One of the many projects I worked on in the Scion, was to utilize the "low speed" resistor as a method to keep the car cooler when it's hot outside, so I simply "tapped" off the AC switch button on the dash to run a relay that grounded the fan thru the low speed resistor - every time the AC was used - or when I was stuck in traffic on a hot day.
This bypass had allowed the AC compressor, then - to work on HIGH Speed when the compressor needed to be on to push the AC to cool the car. The engine took care of the idle up loading as the compressor loaded down the engine - then the low-speed fan side ran so the noise was quieter but I did not need to worry about an overheat condition.
So the fan ran, on two settings all the time, but only high-idle, when the AC was on, this helped me and the car both as needed else the Fan only ran to cool the radiator if things got too hot.
Ok, to add a little more.
Many would balk at these mods, and so they should, for it does change the economy of the vehicle in specific ways.
Firstly, many old-schooled mechanics would have run a cooler thermostat - and many older engines offered this as a way to prolong the life of the motor and also for the environment it was used in - it was treated as if you weren't in the artic you didn't need the hottest thermostat available to use, put in there. For it really did not need all that heat stored in the block to develop efficiency - the hotter the engine - the better the efficiency equation, only works so far.
Each motor-class is different, we're discussing Gas powered so that means the number of cylinders is what I'm talking about here. The more cylinders means the more fuel it will consume, yes, but the extra cylinders means the hotter the motor can get, and the more it can get damaged by the high heat conditions - that smaller motors with fewer cylinders - can easily tolerate.
The problem is in changing the thermostat - does affect the sensor network and how the motor develops the fuel to air mix and profile - it's needs the motor to be in a tighter tolerance limitations of temperature to maintain this profile and efficiency without setting a check engine light and causing poor fuel economy.
As days in the summer are hot and humid - the car engine does not need a "long time" to warm up - so to wait and let it do so, it just wastes gas. So the OEM thermostat would keep the engine pretty hot in the summer - usually the OEM for a typical small-block chevy carbureted car was about 185~190 degrees F - it also had modified radiator systems - which provided more cooling for motors that used Air Conditioning. They called the radiator capacity as by a figure of cores, usually the typical non-AC vehicle used 2-core radiator, while others with AC installed used 3-core.
What are cores when it comes to Radiator?
It's the number of rows of cooling tubes that spanned across the width of the radiator.
The more "cores" you had, made a "thicker" radiator
- making it capable of taking away more heat from the motor
to help keep the motor cooler in a wider range of hotter operating temperatures.
The typical Fiesta uses a 2-core radiator and it's mounted BEHIND the AC condenser unit - which helps the engine "capture" heat when it colder outside - the Defrost setting runs the AC compressor and so the heat exchange function applies here. In cold weather - just like it does in the hot summer when you have the AC on. It takes heat out of the air (Evaporator inside the car) and concentrates the heat differences in the condenser and lets the cool air "cool" the condenser and in doing this, carries the heat away into the air flow and this heated air flows thru the radiators finned cores - transferring this heat into the fluid circulating into the fins of the radiator.
- Since the AC is a reciprocal event, meaning it takes heat away from one side and places it in another - and repeats this process over again. The AC fan being also the Radiators cooling fan, has to run nearly all the time in the summer to keep the heat it has, from overheating the car - so as the heat rises outside - the transfer mechanism efficiency degrades due to the fluids (Freon) used to develop that cooling can't develop the temperature gradient effectively enough to keep the passengers cool. It doesn't take away enough heat in the cabin - because it can't transfer that heat away due to the vapor pressure change in both volume and expansion and heat gradient conversion all become lower as it gets hotter outside.
In some Fords using 3-cylinder Eco-Boost, turbo, and larger 4L and V6 motors of 2.0L displacement and above in size, they also provide a "low speed" fan that runs more and takes the heat off the engine and radiator when the AC system is working but not supplying cooling thru it's compressor running. This helps to lower the heating load and the latency issues that can happen when engine blocks and the cooling system in it - try to equalize the temperatures in the block and cylinder head - the cooling fan helps to carry that heat away so the lower limits of the engines efficiency power curve. This can prolong the life of the motor and reduce the over temperature heating and fluid boiling issues that may occur and damage the system if the engine just ran the Fan at a higher pre-set temperature limit it would use in cooler weather - this upper limit switch setting being applied all the time would make the system cycle and possibly put the engine block to cylinder head seals at risk of excessive overheating and boiling conditions caused by the wider variances in cooling system pressures.
If you can be creative enough, you can take the above diagram and make a similar system using a manual switch to bypass the relay portion and just run a Power Resistor used in a typical FORD V6 Fusion / Mondeo / Taurus Radiator shroud. It is a simple wire wound nichrome wire Coolant Fan Resistor it's a Dorman Part 902-219 - comes with a Pigtail so you can then interleave it into your harness.
You also don't have a "low speed" resistor - for if you are not using any 1,0L ecoboost motor - the vehicle has no relay or part for it. The "low speed" resistor is only meant for the Eco-boost motors and they are 3 cylinder turbos on top of that - so the extra cooling parts added into the Fan circuitry is what drives the efficiency of that design.
So if you can still find one of those Dorman 902-219 dropping Resistor kits, go for it. This can be set up to run the Fan on a low-speed rotation so it's not so noisy when it is doing it's job, then the system is cooled down to make the AC exchange more effective when the AC kicks on.
This is why I did what I did to the Scion to keep it from overheating in stop start traffic I get caught in when I leave work at the 5PM schedule. Just Ran the Low-Speed fan to keep me and the car cooler - then as it needed - the high-speed fan only kicked in when it got really-really hot, which was not as often - nor did I have too many issues with the engines own temperature controls - it gave plenty of heat in the winter so the Low-speed fan option was only for hot summer days and as needed.
Else you'll have to remove the Thermostat and bend the Piston V - arm that works the wax-plug/piston seal - pushing this "arm" into the seal / rod area - moves the Seal down - releases the coolant earlier for the Wax plug only moves so much per degree of temperature in which the wax beings to melt. So it will open sooner, keeping the engine running cooler because the coolant can start pulling the heat out sooner - which can help keep the car cooler from producing less heat - but be warned you'll have to make a judgment call on this cooling system temperature change - for it can set a MIL Check Engine light because the system is not getting "hot enough" per the emissions - just re-bend the V-arm bar back to it's peak to force the wax plug to stay closed longer.
