Bart Bosco: A couple of things to try. 1, Check the battery. Under load, easy to do. Turn on the lights and put a volt meter across it. Should be 12-12.5 steady. If that is good then try the armature output voltage. If you have a generator. should have something like 12 volts out. That means you have a working generator. If that is good then follow the regulator adjustment procedure in the shop manual for a 55 second series truck. If the regulator is bad you can replace it with a 12 volt regulator from any parts house. Then polarize the gen and start testing the voltage at the battery. Should be around 14.5 volts at a high idle. If it is not you may have a bad generator as well. The regulator feeds voltage to the field coils to increase output under load. Permanent magnets will still cause the generator to put out voltage even if the field coils are bad. Just not enough to charge the battery under working loads. That is why the regulator adds voltage to the field coils. Makes stronger magnets when needed. You could measure the field coil resistance but it is not very much just about 2-5 ohms and even then you would not know if the coil is shorted. If the armature is bad no volts out at any time. And that includes the brushes. You can fix the generator. Any shop that does starters can do it for you. The regulator is not hard to find.

Steve H