In generator protections, a low oil pressure alarm is used to alert the operator to the engine suffering from low oil pressure. The low oil pressure can be monitored by either a oil pressure switch, or an oil pressure sender.
An oil pressure sender is an on engine protection device that is used to monitor the oil pressure of the engine. Normally screwed into the engine block, it can output a range of values based on the oil pressure in the engine. Oil pressure sender can output by different methods. Voltage, capacitance or resistance are sometimes used, but normally they are a resistive type when seen in diesel generators. The oil pressure sender is sometimes seen combined with the oil pressure switch in a single unit. Senders are often provided by the company that manufactures the diesel generator, rather than the engine manufacturer.
The range of resistance, capacitance or voltage values output for a given pressure depend on the manufacturer of the sender unit and vary between suppliers.
In addition, an oil pressure sender is often used to ensure an engine isn't running before trying to crank. Because an oil pressure sender outputs different values based on the oil pressure (because when the engine is not running, the engine has no oil pressure), it is checked prior to cranking to ensure the engine does not appear to be running, as engaging the starter motor while the engine is running can cause the starter to fail.
To check an oil sender that works by resistance you need a multimeter reading resistance depending on the sender and also the information relating to the sender, as each manufacturer has different outputs for different pressures. When the engine is at rest, the oil pressure will be zero, so you can measure this using the above information. When the engine runs the pressure will increase and the resistance will change. it may go up or down depending on the sender type. If the sender does not change its reading when the engine runs, check if engine has a low oil pressure condition and/or try another sender.