What is an oil pressure switch?
An oil pressure switch is a protection device for an engine. Its a type of oil pressure sensor. As the name suggests the oil pressure switch is a mechanical device that has a switch inside. The oil pressure switch may be 1pin, 2 pin, or 3 pin. Traditionally most oil switches for protection close when they activate, to show low oil pressure (the oil pressure will be low when the engine is not running!). This may be to the engine block acting as ground, or via a insulated return. You should also have a look at what is an oil pressure sensor and what is an oil pressure sender.
1 Pin Switch
A single pin oil pressure switch has two connections - one on the pin and the other through the body to the engine block. This connection through the body of the switch to the engine block acts as an earth connection. The pin then changes state. Normally the circuit is closed when the oil pressure is low. This is an example of a Perkins oil pressure switch found on most 1103, 1104 and 1106 engines, part number is 2848062.
As you can see this switch has only one pin, so the switch works between ground and the pin.
2 Pin Switch
A two pin oil pressure switch has two connections. These can vary - there are two options.
2 Pin Switch, With One Pin as Ground (Most Common)
One on the pin and the other through the body to the engine block. This connection through the body of the switch to the engine block acts as an earth connection. The pin then changes state. Normally the circuit is closed when the oil pressure is low. The below is a Perkins oil pressure switch, part number T421762 switch found on most Perkins 400 series engines. This switch has a plastic socket that takes a 2 pin plug for the connections.
This switch works between the two pins, it doesn't use its body as the ground connection.
2 Pin Switch, With One Pin Normally Open and One Pin Normally Closed
In this configuration, the each pin is switched. One will be closed when the oil pressure is low, the other open. When the oil pressure rise each will change state.
3 Pin Switch
A 3 pin switch normally has two pins that switch state, one that is normally open, one that is normally closed and a common connection, usually used for ground. Here is the most common oil switch on the Perkins 4000 series engines. It is part number 782/175.
This is a 3 pin switch, it has a triangular socket which takes a 3 pin electrical plug. One pin is ground, one is normally closed and the other normally open.