The Italian and German approach is to put it at the bottom of
the radiator, following the logic mentioned by Randall. I like
that mechanism, too. The Brits and Americans, when they have installed
thermostatic electric fans, are inconsistent, but more often than
not put it at the top - presumably with the thinking that the
thermostat is up at the top, too. Dunno.
The answer also depends on the temperature range of the sensor. If
it's a fixed sensor, you can "adjust" the temperature at which the
fan comes on/goes off by moving it up and down in the radiator.
And, the answer depends on how effective the fan is. If you're constrained
to use a small fan based on clearance issues, you probably want it to
run more and keep the overall temperature of the radiator lower.
Ultimately, the right answer will be determined by how your
car's temp behaves!
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