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When I stated her up yesterday, she was showing low voltage. I just replaced the batteries a couple months ago and it doesn't appear that the belt is slipping. I suppose I'll have to get the alternator bench tested. How do I check it at home?
If you have a multi meter, set it to 20 volts DC. Touch the red probe to the bolt where the big red wire attaches to the back of the alt. The black probe goes to the battery ground. You should read at least 13.5 volts. Be careful not to touch both the alt. case and the bolt at the same time with the probe. Big sparks happen if you do.
If it is low then first verify that all of your engine and frame grounds are clean and tight. Then just go get another FORD voltage regulator. It's on the back of the alternator and I think it was just a couple T15 screws holding it on. It contains the alt brushes and the regulator. It's easy to replace and a LOT cheaper than an alternator. I've never replaced a Ford alternator before.