Engine Coolant Fan Switch


As the name implies, these switches activate several parts of the cooling system, according to the temperature of the engine.



  • Radiator fan: as it is activated, it forces the passage of air through the radiator, assisting with the coolant temperature.
  • Warning lamp and alarms: as they are activated, they inform possible overheating in the cooling system.


Operation Principle

The main component of the engine coolant fan switch (or thermoswitch) is the bimetal disc. Formed by two metals with different thermal expansion coefficients, the bimetal disk is calibrated to snap at a specific temperature. This deformation causes the pin displacement, which activates the on-off silver contacts (Fig. 1).


Important: Some vehicle models use a temperature sender in conjunction with a thermoswitch – in a system called Duplex. This sensor not only activates the gauge that informs the coolant temperature increase, but also informs the temperature excess to the instrument panel (via warning lights or alarms). (For more information, please refer to the Temperature Sensor article.)



When used to activate the fan, the thermoswitch will be located in the vehicle’s radiator. When used to activate a warning lamp or alarm, it might be located near the thermostat or at many other places in the engine.



  • Activates the fan when the coolant temperature in the radiator exceeds the specified limit. This usually occurs when the vehicle is operating in heavy traffic.
  • Activates a warning lamp on the vehicle’s instrument panel or a sound alarm.


When It Does Not Work

  • The temperature of the coolant in the radiator increases so much that it boils, causing the engine to overheat, burning and deforming the cylinder head gasket, damaging hoses, causing the coolant to leak, among other consequences.



  1. The switch does not activate – it is inoperative.
  2. The switch activates out of the specified temperature range.


In both cases, the diagnosis can be made in the manufacture site, with the use of specific test equipment.


Activation Temperatures

The activation temperature is indicated in all product codes.

  • Switch of one temperature with one terminal: MTE-THOMSON Number: 3046.115; ON at 115 °C.
  • Switch of one temperature with two terminals: MTE-THOMSON Number: 705.92/87; ON at 92 °C and OFF at 87 °C.
  • Switch of two temperatures: MTE-THOMSON Number: 717.95-102; ON at 95 °C (activating the first fan speed) and then at 102 °C (activating the second fan speed). Used in vehicles equipped with an A/C system.


For temperature information in Fahrenheit please refer MTE-THOMSON catalog



Important actions when changing the fan switch:

  • Avoid excessive tightening.
  • Bleed air out of the cooling system (remove air bubbles).



  • Make sure the  temperature switch is appropriate for each vehicle model.
  • Never perform maintenance while the cooling system is still hot, due to high risk of burns.
  • At any evidence of overheating, park the vehicle in a safe place and turn off the engine immediately, otherwise you may damage the engine.
  • Check the coolant level weekly, with a cold engine.
  • Always use the specified coolant for your vehicle at the proper rate.
  • Do not fill up the coolant reservoir with plain water, since this dilutes the coolant concentration.
  • Any reduction in the coolant level may indicate a leak in the cooling system. 
  • Perform the temperature sensor preventive maintenance every 30,000 km.



All MTE-THOMSON products are guaranteed for 01 (one) year against faulty materials or manufacturing defects. The guarantee is limited to the replacement of the faulty part; it cannot be extended to cover defects caused by misuse, neglect, accident, or wear and tear. We cannot accept liability for consequential loss or damage which is claimed to have resulted from the use of one of our products. More information at:


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