[rotorwrench]

If it has a diode that looks like a cut out on it, isolate the connections on both sides and ring the diode through with an Ohm meter. It should pass the current one way but not the other. If it's not a diode but a real cut out relay then it needs to be tested. They can be tested with an 8-volt battery with a quick connection to see it it closes the points and allows voltage through.

If the generator motors then it has field and brush connections but it may still have a problem with the 3rd brush or armature. A person needs a growler to test the armature. The rest can be easily tested for field ohms and shorts to ground anywhere.

The ammeter should show a healthy discharge when you turn the lights on with the engine off. If it doesn't, you need a better ammeter. I don't trust the reproductions. They have poor quality control and are not sensitive enough to give a correct indication.

Your battery has to be in good condition or a 3-brush generator won't hold good voltage. The battery controls the voltage of the system. I worry more about the amperage output than the voltage. No amperage output equals a generator that is not working for one reason or another. Check all your wiring over. It had to have good connections between the ammeter load and battery sides. Any bad wires or loose connections will be a problem.