It sounds like the rubber bushing in the tensioner has separated and the tensioner is shot...

It is common for them to fail when being re-tensioned after a belt replacement.


I read a test procedure on ALLDATADIY:

If you secure the shaft that the 19 mm bolt goes through in a vise, you should be able to 'deflect' the tensioner arm by up to 30 degrees and a good tensioner will return to the original position. If it does not, the arm is slipping on the shaft and the tensioner must be replaced.


Over-tensioning a tensioner will ruin it.

Here is how I, a DIY person, set my tensioners. If a pro has anything to add, or correct, please chime in. This method has worked well for me for several years, and in several cars!


Locate the series of marks on the tensioner, near the pointer. One mark is away from the others. Install the pointer directed at that mark. Install the tensioner and tighten the tensioning link until the pointer reaches the first or second mark. I usually set it to the first mark and have had no problem.

Good luck!

UPDATE:
Leave the large thru-bolt a bit loose until the tension is set, then tighten it to lock things down.

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