Join Date: Sep 2001
Location: Milwaukee (Sussex), WI
^^^ 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.
Chris (Sussex, WI)
289 w/AFR 165, T-5z, 3.55T-loc, Granada Discs and 6 bolt Moto-Lita Steering Wheel