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I am wiring the relays for the radiator cooling fans.  It is a Fluidyne radiator and it has two thermostats with the same temperature values.  One thermostat is physically higher up on the side of the radiator than the other.  Should I trigger the fans with only one of the two thermostats or use a thermostat for each fan relay?  And, why does the Fluidyne have two thermostats?

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The reason for the fans is to cool the radiator so my opinion is that you use the one in the leaving the radiator tank. The likely reason there are two locations is  they are copying the original factory layout without knowing why. The original radiator had a vertical baffle in the tank so that coolant flowed across the front and then back across the back. This meant that both sensors were reading the leaving coolant temperature. Somewhere in the 72 area ford determined that it would be more efficient for the flow to be across the bottom and return back across the top. They changed the baffle from vertical to horizontal to make this happen and issued a tech bulletin for dealers to have this done to earlier cars but did not address the fact that now one sensor is measuring entering coolant temperature and the other leaving coolant temperature.

I assume that the aftermarket manufacturers have just copied the original layout without knowing why it was done the way it was.

That is the way it worked on the original radiators. The newer and aftermarket radiators have one sensor measuring the temperature of coolant leaving the engine which will always be whatever the thermostat rating is  and the other will be whatever the coolant temperature is leaving the radiator because the sensors no longer measure temperature of the same coolant.

I was at Wilkinson's today and he recommended that I use both thermostats.  I will be using 75/70 on the inlet (bottom) and 85/75 on the outlet (top).  He said that the fan triggered by the outlet will actually turn on and off with this setup.  My wiring harness has the yellow wire that goes from the radiator fan relays all the way to the fan light in the speedometer and I will be able to see if the outlet fan is cycling.  All of this can be fine tuned once the car is operational.

If you have two radiator thermostats with slightly different temperature values, then it will run one fan most of the time and the other when the engine temperature gets a bit hotter. The factory set up had the second fan coming on at a high temperature than the first fan and when the second fan came on, it would light up the fan light. There is really no point in running both radiator cooling fans at the same time if you only need to run one.

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