ACTION NAPA AUTO Parts - 18

Mechanical fan clutch diagnosis

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MACS

By Steve Schaeber, MACS Technical Editor
he engine cooling system is one of the ﬁrst and most basic
of automotive systems that are taught in tech school, and
rightly so. It's also one of the most frequently serviced
systems in any vehicle. An often overlooked cause of
overheating issues could begin with faulty engine driven
thermostatic cooling fan clutches. Here's a great way to help diagnose
these problems.
We all know the normal ways to test these devices. Making sure
the engine is turned off (and can't be started), grab hold of one of the
fan blades, and try turning it by hand. You should feel resistance,
but the fan should turn in both directions without freewheeling, and
deﬁnitely should not be locked up. Next, check for wobble and end
play by trying to move the fan blades towards and away from the
engine. There should be no movement in this direction. Excessive
wobble, freewheel or lock up indicates a likely faulty clutch in need
of replacement.
Here's another test you could try, but you're going to have to get
yourself a handheld non-contact tachometer. They are relatively inexpensive, easy to use, and with enough practice and testing of many
vehicles, can quickly let you know if there's an issue with the fan
you've encountered. Place a piece of reﬂective tape onto the end of

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Figure 2: The red indicator light helps line up
the tachometer.
Figure 1: Reﬂective tape on the blade is used
to count fan RPM.
one of the fan blades (See Figure 1). Make sure you clean the blade
ﬁrst so the reﬂective tape will stick and not ﬂy off. Next, start the
vehicle, and get ready to measure. Hold your tachometer in such a
position that you can get it to shine down onto that reﬂective tape.
Then, you'll see the fan blade's rotational speed displayed on the
tool in RPMs (See Figure 2). Through my own testing, I've found
that a properly operating fan should spin somewhere around 1100
rpm when idling at normal operating temperature. However, I've
also found a defective fan clutch that would spin at around 550 rpm.
That's a pretty big difference from normal, but not unexpected on
a 15-year-old vehicle. By the way, I did feel some wobble in the fan
clutch shaft, but the rotational resistance really did feel ﬁne. In fact,

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ACTION/NAPA * Spring 2015

had I not measured the rotational speed and compared it to a known
good, I would not have condemned this clutch as a contributor to the
overheating.
Just how fast is a mechanical fan supposed to spin? Ever try looking up the speciﬁcations for this? It's not a commonly published spec,
but I have heard of this being mentioned once or twice before. If you
know of one, please let me know. I'd love to check it out (and mention
it here in Cooling Corner). Otherwise, what I've found works best
is practice. Using this technique on several different vehicles gives
me a "database" of my own that I can reference. Now sure, I'm not
going to ﬁnd the same readings on every one, most likely because
of the differences in fan and pulley diameters, belt length, number
of pulleys and base engine rpm speed differences amongst different
vehicles. But generally speaking, you can ﬁgure out an average for
your own purposes. ❆
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