A fan clutch is a heat-sensitive device that controls the cooling fan on an engine. Most vehicles today rely on an electric cooling fan assembly mounted to the back of the radiator. But older vehicles featured a fan that was mounted to the engine. In between the cooling fan and the engine was a fan clutch that allowed the speed of the fan to match closely with that of the engine.
The goal of a cooling fan clutch is to ensure that the fan only runs when it is needed. An engine should operate within a specific temperature range. The cooling fan, whether on a modern vehicle or older engine, helps to draw air through the radiator and ultimately cool down the engine. When the fan is not needed, a computer module shuts it off (in a modern vehicle) or the fan clutch prevents it from spinning (in an older vehicle).
If your engine runs below or above normal operating temperatures, the fan clutch could be the culprit. That is because the clutch responds to heat from the engine to speed up or slow down as needed. If the clutch is not responding as it should, it will either cause the engine to cool down too much or overheat.
A fan clutch that seizes up will remain engaged. You might hear a loud roar from underneath the hood as the fan picks up excessive speed. Your engine might feel as though it lacks power. When the engine is off, you might notice that the fan will not turn freely. Conversely, a failing fan might not spin at all - or spin very slowly - when the engine is running.