oil sensor

I have 6-71's on my 1989 38 Hatteras. I noticed a significant oil leak coming from the hose fitting that is located near the transmission. the fitting is mounted on a "T" that has a sensor/pressure switch. I am replacing the "T" and all fittings including the sensor/pressure switch. The old sensor/pressure is small with connectors for 2 wires. I did not know the psi of the old switch. A local marine shop sold me one that has connections for 3 wires although he said we would only use 2 wires. (one connection is for the ground wire and the second wire is to be connected to one of the remaining 2 connectors. One connector is marked for open all the time and and the other would be for leaving the switch close all the time) can someone help me on this? should this sensor/pressure switch be open all the time or closed all the time? also we did not know the PSI so the switch he sold me was for 4psi. he said for the size hose I had it could not be more than 4 psi. I'm lost. Any help would be appreciated.

First off, the size of the "signal line" makes no difference. If this is for an engine oil alarm and or light, it should give a warning at about 15 psi and below, not 4. Check your old one if you still have it and see if there is a number stamped on it.The switch should be "normally closed" so that it will complete the warning circuit when the oil pressure falls below minimum. I suggest that you continue to shop for a proper switch at another parts vendor as they obviously aren't helping you at all. Many three terminal "switches" are actually "gauge senders" and are not meant for warning circuits. You need to get the right replacement so that you end up with the information the circuit was designed to provide. Good luck.