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I replaced my alt and prior to doing so I locked out the belt tensioner by pulling on the belt and pushing the lock tab on the pulley in place. Now once the new alt is in, I cant get the tensioner to unlock.
There is a opening for a 1/2" drive on the tensioner, you might be able to get a breaker bar or a 1/2" wrench with a pipe extension on it, the angles can be a little tricky though...