That's a pressure switch, used for idiot lights, manifold heaters among other things. Three terminals, likely common, normally open and normally closed. Your pressure sensor is the round can right above the switch behind that bundle of wires. I think the sensor just uses a wiper arm across a resistive element. You can try using an ohmmeter from the terminal to a good ground and see if it changes with pressure changes. Or just say the hell with it and plumb in a mechanical gauge.
1986 CJ7, 4.2 w/4.0 head, TFI-HEI hybrid ignition, Clifford manifold w/Holley 390 w/cold air intake, OBA, 4.5" lift, Woody CV shaft and Tattons in front, 4.10 gears - lunchbox in front, Truetrac in the back, twin-sticked, blower upgrade for running topless, trying to keep it simple.
It's just a Jeep, and if you don't wheel it once in a while, it's not even that.