Low Oil Pressure Switch
This is the protection contact that changes state when engine oil pressure falls below a safe threshold. In common use the contact stays open while pressure is sufficient, and closes to chassis when pressure drops, triggering the warning or shutdown chain. Because loss of oil pressure is one of the fastest damage-producing faults, the response must be immediate.
Oil pressure keeps an oil film on the bearings. When this film is lost, metal-to-metal contact begins and damage progresses in seconds, not minutes. For this reason oil pressure and coolant temperature form the basic protection pair of engines.
The switch's behaviour must be understood correctly. When the engine is stopped there is no pressure, so the contact is in the fault state. For this reason the protection circuit is arranged so that it is disabled for a short time during cranking and initial running; otherwise the engine could never start running.
Its relation to speed control is this: on the GS740 and GS740-3P an engine protection input is not declared. That is, this switch must be connected to the engine's own shutdown chain, not to the speed control kit. The presence of the kit cannot be used as a justification for simplifying the engine's protection equipment.
Context in which this term is used: Electronic speed and throttle control on a diesel engine