I got a 1990 F-250, at idle, the voltmeter reads right in the normal range, at fast idle, it reads in the red almost to 18 volts, I also checked it with a bttery tester and it reads the same. If the truck sits over night, the batteries run down. It seems like something is drawing on it, but I dunno what. I can't understand why the volts are reading so high. Could there be a short in the alternator running the batteries down and causing it to overcharge? or possibly the regulator? The only thing I recently had apart was the steering wheel off, but, I unhooked the connector under the steering column to cut the power to that stuff, and it still has the problem. I thought maybe the short was in the column, but it shouldn't be with no power going to it.

 

i'd pull the alt and get it tested. and the battery. the regulator is probly bad and most likely took out the battery.

 

I checked the alternator, it's good, changed the regulator, it stopped the overcharging issues. Hopefully the thing will start in the morning, that's all I have left to see. If not, i guess there's a short somewhere else.