Q: I was just told that a tensioner or pulley could have caused my alternator to go bad and if not replaced could make the new one go

Pulleys do go bad because the grease in the pulley bearing dries out, the ball bearings and bearing races get pitted and then the pulley becomes noisy. Sometimes, the pulley will actually seize. However, it will be EASILY evident if a pulley bearing is bad. So, all you need to do is have them PROVE the failure to you. If the pulley turns SMOOTHLY with no resistance and there is no noise, then there is nothing wrong with the pulley. Tensioners combine a pulley with a "tensioning" mechanism. If the pulley on the tensioner is good, to check the tensioning mechanism, just ask the shop personnel how many pounds they are able to measure (using a Gates gauge) as "belt tension" with the tensioner installed. If they can measure an installed belt tension of at least 80 to 110 pounds on the serpentine belt, you are fine. As far as alternator failures, even if all pulleys seized, the serpentine belt snapped and all the accessory drives came to a crashing halt, such would have no effect whatsoever on the mechanical or electrical function of an alternator. Alternators are DRIVEN by the serpentine belt. All that happens when the belt disappears or pulleys seize is the alternator stops turning; there is NO possibility of damage to the alternator. Once you re-apply a new belt, or new pulleys, the accessories start turning again, just like before and the alternator produces current to charge the batteries. As noted, pulleys do wear out...worn pulleys do cause noise but all that is beside the point. Insofar as your concern, you CAN confirm if they are telling the truth by just simply having them PROVE the alleged failure to you. If they cannot prove it, or are unwilling to prove it (don’t buy the "insurance rules don’t allow you in the shop trick), then do not replace the pulleys. In the future, you can save a lot of money by having a repair like this, as well as maintenance, performed on a mobile basis. So, please keep us in mind and if additional concerns arise related to your current issue, please don’t hesitate to inquire further.