Originally Posted by scholman
The vehicle is a 1976 GMC Sprint, 350
I start the engine and it runs like a top. I rev it up to about 2,000 RPM and the Alternator light comes on. I thought it was the alternator so I put a volt meter at the battery with the car off. Voltage reading is 12.35. Start the engine and the voltage is 12.28. Engine running no Alt light. Rev it up the voltage jumps to 13.84 AND that is when the light comes on. As long as the alternator is charging the light is on. When I disconnect the two lead plug going to the alternator the voltage drops to 12.8 but the light is out. Reconnect the connector and the voltage stays the same until I rev the engine. Then the voltage goes up and the light comes back on. I have not run into this before so any help would be great.
Check the two wire plug at the alternator. It sounds like the terminals are corroded or the wires bad causing you to have to rev the engine to get output.