How does it work? Does this wire get a ground connection when there is no oil pressure?

 

Since it's attached to the engine block, it's probably using it's casing to ground thru the block.

DW

 

there's simply a spring inside the switch.. when the oil pressure is too low, the spring is closed, letting the switch connect to ground (and the oil light comes on).

when the pressure rises, it releases the ground connection and opens the circuit.. light goes off.

 

Cool...thanks. The OEM one grounds itself. I'm replacing the oil pressure switch with an aftermarket unit. The aftermarket one doesn't ground itself. I had to connect a ground wire to one of the two terminals.