I have a charging issue with a '99 F-250 Diesel. The truck went completly dead and would not start. The battery's were charged and tested as in good condition. The alternator was also tested while running and tested @ 12.5 volts. I need to know if I have a bad batterry or a bad alternator. If I am not mistaken the alternator should be running @14.5 volts charging up the battery's. I am not sure if I have a short causing this or the alternator is bad. If there was a short then I would not have a voltage reading at the alternator, right? Where should I start?

 

12.5 volts would indicate a no-charge condition. Even if the batteries were bad the running voltage would be higher.
Pull the alternator and have it bench tested. If it checks bad, you should be able to replace the regulator and/or brushes to save some as opposed to a complete unit (if you're interested in going that way). Easier to replace the whole unit.

 

"If it checks bad, you should be able to replace the regulator and/or brushes to save some as opposed to a complete unit (if you're interested in going that way). Easier to replace the whole unit."

Yea, I bet an alternator for a diesel is not cheap.

 

A diesel alternator is about 180 to 200 dollars, for a stock replacement, but the real kicker is, if you get a motorcraft alternator, is has a lifetime warrenty, and the oem ford is a motorcraft, but no warrenty. So i geuss you will only have to buy it once, but still, they should have warrenties from the factory, if they are using the motorcraft alternator.