Engine Switches and Senders

Most boat operators are familiar with their gauge set and are aware that there is a device on the engine providing a signal to the gauge that allows it to provide useful information. However, many boaters are unfamiliar with the nature of those engine mounted devices. 

Most indicators that are either off or on, like an over temperature light or a low oil pressure alarm are simply a specialized switch.  The state of the switch will either be normally open or normally closed and the condition being monitored will change that state. For example, the low oil pressure buzzer that is heard when the ignition is turned on before the engine is started is normally closed. When the oil pressure increases beyond a set point, it actuates a switch to open the circuit and the buzzing stops.

Gauges that have analog incremental variation are typically variable resistors. This includes the fuel senders, temperature senders, and oil pressure senders.  Voltage is supplied to the gauge and the sender interprets the feedback from the sender based on the resistance to ground. For example, a typical North American standard fuel sender has a resistance value of 240 ohms when the tank is empty and 33.5 ohms when the tank is full. As the float in the tank drops, the resistance to ground potential increases which is interpreted by the gauge and converted to a readable value.    

- Lyle
PYS Electrical Technician