In generator protections, a high coolant temperature 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 pressure of the engine. Normally screwed into the engine block, it has two states, either open or closed. Traditionally a coolant temperature switch is closed to earth on fault (when coolant temperature is high) and open when the coolant temperature is below this maximum requirement. Engine manufactures determine what the level of low oil pressure is and when the engine should be shut down to prevent problems or catastrophic failure. Switches are often provided by the engine manufacture, rather than the generator manufacturer.
To check the coolant temperature switch you need a multi-meter reading resistance. When the engine is at rest and the engine has not been run to the point it has overheated, the coolant temperature switch should be open to earth. When the engine runs it should then not open unless the coolant temperature is high. If the coolant goes over its maximum temperature then the switch will close to earth indicating a fault.