Measure the voltage on the output terminal while the engine is running. It must be between 13.75 and 14.75 volts. If it is high, it is charging but there's a break between there and the battery positive terminal, usually due to a blown large bolted-in fuse. If the voltage is low, the same as the battery voltage, check for voltage on the smaller wires. Holler back with those colors and voltages with the ignition switch on and the engine off and with it running.