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Hello, maybe somebody could point me in a right direction. Recently I replaced alternator on my ISX CM870. Delco Remy SI34. I removed it from another truck. Today when removing the belt I noticed that if I rotate pulley by hand it kind of wobble a little bit from side to side. Why it could be that way? My old alternator doesn't have a key , so I didn't install any. Bad pulley? Bad alternator? Thanks!

Take the pulley back off and remount it. It might be cocked sideways and not pressed all the way on. While it's off put the alternator in a vice and spin the shaft to see if it spins true. If all checks out remount and try again.

Make sure alternator is mounted horizontally, when redoing pulley, or you will damage internal components. As per delcoremy instructions.

thank you guys, i will remove it and put back on. will see