What are the symptons of the belt tensioner being bad? I have ruled out the harmonic balancer by taking belt off and cranking engine. No noise. I've ruled out water pump. Bearing is tight and has no water leak. Alternator seems to be good , power steering pump, air conditioner compressor, all seem to be okay. Tensioner seems to be bouncing on the belt. Any advice?

 

There are what you might call index marks on the tensioner and if the reference mark doesn't remain within the group of lines when against the belt , it's time to replace it.
Many times the tensioner is frozen due to rust and can't do it's job. Make sure it is free to move against the belt.
Believe it or not an engine performance problem will cause the tensioner to bounce around. Make sure the ignition system is working up to par.