If your tensioner is already bad, try doing this:
http://www.peachparts.com/shopforum/tech-help/368627-belt-tensioner-shock-upgrade-w201-possibly-others.html
As far as I can tell, this strut like rod adds great tension to the belt. Get a spring that you can wind around the rod like a coil over. It just so happens that I used a spring from an umbrella that was the perfect fit and had the perfect compression: not too soft and firm enough to add proper tension on the belt without being extremely tight.