roller skate bearings it's time to replace, etc.

With all that corrosion on yours in the pic, that should tell you something.

Also, don't take it for granted that the old belt shredded and didn't end up winding at least some of it around the inside of the crankshaft drive pulley, as that is fairly common when the belt comes apart like that. Hopefully it didn't damage anything else.

As DIY repair, it's not difficult to do at all, and you can buy a complete service 'kit' for the S-belt system that has all the wear components, etc. to make it simple. It's going to cost quite a bit more to have any shop do it, and you don't wanna take it to Audi Stealer for this basic service...I won't call it a repair as the belt and entire system are wear parts.

Good luck!!! I just did this service on my own Q7...everything I took off the car was original from the factory @ 130k miles (except the serpentine, which had been replaced at least once prior, perhaps twice), and was worn out.