How to Test a Fan Clutch

by Dan Ferrell
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You might not think it, but your car's belt-driven cooling fan can take a lot of power to turn. When James Watt needed a calibration for how much work his new steam engine could do, he measured how long a horse could pull a given load for a certain period of time. If a horse could plow one acre in an hour, that was "one horsepower." A mechanically driven fan can suck 10 or more horsepower from your engine when it's running -- that's a lot of Clydesdales wasted. For this reason, manufacturers long ago started using temperature-controlled clutches so that the fan only came on when the engine needed help staying cool.

Step 1

With the engine off, inspect the fan blades carefully. Make sure the blades are all in good shape and secured to the fan hub. Replace the fan if necessary.

Step 2

Look carefully around the fan clutch for signs of oil leakage. Fluid-coupling and thermostatic fan clutches are filled with silicone-based oil; if you find a leak, replace the fan clutch.

Step 3

Grab the fan assembly with your hands and move it back and forth. If the clutch or fan have excessive play, remove the fan and clutch and check if the water pump shaft -- where the fan-clutch assembly mounts -- displays excessive play. If the water pump shaft checks OK, replace the fan clutch.

Step 4

Try to spin the fan with your hands. A healthy fluid-coupling clutch should offer some resistance to fan rotation, because the silicone oil is thick and viscous. If the fan spins without resistance or is hard to rotate, replace the clutch.

Step 5

Turn on the engine and let it idle. If you have a thermostatic clutch type -- look for the thermostatic spring at the front of the clutch -- the fan should not turn until the engine reaches operating temperature. If the fan rotates when you turn on the engine, or the fan does not work after four or five minutes when the engine reaches operating temperature, replace the clutch. Keep an eye on the temperature gauge indicator on the instrument panel.

Step 6

Test the fluid-coupling fan clutch visually with the engine running. This type of fan is designed to slip at high engine speeds. Have an assistant rev up the engine. At high speeds, you should see the fan slowing down or stopping.

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