How do you fix a

hi. i would be glad to help.
the belt tensioner should have a little up and down play in it while the engine is running. but not alot and no side to side play. if its bouncing up and down real bad or makes alot of noise or is not keeping the belt tight it needs to be replaced. this is a very easy project that takes about 30mins. i would be happy to assist you in replacing it if need be. also, check your belt for cracks. this would be a good time to replace it if needed.