A frightening test from way back in the day was to use a piece of soft wood, like white pine. You were supposed to "gently apply" light pressure to the back of the fan blade with the engine idling on a hot engine. The fan blade was supposed to chip away pieces of the wood and continue turning. If the wood easily stopped the blade the clutch was shot. The good old days of analog testing! Pretty sure this test was from Gates, Dayco or possibly Hayden. Been many decades. My test is on a hot engine, 200F+. Gently raise the RPM's, the fan should turn close to engine speed and sound like a wind tunnel. Shut engine off and turn by hand. It should turn pretty tight.
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