Bain is correct as it is a temp sensor that would affect the voltage regulator activity. The idea was to alter the voltage as battery temp changed. The voltage needed to charge battery without overdoing it and risking electrolyte evaporation varies with the temp of battery. This noble engineering feat was wrought with troubles though as the sensor could (and most times did) get attacked by acid and the temp sensor value would be wrong. The result was exactly the thing the sensor was there for, i.e., it would overcharge. There was a service bulletin 13 yera ago saying to disconnect the sensor plug on back of alternator. The voltage regualtor would revert to internal temp regulation when the sensor resistance went infinite.