When you replace the factory ECU controlling a GM LS or late-model truck motor don't forget that's where your alternator gets its "excite" voltage required to produce power.  This pigtail harness is exactly what you need to wire up excite power to your GM Alternator with 4-pin Oval connector.  This 8-foot long harness is sufficiently long for most applications.  Wire this to a switched 12-volt source that is powered when the ignition is in the "run" position.  The inline 560 ohm, 1/2 watt resistor allows you to run this through a "Charge Light" indicator (that will illuminate if the alternator stops charging.)