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Thermo switches and water temperature sensors
Basic information

This series of devices prevent the engine from overheating by activating radiator cooling fans, measuring the coolant temperature and controlling the level gauges and warning lights on the engine control unit.

They can have up to four terminals and be installed on the radiator, the cooling system tubes or the thermostat, so that the coolant flows across the sensing element (bimetal disc or thermistor).


Component parts and working principles

Thermo switches are activated by a concave or convex bimetal disc, which opens and closes contacts located inside them, and are divided into two categories:

- tipo circuito N/A normalmente aperto
​- tipo circuito N/C normalmente chiuso

The disc changes shape as the temperature varies.

Once the exchange temperature is reached, the heat-sensitive disc “snaps”, thus closing the electric circuit.


Main causes of failure

- Oxidation of the contacts

- Obstruction of the bimetal disc

- Thermistor malfunction

- Loss of air-tight seal, with penetration of water or moisture into the switch

- Loss of elasticity in the connector gaskets


Effects of failure

If the disc is stuck with the electrical circuit open, the switch cannot activate the electric fan, causing overheating of the liquid with the possibility of serious damage to the engine.

If the electrical circuit remains closed, the fan continues to run, causing loss of power.

If the sensor sends wrong information to the engine control unit, error messages are produced, with the risk of fuel injection regulation problems.


Facet thermo switch and temperature sensor types

Facet technology keeps pace with the evolution of thermal management, offering a wide range of solutions: from simple disc switches for electrical-mechanical fan control to the most sophisticated temperature sensors fitted with thermistors, which send temperature information and alarm signals to the engine control unit.

Thermo switches for electric fans

These are the simplest type of thermo switches. They are fitted with a concave or convex bimetal disc and are installed on the radiator or in the cooling system pipes. The bimetal disc is a sensing element that changes its state as the coolant temperatures varies. When it reaches a predetermined temperature, the disc snaps, thus closing the circuit that activates the radiator cooling fan.

There are two types of thermo switches for electric fans:

- For single-speed (single circuit) electric fans 

- For dual-speed or double electric fans (with several circuits) 

Control switch for water temperature warning light

This thermo switch, which is a more evolved version of the simple bimetal disc model, in addition to the activating the fan, also indicates when the coolant has reached a limit temperature by switching on a warning light on the vehicle instrument panel.

Water temperature sensors with warning light

These sensors contain both a thermistor and a bimetal disc; in addition to measuring the coolant temperature, they also activate a warning light on the engine control unit.

They communicate with needle gauges on the engine control unit and activate warning lights when the temperature of the liquid rises beyond a certain level.