How to Tell If Your Fan Clutch Is Going Bad

engine cooling fan with fan clutch
A new radiator fan and thermal fan clutch, ready to keep your engine cool.

Panoha / Wikimedia Commons

Nearly unheard of on modern cars with electric engine cooling fans, people sometimes still refer to the accessory drive belts as “fan belts.” This term harkens to the day when cars and trucks used drive belts to drive the cooling fan or water pump. The engine cooling fan, made of plastic or metal, isn’t hard-connected to the fan pulley or water pump, but instead is soft-connected via the fan clutch. A properly-functioning fan clutch is critical to engine reliability, as it works to maintain a safe temperature range. A failing fan clutch could leave you stranded with expensive repair bills, so pay attention to these symptoms.

1989 Toyota 4.2-liter Turbodiesel
Here, you can see where the fan and fan clutch are driven by the engine. Morio / Wikimedia Commons

Fan Clutch Types

There are three types of fan clutch, depending on the vehicle’s design. A small amount of resistance always keeps the fan spinning, but it’s basically free-wheeling until the fan clutch engages. When the fan clutch engages, it spins up greatly, increasing air flow through the radiator and over the engine.

  1. A thermal fan clutch reacts to temperature of the air rushing over it from the radiator. As the radiator heats up, the air passing through it also heats up. This heats up a small bi-metallic coil in the face of the fan clutch, releasing an internal valve. The valve releases heavy silicone fluid, which locks the fan blades to the fan pulley.
  2. A torque-limiting fan clutch reacts to engine speed. A centrifugal valve opens to allow the flow of heavy silicone fluid, locking the fan blades to the pulley. At idle and low engine speeds, this fan clutch is fully engaged, gradually disengaging as engine speed increases. Depending on the application, it may freewheel above 2,500 to 3,000 rpm.
  3. An electronic fan clutch functions similar to thermal and torque-limiting types but isn’t modulated directly by temperature or speed. 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.

    Symptoms of a Bad Fan Clutch

    If the fan clutch is going bad or has failed, there are a couple of ways you may notice. Paying attention to your vehicle is the first step in an accurate diagnosis, effective repair, and reliable vehicle.

    1. Engine overheating at low speed or when stopped is the most-common fan clutch failure symptom. When cruising, the vehicle moving through the air pushes air through the radiator, cooling the engine. At a stop, though, because the fan clutch doesn’t engage and air isn’t forced through the radiator, the engine can’t dissipate excess heat.
    2. Insufficient heat in winter is another common problem but caused by the opposite fan clutch failure. If the fan clutch seizes, it remains engaged all the time, cooling off the engine too much.
    3. Excess fan noise at high speed is caused by the same problem, a seized fan clutch. This can cause bearing damage, radiator damage if the blades flex too far, or even shatter a plastic fan.
    4. Fan spins after engine shut-down might indicate a weak clutch. Silicone fluid leaking from the fan clutch would cause this problem.

    How to Check the Fan Clutch

    When the engine is off, there are a few things you can do to check the fan clutch:

    1. Spin the fan. Electronic types might freewheel, but most fan clutches should spin with a little effort. If the fan spins more than three times, you may have a bad fan clutch. If the fan doesn’t spin at all, the bearing may be seized.
    2. Wiggle the fan. Try moving the fan forward and backward. If it wobbles more than a quarter-inch or you feel it clicking, this may indicate a worn bearing.
    3. Inspect for leaks. The key to fan clutch function is its heavy silicone oil, whose viscous properties allow for soft engagement to drive the fan. If it leaks out, the clutch will weaken and eventually fail.
    4. Check fan speed. You should be able to audibly detect when the fan clutch engages, with an accompanying rush of air. At idle, as engine temperature increases, a thermal fan clutch will engage at a certain temperature. Torque-limiting fan clutches will disengage at a certain engine speed.
      1. Alternatively, an optical tachometer can be used to measure fan speed. As engine temperature increases, look for a sudden jump in fan speed.
    1. Scan tool. Some electronic testing and knowledge of a scan tool may be required to diagnose electronic fan clutches and the sensors used to determine fan engagement.

    The internal combustion engine generates a lot of heat, which the engine cooling system is tasked with moderating. A little heat is a good thing, improving fuel vaporization, performance, and fuel economy – it also warms up the cabin in the winter.

    On the other hand, too much heat can be a big problem, melting non-metal engine parts and rendering lubricating oil useless. An overheating engine could literally weld itself into scrap metal.

    If your fan clutch has gone bad, replace it as soon as possible to prevent engine damage. While you’re at it, consider the performance and fuel economy benefits of upgrading to electric cooling fans.