the switch is an on/off type,its on with no pressure,turn the key on and the lamp should light,no oil pressure,you start the engine and oil pressure opens the contacts and your tell tale lamp goes out,to test it,remove it from the pump,then with a globe or meter,the main body will have full continuity to the terminal connection,the globe should light if not the switch is buggered,if you can blow air into the sensor hole in the thread at the same time the globe should go out,these are set a a specified pressure for different cars or bikes,its not real important to get the right one,you dont want a tight one that flickers at idle though,even though your oil pressure is ok at idle the sender is set at higher pressure for some other model car or bike and speaks with false tonque.