In generator protections, a low coolant level alarm is used to alert the operator to the engine suffering from coolant level. The coolant level is normally monitored by either a switch mounted in the radiator. Commonly coolant temperature is also measured separately, by either a coolant temperature switch or a coolant temperature sender.
A coolant level switch is an on radiator protection device that is used to monitor the coolant level in the engine cooling system. Normally screwed into the radiator header tank, it has two states, either open or closed. Traditionally a coolant level switch is closed to earth on fault (when coolant level is low) and open when the coolant temperature is below this minimum requirement. Engine manufactures or radiator manufacturers determine what the level of coolant is appropriate and when the engine should be shut down to prevent problems or catastrophic failure. Switches are often provided by the engine and/or manufacture, rather than the generator manufacturer as a standard protection device.
There two common different types of coolant switch. The simplest is a float switch, which changes state as the float raises or lowers. A good example of this would be a Fozmula T/LL80. With its two wire system, the circuit closes on fault and should be open at other times. Float switches are simple devices, which in most cases function well, but because they are mechanical in nature jamming or during up over time, especially if there is contamination in the coolant can cause a lack of protection.
The second type is a Capacitance Type level switch. Again Fozmula provide a good example with their S285 Level Switch. A 4 pin switch, containing a factory programmable microprocessor, the switch offers sink to ground or source voltage outputs.
To check the coolant level switch you need a multi-meter reading resistance and voltage. The first two wire system you can measure if the circuit is open or closed using the resistance setting. When there is no fault (ie the coolant level is OK) it should be open. It closes on fault. You can take out the level switch and move the arm up and down to check its working.
The second type, if the system is using a sink to ground setup, you can measure it like the first. Again you can test it by dipping the unit in coolant and removing it again. For voltage you can measure the voltage between the output and the input pins.