There’s an inexpensive way to determine the fan speed if you want to go to the trouble yourself of buying the gadget below. On my 40+ year old Ford truck the fan speed is entirely dependent on the engine rpm, but on newer vehicles the fan speed varies depending on how much cooling is required. That’s done to improve mpg and keep the engine coolant at a more constant temperature for emissions purposes if nothing else. Some vehicles use an electric operated clutch, and others use a thermal operated clutch. Either one, the fan speed should measure lower when the engine is cold than when the engine is hot.
When did thermal fan clutches first become available on automobile engines anyway?
I see some patents from the early 80’s, but apparently it was invented before that. Not sure what year the first production vehicle came equipped with one.
“The fan clutch was invented sometime in the 1960’s and it was not until the 1970’s when it was in production fully. The primary objective for making clutch fans in vehicles was for saving engine power to get better gas mileage for the engine. In the past, when there was no fan clutches, a fixed fan was attached directly into the engine. Not only was this an energy waster, it was also a noisy contraption. The fan clutch, on the other hand, does not produce unwanted noise and saves engine power as it can disengage partially when the car stops running.”
I would put it: “…saves engine power as it disengages partially when the engine is warming up or forward velocity provides sufficient air flow through the radiator.”
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OK, so I have a different Tach. Mine sees the light between the fan blades. So by aiming at the blades while running it is counting the blades or gaps between the blades. Now since this unit is designed to see exactly 2 blades per revolution and the fan on the E-150 has 10 blades a little math needs to happen. I have calculated that at idle with the temp gauge at normal the fan is spinning at about 900 + or - 50 rpm’s. Which at idle the engine rpm is most likely about the same. So if I am understanding the thoughts correctly when the rpm’s increase to travel speed the fan rpm’s should increase as well. But it will never exceed the rpm’s of the engine.
The fan speed will increase to the rpm’s of the engine only when the clutch gets hot enough to fully engage the fan or somewhere in between.
I ran the engine rpm’'s up and the fan speed remained at about 900 + or - 50 rpm’s.
So I can only “assume” that the fan clutch is doing what it is designed to do.
Honesty I would feel better if the fax rpm’s were considerably lower when the engine is cold. But I am guessing the mechanism has some internal resistance and therefore the shaft speed passes through to the fan itself.
FWIW it is a bit of a mystery to me how the fan clutch senses the temperature so that it can engage or disengage depending on engine temperature. Or is it picking up the temperature of the air passing through the radiator?? If yes, then if the radiator is super efficient then it would never engage. If the radiator is less than efficient then it would engage all the time. Seems to me it would need to be a delicate balance as well as some excellent serious mathematical engineering, even for Ford.
Les
Even if the fan clutch were 100% engaged, the fan rpm wouldn’t necessarily be the same as the engine rpm. It would depend on the pulley gearing. From what I can see the proper test is that the fan – if the clutch is working – shouldn’t turn faster than 1500 rpm when the water pump pulley is turning at 3000 rpm. The engine rpm (crankshaft pulley) would be about 2300 rpm when the water pump pulley is 3000 rpm. This info for the 2005 4.6L, may differ for other model years and engines.
With the engine cold (and turned off) you should be able to spin the fan by hand and note only a light resistance to turning. No resistance or very high resistance indicates a problem.
You can look up how it works on the internet, try Wikipedia. I presume inside the fan clutch there’s a spring that presses more forcefully or less forcefully depending on the air temperature moving past it. There’s various ways they could use to make that happen, could be like a thermostatic spring gadget use in carburetors choke systems, or could be the expanding wax method used in vehicle thermostats. From what I can see the way to get the clutch as engaged as possible is to run the coolant at 205 degrees F, block off the sides of the radiator and in front of the radiator grill, AC at MAX, Blower High, with the water pump at 3000, the fan should be 2300 or higher.