To clarify, your timing belt is not associated with your alternator. It is entirely concealed and tension is non adjustable; it is set by a tensioner. This is no the belt you're working on.
The serpentine belt (or alternator belt) has to be over tightened to stop it from squealing. Going by a best practice or factory setting will not stop squealing on this engine. It is a poorly designed system. Trial and error is the only way to stop it. Still squealing? Tighten it more. But I only recommend doing this on a new belt. Some claim they've destroy alternator bearings and water pump bearings by over tensioning. I have not had this issue, but I set tension on new belt that wasn't worn or glazed.
This is how the FSM says to check your belt deflection...
I personally find it difficult to reach and even see in there at an angle to try and measure 10 mm... not to mention trying to guesstimate pushing my finger down with 22 lbs. of force...
I just reach in and twist the belt as mentioned.
PS... I cheat a little and don't bother loosening the bottom pivot bolt on the alternator. The adjuster bolt has enough power to move it. You can help it along a bit by pushing on it. I've wedged a hammer handle in there to help it along.
That way you don't have to lift or crawl under your car.
One guy here on the forum striped that bottom bolt and opened up a huge can of worms.
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I just had new belts put in yesterday but are squealing like there is no tomorrow. I'm trying not to drive the car as I don't want to glaze the belts. a few minutes of terrible squealing cause the belts to glaze?