Fan Clutches. Arrrrrgh!
Many vehicles depend on a mechanical fan driven from the engine. Most have some sort of "clutch" or drive device to control the fan. This allows the fan to run at reduced speed when less airflow is needed, and to provide additional airflow when things get hot.
Over and over, we hear on this forum " the fan clutch was checked, it is ok" , or "it blows strong" . What follows is a post from another thread, which details HOW to check a fan clutch. If you are in doubt about your clutch, read this, then observe your fan operation. Then replace your clutch, and check it again.
How EXACTLY was this fan clutch "checked"?
Here in Las Vegas, I see it all the time - the vehicle does not boil the radiator, so the ASSumption is that the clutch is OK.
At high ambient temps, airflow over the condensor is crucial to good performance. Most fan clutches have 2 stages of "drive". The first stage is the "fluid coupling", which depends on a fluid inside the clutch with a viscosity that changes with temperature. The second stage involves a bimetallic spring (like in a household thermostat) To mechanically decrease the clearances in the clutch, which causes higher fan speeds.
Just like the oil in your crankcase, the fluid in the clutch breaks down with use. it "shears" the high viscosity fluid, and it becomes thin - no longer driving the fan well. This also affects the "locked" performance of the fan, since most new fans do not actually incorporate an actual mechanical lock. With the fluid thinned way down by wear and heat, it is also more prone to leak out of the clutch assembly.
The first sign of failure, is less noise - especially on cold start up. On a cool morning the fan should make a racket for a few seconds at least. If it spins with a fingertip when cold, it is shot. When the vehicle is left to idle on a hot day with the A/C on, it should "lock" the fan clutch within 3 minutes or so. Again, when you accelerate away from that long stoplight, you should hear the fan roaring.
For a more scientific test of fan speed, I use a model airplane tachometer. A simple photocell tach, available at hobby stores. The actual blade count and fan speed are not important, so no complicated math is required. Simply check the fan 'speed' at idle - the tach will give you a number (N). Now, increase engine speed to twice the idle speed used for the first test. If the fan was driven 100%, the model tach should read twice the original reading, or 2N. With a bad fan clutch, I have seen as little as 5% increase in fan speed. Let the vehicle idle with the A/C on for several minutes, so that the fan clutch should be 'locked". Measure the speed with the model tach, and then have an assistant raise the engine speed to double the idle speed. The fan should run AT LEAST 75% faster than idle. if it is only 50% faster, it is junk.
Just because a vehicle does not overheat, does not mean that the fan clutch is operating correctly.
Fan clutches are usually not terribly expensive. The damage that can be done to other systems by high operating temperatures can be very expensive. A failed fan clutch can add 50 degrees to your underhood temperatures. This kills belts, hoses, and electronics. It can cause your automatic transmission to run hotter, leading to failure.
Some manufacturers clutches are repairable. I believe Toyota still sells the viscous fluid at the dealership for renewing the clutch. If your fan clutch is bolted together, then it may be repairable. Most domestic clutches are pressed together at the factory, and can not be serviced.
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.
thanks bohica2xo very good info, what about a clutch-less fan, end of problem right?
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