won't start: battery okay, one loud click

If it shows 12.9V, the battery is not OK. Get the battery load tested.

 

Back from the dead thread

 

If the contacts inside are not installed flat it can give the same symptoms of worn out contacts. If you have the starter off, just take the back off and check the contacts. When you install the big cable to the outside of the starter, if you tighten the nut too much it can rotate the contact inside too. The stud that the cable gets installed onto is part of the contact, so that's why it can rotate.