Daz, it's important to loosen the 19mm center bolt on the tensioner 1/2 turn or so BEFORE turning the adjuster CCW to loosen the drivebelt. The horizontal center bolt locks the tensioner once the drivebelt is properly tensioned. If you failed to do that, chances are you ruined the tensioner ... order a new one and chalk it up to inexperience. Instead of replacing the alternator you may only need to replace the voltage regulator which bolts to the back of the alternator. When the voltage regulator brushes wear down too far the alternator fails to recharge the battery properly.

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Fred Hoelzle