The thermostatic fan clutch is a very nice invention....when it works right. Most of the time it does. When it fails the symptoms may vary from overheating only at idle or highway speeds to excessive fan noise or “cavitation”. It can also destroy a perfectly good water pump.
Theory of Operation. The clutch is, essentially, a torque converter, very similar to the converter in an automatic transmission. Prior to itsan. Fast engine rpms, fast fan. The problem with this is, at idle the fan barely moved enough air to cool the engine. At high rpms the fan would cavitate. That is, it would create a partial vacuum in the wake of the blades. This would cause an annoying “Roar”. Not to mention the aerodynamic drag that robs horse power. At 4000 rpm a large fan could take 10 or 15 horse power right of the top of available energy. The thermostatic fan clutch allows the fan to slip when the engine is cold and spin faster when the engine is hot. The problem that had to be overcome with a viscous fluid clutch is that the fluid becomes less viscous as it heats and, thus, lets the clutch slip more. Exactly the opposite of what you want. To overcome this, all clutches have an external bi-metalic coil spring that, proportionally to temperature, opens or closes a bypass valve in the viscous fluid circuit.
Symptoms of Malfunction. Symptoms can vary widely. If the clutch has lost fluid it may slip excessively. It may allow overheating at low rpms but allow adequate cooling at highway speeds. It may do just the opposite. Or allow over heating under any driving condition. Parts inside may come into contact after heating up and over spin the fan generating excess noise. Look for oil on the outside of the clutch body. Grab the fan (with the engine off!) and move it side to side while looking for excessive moings. Rotate the fan by hand. It should turn fry to give it a good spin. If it rotates more than half a turn it may have lost fluid. If you have an overheat problem that defies diagnosis, it may be the thermostatic clutch. Something else to think about. The clutch is “stack mounted" on the water pump. If the clutch is bad, and out of balance, it can and often will cause premature failure of the water pump bearings. Considering the clutch is often between $30 and $60, and you have a mysterious overheat problem, it is worth the price to replace. Your water pump may thank you.
Replacing The Clutch. Replacement is easy. Remove the black plastic cowling from around the fan. There is the one bracket on top and several metal clips securing the bottom. Hang on to the clips. You'll have to jockey the cowling around to get it past the fan. Next remove the four 10mm nuts holding the clutch in place.
To reach the nuts on the side hardest to reach, have the car in 5th gear with the parking brake off. Push the car with your knees until the clutch rotates around to where you can reach the nuts. Be careful not to disturb the pulley, however. The pulley is held on by the same nuts. If the pulley falls off you may have to loosen the fan belt to get it back together. Once the clutch is off reverse the order of disassembly to install the new one. Put this metal clips back on the bottom. The top edge of the cowling hooks onto a groove on top.