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Discussion Starter #1
Looking Radiator fan switches they typically have a built in ON temp and the OFF temp 10-15* lower. Typically 185* ON / 175* OFF. I've seen 200* ON, 185* OFF too.


The Thermostat I installed is 195*.



Why would I want the fan to come on before the t-stat is open, or even keep running with the t-stat closed?

Wouldn't it make more sense for the fan to kick on closer to desired operating temperature?

Say 210* ON/ 200* OFF?


There are adjustable switches too. I'm just wondering what I am missing for the default temps to be set so low.
 

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If it were me, I'd set the fan to come on at 210 and go off at 200. Even at 200 your thermostat may not be completely open and people may say you should set it higher so that it's never on while the the thermostat is partially closed. However, my car starts running noticeably different around 220.

I'd like to talk a little more about that last sentence of mine. I've bought into the whole 195 degree thermostat thing experienced members espouse on here, however my personal experience is that I can tell when my car hits 220 that it's a little off. It starts missing at idle and just generally doesn't seem as smooth. Maybe that's due to the inadequacy of some other part when that part is hot, not sure, but 220 is not good for me even though 220 isn't out of the ordinary for a lot of others.
 

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Discussion Starter #3
I tested the 195* t-stat in a pot of water and it did not open until about 205*. The thermometer I used is my meat thermometer I use for smoking and near maxed out at these temps. I'm not confident in its accuracy. But figured that if the t-stat was full open before boiling I should be good.
 

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Don't forget that the boiling point of water is higher under pressure so 205 is further from boiling in a cooling system than an open pot of water.
 

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I tested the 195* t-stat in a pot of water and it did not open until about 205*. The thermometer I used is my meat thermometer I use for smoking and near maxed out at these temps. I'm not confident in its accuracy. But figured that if the t-stat was full open before boiling I should be good.
The rated temperature is the point at which the thermostat begins to open. Fully open can be 10*~20* higher.
 

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Looking Radiator fan switches they typically have a built in ON temp and the OFF temp 10-15* lower. Typically 185* ON / 175* OFF. I've seen 200* ON, 185* OFF too.


The Thermostat I installed is 195*.



Why would I want the fan to come on before the t-stat is open, or even keep running with the t-stat closed?

Wouldn't it make more sense for the fan to kick on closer to desired operating temperature?

Say 210* ON/ 200* OFF?


There are adjustable switches too. I'm just wondering what I am missing for the default temps to be set so low.

You would be right...if the fan were cooling the coolant in your engine...but its not...its cooling the coolant in your radiator.
 

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You would be right...if the fan were cooling the coolant in your engine...but its not...its cooling the coolant in your radiator.
The whole purpose of the radiator, fan and thermostat is to regulate the coolant temperature in the engine. I am not sure why anyone cares what the temperature is in the radiator so long as it is sufficiently cooler than the engine to maintain desired engine operating temperature . Coolant will be at it's hottest as it exits the engine. That's were I want to know what the temperature is for purposes of regulation.

With a 180 degree T-stat I want the fan to go on at 200 and off at 185. (taken at the T-stat housing or intake manifold)
 

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The whole purpose of the radiator, fan and thermostat is to regulate the coolant temperature in the engine. I am not sure why anyone cares what the temperature is in the radiator so long as it is sufficiently cooler than the engine to maintain desired engine operating temperature . Coolant will be at it's hottest as it exits the engine. That's were I want to know what the temperature is for purposes of regulation.

With a 180 degree T-stat I want the fan to go on at 200 and off at 185. (taken at the T-stat housing or intake manifold)
Yes, I understand the coolant from the radiator flows to the engine(obviously). I guess I should have spelled out that there is a delay in the time it flows into the radiator, is cooled by the fan, then flows into the engine, meaning t-stat opens fully(usually about 195 degrees) so the water coming out of the t-stat may be 195...but the water entering the engine may be 175. This is important because many fan switches are located at the BOTTOM of the radiator in OEM applications...not at the t-stat housing(though some are, depends on the switch) so the switch at the bottom sees 175 and turns the fan off...while the water exiting the t-stat is still 195

In any case...you might well able to adjust the desired fan switch operating temp with an aftermarket ecu(if you have one) or if you dont with either a rheostat or possibly adding a resistor(probably a resistor would work, since I think the fan switches operate on resistance and not voltage) though what resistor you would need is something I couldn't tell you.
 

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^^^ Yes, some modern cars have a coolant sensor in the radiator that sends a signal to the ECU. The ECU uses that info to determine when to turn on and off the fan. While I do not know all the code in an ECU, I expect other information also plays into the decision to turn the fan on or off. It's not a simple switch like one is likely to have on a vintage Mustang.

If someone chooses to put a switch at the radiator exit, I would suggest they install some type of controller so that they could vary the on/off points to keep the engine in the desired temp range. There are too many variables at play to say that one set temperature of coolant exiting the radiator will provide proper engine operating temperature. Using your example, 175 degree coolant entering the engine might be good to keep a 289 at 195 degrees under ordinary driving, but not a 408 stroker being pushed hard. That stroker likely would raise them more than the 20 degrees of the 289.

One can get there either way. It just seems turning on and off via radiator temp would take some trial and error until one finds the right temp differential.
 

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Discussion Starter #10
This is just a 5.0HO. The radiator fan switch is mounted in the thermostat housing and operates independent from the EEC. There is an ECT (temp sensor) for the EEC but it does not control the fan.
 

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One other thing to keep in mind. I play with EFI a lot...in particular...Megasquirt(which does have a fan controller you can set at whatever temp you want) and you would be absolutely shocked at how quickly coolant heats up in the engine...my experience comes mostly from forced induction tuning(turbocharged applications in particular, so there is more heat) but just laying into the throttle for a few seconds will make that coolant temp jump 20+ degrees...I usually use a 195 degree thermostat...but when in the throttle with t-stat fully open and plenty of airflow at 60+mph its not uncommon to see temps of 215 degrees at the t-stat housing sensor. The nice thing about standalone ECUs is they give you a realtime window into what the engine is actually doing at any given time...it gives you a good idea of things you would never even consider like the time it takes coolant to heat up and cool down in different conditions.
 
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