Why does fan cycle?

For a large variety of reasons, one wants the engine to operate above 212ºF. Water evaporates out of the oil, ZDDP in the oil forms the protective film, acid neutralizers work better, the oil cleans the piston rings better, certain substances in the antifreeze protect better at elevated temperatures, engine performs better,...

About the ONLY thing the engine wants at a temperature lower than 212ºF is the intake air, and here lower is better with 0ºF being ideal.

One could (COULD) make a fan that runs continuously, here one would want a variable speed control and run the fan such that the fan comes on at 212ºF, and then changes its RPMs based on keeping the water at 212ºF. Variable speed fans are significantly more expensive than one speed fans plus a thermostat. So, it is cheaper as currently designed.

I have thought about wiring in a circuit that spins the fan just below audible levels for city driving in Texas summer heat. A switch and a 10 ohm resistor should about do the trick.

 

Yeah what he says the engine needs to be at temp


I use the Flex-a-lite 33054 controllers.
Reckon it would be easy to wire up and program a simple kiln logic controller with a SS relay that would be far better

 

Once the LH fan comes on, how long does it stay on for? And how long before it comes on again?

 

The radiator thermo-switch monitors the coolant temperature after the coolant has passed (been cooled down) through the radiator, not the engine temperature, thus you cannot monitor the switch "on" and "off" temperature points by looking at the engine temperature. The engine temperatures will be approx. 10F higher than the temperatures at the outlet of the radiator that the thermo-switch sees.

 

such a simple concept that I wasn't grasping, thank you for putting it so succinctly! I just replaced my RH radiator on my 348 due to a small leak; I also went ahead and installed a new radiator fan thermo switch. Upon idling in the garage to validate the repair I found that my fans switched on at a higher temp (on the temperature gauge in the dash) than I expected based on the info in the WSM. Your explanation makes sense now and it's reassuring to know that it sure looks like my cooling system is doing what it should be doing. That's why Fchat is so valuable.

 

As I understand, you noticed that the new thermo-switch comes on at a little higher temp than the old one. If this is the case, it is nothing to worry about; the new thermo-switches have been "upgraded" to (radiator outlet temp) 88C on, 82C off as compared to the original 85C on, 76C off. The new switches come on at just 3C higher temp than original which is not an issue at all. The big advantage of the new switches is at the "off" point - 82C compared with 76C. The "off" point of the original switch at 76C was (unnecessarily) too low and the fans would take ages to go off. With the new range 88-82, the fans cycle nicely. I also replaced the thermo-switch on my 348 with the new type about a year ago - much better.

 

This can be done easily and fairly cheap at the cost of a happy meal and maybe some electrical knowledge. I don't know what motor inside the fan but rumors circulating on Fchat suggests it's a D.C. Motor. Until I see one, it remains a "rumor".

Speed control of a D.C. Motor can be done by varying the net electrical power supplying the motor. It's a combination of volts and amps rated in watts. One method to slow or speed up a D.C. Motor is to lower or raise the voltage. Increasing or decreasing the desired voltage with respect to the supply voltage for the purpose of speed control does require addition electrical devices. It can be costly and the efficiency factor drops. For this and other reasons, manipulating the supply voltage is not economically sound.

Another way to obtain speed control of a D.C. Motor is to limit the current to the motor. Technically speaking a fan can run all day at different speed depending on the temperature. Now the saying goes, if something is not broken why fix it. It would be interesting to see how many Fchatters want that for their car. With enough demands, I can get the supply. Time and economic incentive is at play.