If the switch is in the top hose it will switch the fan on regardless of vehicle speed and therefore take no account of natural air movement/cooling.
Surely the bottom hose is the more logical position as the fan will only be switched on if the radiator has not done enough without help?
Okay I accept what you say, the fan I am using is 2 speed unit, I will probably put a lower temp sensor in the top hose for the low speed and a higher temp sensor in the bottom hose for the faster fan speed.
The original viscous coupling is not an on/off switch I believe it slips less/drives more as temp of air through radiator rises, therefore reacting to the situation rather than just overcooling.
Thanks for your thoughts.