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You're right that there are two fans, but your description of when they turn on is incorrect.
Whenever the fans are supposed to be running, it should always be both of them either on high speed or low speed. If ever only one is running, there is a problem with the fan motor, or the wiring. (This is for US vehicles - some markets only have 1 fan and as such they behave differently)
The fans are controlled by either the refrigerant pressure, or the engine coolant temperature.
Refrigerant pressure -
Low speed will come on when the refrigerant pressure reaches 9 Bar (130 psi). They will switch to high speed when the refrigerant pressure reaches 18 Bar (260 psi).
Coolant temperature -
Low speed will come on when the coolant temperature is 100°C (212°F), they will switch to high speed when the coolant reaches 113°C (235°F).
When the fans are running on low speed, the two fans are connected in series with each other. Since they are in series, each fan does not see the full 14 volts. They each see half the total voltage, and as such they run at a reduced speed. If one fan motor is broken, neither fan will come on since the complete circuit is open.
When high speed kicks in, both fans are connected in parallel with each other. Now they both see the full 14 volts and run at full speed. If one fan motor is broken, the other fan will still run.
fusionrx - when the car is idling, do the fans run at all? Low speed is pretty quiet, but high speed is much louder so it should be easy to tell which speed is running.
Michael
posted by 65.30.16...
, Wed, 11 Aug 2010 08:38:29, Tue, 10 Aug 2010 16:33:00
, Tue, 10 Aug 2010 17:18:11
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