What is the reason for the car fan not rotating at low speed but rotating at high speed?
3 Answers
Car fan not rotating at low speed but rotating at high speed may be due to a damaged thermostat, abnormal circulation of the vehicle's cooling system, causing the temperature control switch to malfunction, which can also lead to this situation. The last possible cause is a faulty engine coolant temperature sensor. Below are additional details: Principle: When the coolant temperature is below 70 degrees, the thermostat remains closed. At this point, the engine's cooling system only has a small circulation loop, without a large circulation loop. This means the water from the engine's outlet pipe passes through a small bypass pipe directly into the cylinder head for internal circulation. Specific reasons: At this stage, the coolant does not pass through the radiator. As the temperature continues to rise to nearly 80 degrees, the thermostat gradually opens until it is fully open. Then, the engine's coolant flows through the main outlet pipe to the radiator before entering the cylinder head for large circulation. If the engine temperature continues to rise, the temperature-sensitive switch installed on the thermostat cover or radiator will activate the fan's low-speed switch, causing the fan to rotate at low speed and draw air from front to back for cooling.
I've encountered the issue where the car fan doesn't spin at low speed but works at high speed before, and I think it's quite common. Usually, it's because the fan's relay or resistor module is faulty. At low speed, it's needed to control the current, but if the resistor fails or gets clogged with dust, the fan won't spin; at high speed, the circuit supplies power directly, so it works normally. Another possibility is that the temperature sensor isn't sensitive enough and only triggers the high-speed mode at very high temperatures. If this issue is ignored, the engine might overheat and get damaged, and fuel consumption could also increase. I suggest you go to a repair shop as soon as possible for a check-up—they can pinpoint the problem with their tools. Regularly cleaning the dust around the fan can prevent such failures. Don't delay; safety comes first, or the repair could end up being more expensive.
As a veteran driver with 20 years of experience, I've found that when a fan stops working at low speeds but operates normally at high speeds, it's mostly due to electrical issues. The fan motor relies on a resistor to reduce speed in low gear. If that resistor ages, wears out, or oxidizes, the low-speed circuit gets disconnected while the high-speed setting continues to function. Loose electrical connections or burnt relay contacts are also major culprits. This reduces cooling efficiency, and prolonged operation can cause engine temperatures to skyrocket, potentially leading to cylinder explosion. Let me remind you: if you notice any fan abnormalities, don't push your luck—shut off the engine immediately and seek professional help. Trying to save on repair costs now could lead to much bigger troubles down the road.
