I acquired an old junker and have it running now. The alternator, battery and battery cables are new. The truck starts fine and runs OK. If I step on the accelerator though the warning light comes on. I tightened the belts and cleaned the connections on the regulator. The battery shows 12.4 volts with the engine off. 14.5-15 volts when running. Drops to about 13.5 when the light comes on. I am suspicious of the regulator. The first spade in the regulator shows 12.5 volts if I test between the spade and the connector with the key on but the engine off. So I think that the harness is OK at least to the regulator. I don't know what to try next. Thoughts?

 

Replace the regulator.