If it is only an idiot light device, it will be a closed circuit when there is no pressure against it. That's why it lights up nice and bright when you turn the ignition to "on". Once it receives pressure of 7 or 10 pounds, the switch will start flickering on/off (making the idiot light flicker also) and then when pressure exceeds about 12 to 15 pounds, the circuit will be completely open and the idiot light will be off. If you just check between the wire tab and ground with the engine and ignition off, you can tell. If the wire tab when connected to ground is a closed circuit, you have an idiot light device.
