How to Test For a Worn Fan Clutch

This is the documentation that came with the new fan clutch I recently had to replace. Remember that these words are not mine - they come from the manufacturer of the fan clutch (Amgauge - Arlington TX 76011)

1. Excessive fan clutch play. Manually move the tip of the fan back and forth. If there is more than 1/2" play the clutch bearing is worn. A new fan clutch should be installed.
2. Greasy buildup. Look for greasy dirt on the fan clutch or a fluid leak around the outer seal or thermostat (spring). If any visible fluid leakage is noticed, a new fan clutch should be installed.
3. Excessive slippage. A hot engine in the 'off' position which allows easy revolving manually, from 1 to 1.5 revolutions of the fan indicates a weak fan clutch. A new fan clutch should be installed.
4. Freezing up. If the fan blade can not be rotated by hand, or if hand rotation makes a harsh, rumbling noise, the fan clutch bearing has failed and a new fan clutch should be installed.
5. Vibration. Any vibration along the water pump shaft means a worn bearing in the water pump or fan clutch. The fan clutch should be carefully checked when a water pump is replaced. If the water pump is replaced, at the same time a new fan clutch should be installed.

The fan clutch is designed to provide limited fan movement when the engine does not require cooling, therefore saving fuel and reducing fan noise. The graph shows how fan speed increases in direct proportion to RPM's, whereas with a fan clutch the engine still generates fixed RPM's while the fan drives at a lower speed such as highway driving, cooling effect from the fan is not required due to the air flow through the radiator cooling fins.