2005 chevy tahoe 5.3 v-8, the a/c belt shifted over off the pulley a little but not completely off. It's making noise, I tried to correct it by using 3/8 drive. I pulled up on it and it moved but when pushed down on it, it did not move. I couldn't take it off by making pulling on the tensioner it made it tighter. I thinking I need a new a/c tensioner?

SOURCE: Shift lever

stretched shifter cable i believe

SOURCE: Changing Serpentine Belt on '01 Impala LS (3.8L)

Can u turn the nut in the opposite direction that u have been trying? U need a lot of leverage to get enough tension off to remove the belt. If u have enough room, add a pipe to the end of your wrench, u will need a lot of room to move the tensioner enough. U might have better luck buying the wrench made for that purpose at a parts store. I think I paid about 25 a couple years ago, I couldn't fit a regular socket and wrench on the tensioner, it was a very tight space.

SOURCE: need to install a serpentine belt for a 1995 Chevy Cavalier..how?

There is a Large Pulley above the Tensioners Small Pulley. This Larger Pulley has a Bolt in the Middle of it. Geta Socket and Wratchet and Tork this Bolt and it will Lower the Tensioner so that you can Reinstall it.

SOURCE: tension pulley tightness problems

the easist way is to loosen the alternator and then pull and tighten it back up

SOURCE: 2005 chevy tahoe dies

you might have the alternator checked or simply replace it.. i would also replace the battery if its more than 2 years old