my 1990 toyota celica new "move" is that the battery light in the dash comes on intermitent, the alternator is ok, and so the battery. I allready change the plug for the alternator and check all wires. Seems like the alternator belt get loose, but i really dont believe so, so myself has tihgten it. Did you think that i need to throw a cable directly from the alternator to the positive of the battery?
If the belt is loose and slips on the pulleys you will get the battery light. How old is the belt? Does the '90 celica come with a belt tensioner? You might want to swap that out.
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