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Usually, is due to the temperature Sensor switch, is the 1 only responsible to send signal to activate the power relay to on/off the fan to cool down the radiator. The temperature sensor switch is actually a pin made of copper, the theory of this pin is simply contact with the radiator water, when the water temperature rise up, the copper sensor expand and made internal current connect and send signal (-12v) to the power relay and activate the fan, once the water temperature has go lower the copper sensor contract, internal connection disconnect and signal cut off, fan off. so, it is very likely the copper sensor expand n contraction is not function accurate, that is why it take longer time to disconnect current for the fan to cut off.
Power relay. in some circumstance, inside relay coil jam and cause undiscontact of current. but is also quit unlikely.
thermostater. This device is actually a valve install inside 1 part of the water vent. the the main function of this device is to help build temperatue during winter. engine usually need to heat up to reduce piston friction. so is the same paincipal apply, thermostarter were block radiator water for circulation before the temperature build up, and it were expand for radiator water to circulation after the temperature has build up. (Heat expand, cool contration) no current apply.
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