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The illustration above depicts an independent fan drive control system. The electrically controlled solenoid valve engages and disengages the fan drive by regulating its air pressure on and off.

The electrical circuit to the solenoid valve contains three switches. Any of these three switches may activate the solenoid and engage the fan drive:

• a thermal switch sensing the engine coolant temperature.
  
  
• a Refrigerant Pressure Switch in the vehicle's air conditioning system.
  
  
• an Optional Manual Override Switch located on the vehicle's dashboard.
  

The DriveMaster fan drive is spring-engaged and disengages when air pressure is applied.

EXAMPLE. When the engine coolant temperature rises above the thermal switch's set point, the thermal switch activates the solenoid valve, which shuts off air pressure to the fan drive (supplied by the vehicle's air reservoir) and engages the fan.

Most newer vehicles are equipped with an Electronic Control Module (ECM), which is a small computer that monitors and controls all engine operations, including the fan drive. If a vehicle uses an ECM, there is no direct connection between the sensors and the solenoid valve for the fan drive. Information from sensors and switches is fed into the ECM which controls the solenoid valve.

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