this pully broke on my 2003 monte carlo ss and need to know what one it us cuz my husband and his friend bothe said something different and how to replace it cuz i like to work on my own vehicles.

i already removed the altinator

 

Thats the belt tensioner pulley. Just go to the parts store / rock auto and ask for the belt tensioner assy. Its that whole aluminum piece that sits under the alternator. Should be under $100.

Also make sure you get coolant elbows if it doesn't come in the package. They absolutely will break when you take this piece out