could also have a bad diode.but if it's like the fords of years ago you have to replace all of them.and those diode bridges will cost about as much as a rebuilt alt.years ago you could buy all the diodes separate.positives and negatives and just replace the bad one.then somebody got the brilliant idea to combine em.so now you replace all of em when it's just one that usually causes the problem.this pisses me off.just recently i had to replace the alt in my suburban.it acted like a bad regulator.so i went to get one at the parts house.turns out they are not "servicible". turns out they solder the leads from the winding thru the back case.so in effect you can't pull the alt apart.i kept the core and i'm gonna break those solder joints and see if in fact the reg can be changed.