Engine Coolant Fan Switch


They are as the name said switches that activate several parts of the cooling system, according to the engine’s temperature.



  • Radiator Fan: activates the radiator fan, forcing the passage of air through the radiator, assisting on the coolant liquid’s temperature.
  • Warning lamp – Alarm: activates a warning lamp or alarm, informing a possible overheating in the cooling system.


Operation Principle

The main component of the engine coolant fan switch (or thermoswitch) is the bimetallic disc. Formed by two metals with different thermal expansion coefficients, the bimetallic disk is calibrated to snap at a specific temperature. This deformation causes the pin displacement that closes the silver contacts. (ON–OFF). (Fig.1).
Important: Some vehicle models use a Thermoswitch in conjunction with a temperature sender, MTE-THOMSON calls DUPLEX. This switch not only informs the temperature excess to the car panel (warning lamp or alarm) but also activates the gauge that informs the coolant temperature situatio. (For more information please refer to the Temperature Sensor chapter).



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 many other places in the engine.



Used to:

  • Activate the fan when the coolant’s temperature in the radiator exceeds the specified limit. This usually occurs with the vehicle operating in heavy traffic.
  • Activate a warning, which can be either a warning lamp in the vehicle’s instrument panel or a sound alarm.


When it does not work.

  • The coolant’s temperature in the radiator increases so much that it boils, causing the engine’s overheating, burning and deforming the cylinder’s head gasket, damaging to hoses, leaking of coolant liquid, etc.



  1. The switch does not activate – Inoperative.
  2. The switch activates out of the specified temperature.

In both cases, the diagnostic can be performed in the factory, with the use of specific test equipment.


Activation Temperatures

There is an indication of the activation temperature in all product codes.

  • Switch of 01 (one) temperature with 01 terminal: MTE Number: 3046.115: ON at 115°C.
  • Switch of 01 (one) temperature with 02 terminals: MTE Number: 705.92/87: ON at 92°C and OFF at 87°C.
  • Switch of 02 (two) temperatures: MTE Number: 717.95-102: ON at 95°C the fan first speed and ON at 102°C the fan second speed.
    Used on vehicles equipped with an A/C.







Important actions when changing the fan switch:

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



  • Always check the right temperature sensor for each vehicle model.
  • Never perform maintenance repair while the cooling system is still hot. There is a high risk of burns.
  • At any evidence of excessive temperature, 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 the cold engine.
  • Always use the specified coolant and the correct rate for your vehicle.
  • Do not fill the coolant reservoir only with plain water, because this will dilute the coolant concentration.
  • Any reduction in the coolant level may indicate a leaking in the cooling system.
  • Perform the preventive thermostats maintenance every 30.000 Km.



All MTE-THOMSON products are guaranteed for 01(one) year against defects from faulty materials or fabrication. The guarantee is limited to the replacement of the faulty part; we cannot extend the guarantee 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:


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