I have a 1977 F-250 and the charging system is not working at all. I have had the alternator and battery tested and both are in good working order. The battery will hold a charge as long as the pickup is not running. I have measured the voltage at the battery and come up with just over 12v while not running and about 14.5v when it is running.

Where should I go from here?

 

It sounds like your alternator is intermittently failing to charge the battery. My guess is a bad connection somewhere. Have you removed the battery cable terminals lately?? Cleaned & lubed them?? I think if they were the problem, you would have STARTING problems also, but then again, you probably DO have starting problems. I would also check ALL the connections on the alternator, all connections remote from the alternator, and all connections at the starter solenoid. Make sure they're all clean & snugged up, no breaks anywhere. Let us know what happens.

 

Also check the fan belt to be sure it is tight. I had one that was semi-loose. It would be ok until the lights were on.

 

Mike W , GOOD POINT! I've had that happen. Sometimes the belts don't squeal, so you don't even know they're slipping.