At idle, as engine temperature increases, a thermal fan clutch will engage at a certain temperature. You should be able to audibly detect when the fan clutch engages, with an accompanying rush of air. If it leaks out, the clutch will weaken and eventually fail. The key to fan clutch function is its heavy silicone oil, whose viscous properties allow for soft engagement to drive the fan. If it wobbles more than a quarter-inch or you feel it clicking, this may indicate a worn bearing. If the fan doesn’t spin at all, the bearing may be seized. If the fan spins more than three times, you may have a bad fan clutch. Electronic types might freewheel, but most fan clutches should spin with a little effort. Instead, the engine control module (ECM) or fan control module uses various sensors to determine when to engage the fan clutch, such as at engine idle, low vehicle speed, air conditioning compressor engagement, or above a certain engine temperature threshold. An electronic fan clutch functions similar to thermal and torque-limiting types but isn’t modulated directly by temperature or speed.Depending on the application, it may freewheel above 2,500 to 3,000 rpm. At idle and low engine speeds, this fan clutch is fully engaged, gradually disengaging as engine speed increases. A centrifugal valve opens to allow the flow of heavy silicone fluid, locking the fan blades to the pulley. A torque-limiting fan clutch reacts to engine speed.The valve releases heavy silicone fluid, which locks the fan blades to the fan pulley. This heats up a small bi-metallic coil in the face of the fan clutch, releasing an internal valve. As the radiator heats up, the air passing through it also heats up. A thermal fan clutch reacts to temperature of the air rushing over it from the radiator.
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