There seems to be a lot of questions surrounding the operation of the Coolant Temperature Switch (located on the lower left side of the radiator on pre '91 XJs) that turns on the electric fan when the engine gets hot.  Many people believe their switch may be defective because it fails to operate when things start to heat up, but this is not usually the case.

     The reason it doesn't come on when it seems like it should is because the switch is mounted on the "cool side" of the radiator and is monitoring the temp of the incoming coolant after it has been cooled by the radiator, instead of monitoring the coolant as it leaves the head (a more accurate indication of the engine's true temp).  This is not an efficient location to monitor the coolant because it assumes that if the incoming coolant is cool, then the engine must be running cool.   This is not always the case and it does not take into consideration all the variables which can effect the system, like coolant flow rate or the outside air temp, both of which effect heat transfer, heat dissipation, and the overall efficiency of the system.  This explains why, that even with a 200º temp switch, the fan won't come on until the engine is much hotter that.  If the incoming coolant is already 200º, how hot will it be by the time it exits the engine?  Remember the normal operating temperature of the 4.0 is only 190º!   If the engine is running hot, then the coolant needs to be cooler,  ...regardless of the temp of the incoming coolant!

    A good example of how easy it is to trick this setup is in stop and go traffic or slow moving drive-thru lines.... at low RPM the water pump is not moving very much coolant through the system, so the coolant in the engine gets hotter as it sits in there longer, while the coolant in the radiator has plenty of time to cool off before it passes by the temperature switch.  The engine will have to get very hot before the incoming coolant will trip the switch.

    This problem was addressed in the later system ('91 and on) by locating the temperature switch in the thermostat housing, where it can monitor the coolant as it exits the engine, and turn the fan on before the engine overheats. However for those of us with older cooling systems the easiest solution to this problem is to install a  Manual Override Switch that can be used to activate the electric fan whenever the temperature starts to creep up on you.
 
 

Care and Feeding of the closed Cooling System

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