ammeter 1937 ford truck

[37 ford firetruck]

Resurrected a 1937 ford flathead that was dormant for 20 years and poorly maintained prior to that. I cleaned the connections on the gauges and the ammeter is registering zero when running and discharge when I turn the lights on and the engine is running. The gauge returns to zero when the lights are off. Does this mean the ammeter is simply showing a load or that the generator is not charging or keeping up with demand?