In generator protections, a high engine coolant alarm is used to alert the operator to the engine suffering from high coolant temperature. The coolant temperature can be monitored by either a coolant temperature switch, or a coolant temperature sender.
A coolant temperature switch is an on engine protection device that is used to monitor the coolant temperature of the engine. Normally screwed into the engine block, it has two states, either open or closed. Traditionally an coolant temperature switch is closed to earth on fault (when coolant temperature is high) and open when the coolant temperature is below this maximim requirement. Engine manufactures determine the level of the high coolant temperature is and when the engine should be shut down to prevent problems or catestrophic failure. Switches are often provided by the engine manufacture, rather than the generator manufacturer.
To check the engine temperature switch you need a multimeter reading resistence. When the engine is at rest, the switch should be open to earth, when the engine runs and overheats it should then close. If the switch does not change its state when the engine runs, thats good! it means your engine probably hasn't overheated!