It's interesting how many people are checking their fan clutch by spinning it by hand with the engine off. This is meaningless. It has to be checked with the engine running and at operating temp, and preferably on a warm summer day to ensure the bimetal strip on the clutch is heated. There are basically 3 valid tests:


1) The "roar" test: With the tranny in Park, raise the hood, and slowly rev the engine to approx 3000rpm. The fan should be roaring loudly. If you're not sure, it's probably NOT working correctly. Compare to when the engine is cool (not warmed up yet), on a cool day, to see what it sounds like when the clutch is not engaged. The clutch should disenage by 3500-4000rpm, this is also easy to hear, and when you let the RPM's slowly drop back to ~3000, you should hear the clutch re-engage. The FSM specifies the "roar" test, btw.

2) The "stop" test: When the clutch is engaged, and you shut off the engine, the fan blade should not freewheel or spin at all. It should stop immediately (as shown in this video). If it spins at all (or worse, if it spins for 5-10seconds before stopping!) the clutch was not engaged, and is likely dead. A second person is needed for this test, they turn the key off while you observe the fan blade.

3) The "tach" test: Using an optical tachometer ($30 on sale from Harbor Freight, click here) with reflective tape on the fan blade, and also the drive pulley (not possible on all engines), you can determine if the fan blade RPM is increasing proportionally with engine RPM. If the fan blade remains at a steady speed (300-600rpm or so) as engine RPM increases to 3000rpm, the clutch is not engaged. Some data from my M119 is below, other engines will vary a bit since the crank/fan pulley ratio may not be the same, but you get the idea:

Car #1 - Bad Clutch:
With the engine at ~650rpm idle in park, pulley speed was ~810rpm, actual fan blade speed was ~275rpm (only ~35% lockup).
With the engine at 1500rpm idle in park, pulley speed was ~1900rpm, actual fan blade speed was ~500rpm (only ~25% lockup).
With the engine at 2500rpm idle in park, pulley speed was ~3200rpm, actual fan blade speed was ~500rpm (only ~15% lockup).


Car #2 - Good clutch, NOT engaged:
With the engine at ~650rpm idle in park, pulley speed was ~800rpm, actual fan blade speed was ~325rpm (~40% lockup).
With the engine at 1500rpm idle in park, pulley speed was ~1900rpm, actual fan blade speed was ~410rpm (~22% lockup).
With the engine at 2500rpm idle in park, pulley speed was ~3100rpm, actual fan blade speed was ~980rpm (~32% lockup).


Car #3 - Good clutch, engaged:
With the engine at ~650rpm idle in park, pulley speed was ~800rpm, actual fan blade speed was ~775rpm (97% lockup).
With the engine at 1500rpm idle in park, pulley speed was ~1900rpm, actual fan blade speed was ~1750rpm (92% lockup).
With the engine at 2500rpm idle in park, pulley speed was ~3200rpm, actual fan blade speed was ~2600rpm (81% lockup).

YMMV, etc...

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