the battery tested right and thats not a knock sensor its a cylinder head temp sensor it told me that in the efi bible in the 1981 280zx manuel i looked up and when i ordered it from msa they much have switched it to a knock sensor in later years but not in the 81 motors which i have its the cylnder head temp switch and the alternator i am using is a hitachi and should be a internal regulator the old cylinder head temp sensor i pulled out had burnt on coolant and i am seeing white smoke from my exhaust so i believe he head gasket went out because i did over heat the car once not too long ago so right now i am thinking that the coolant is hitting the sensor messing up its input with the ecu but thats gust a guess