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Mechanical Cooling Fan Clutch Diagnosis

Cooling Fan (mechanical)

Last week, we talked about electric cooling fans. Today, we're going to look at mechanical cooling fans. Most have a clutch to engage and disengage them as well as for reducing their noise and vibration during operation. Most vehicles that do not have an electric cooling fan system will have a clutched mechanical fan.

Some mechanical cooling fans are run from the accessory belt on the front of the engine while others will be directly driven by the timing belt. In the old days, the cooling fans ran whenever the engine turned, no exceptions, and were generally inefficient. They had a large parasitic draw, often at more than 10 horsepower. Now, with clutches and better engineering, these fans can draw little or no power from the engine during most of the engine's operation.

How the Clutch Works

Before we can diagnose a faulty clutch, we'll need to know how it works. There are two types of fan clutches in common use. One is thermally-activated and the other is not. Non-thermal clutches will usually activate or de-activate based entirely on engine RPM. If the engine is turning at about idle speed (600-1,000 RPM on most gasoline applications), the fan will engage. When RPM raise above that point (or somewhere near), the fan disengages as it's assumed that the vehicle's movement is providing enough air for the radiator to cool.

A thermal fan will be activated by the temperature of the air immediately surrounding the fan clutch. Since this is usually directly behind the radiator and ahead o the engine block, that location is often the hottest part of the engine bay. The clutch has a bi-metal coil spring that is temperature-sensitive. When the ambient temperature reaches a certain point, usually 200-230 Fahrenheit, the spring opens, allowing a small fluid reservoir within the clutch to be released into the fan's clutch to separate the clutch and disengage it. When the fluid is cool, it retracts into the reservoir, causing the fan to slow down or stop moving from engine power.

Note: when working with the cooling fan, the engine should be off and the key out of the ignition. Otherwise, serious injury could occur.

Common Problems With Fan Clutches

When engines overheat, it could be a number of things causing the issue. The fan clutch is relatively easy to rule out if a quick check is made.

Clutch slippage is the most common problem and can be diagnosed with a couple of quick tests. Start by pulling the fan (evenly on two opposing sides) away from the engine to attempt to separate the clutch. If it does not release and the vehicle makes excess noise at highway speeds, you likely have a bound clutch.

Turning the fan by hand should come only with stiff resistance. Allow up to half a turn without much resistance, but any more than that and you may have a problem. If it won't turn at all, you may have a bind. If no resistance is had (or if it's only nominal), you probably have a slipping clutch.

Finally, if there is any front-to-back wobble in the fan at all, you should replace bearings, as they have likely gone bad, which is an early indication of failure.

Another trick is to mark one edge of the fan's blade and, using all safety precautions, starting the engine and utilizing a timing light to check the spin rate of the fan under known RPM loads.