The Auxiliary fan is powered by the Coolant Fan Relay, which is controlled by the Radiator Temperature Switch mounted on the lower left side of the radiator, as well as the Air Conditioner if so equipped, plus the Defroster on A/C models. They all supply 12 volts to the Coolant Fan Relay, which turns on the fan.
Since electric motors have a very heavy initial current draw, it is not advisable to wire a switch directly to the fan. The best way to add a manual override is to wire a switch into the control circuit of the relay to turn the relay on. The easiest place to tap into the circuit is at the relay itself, which (on pre '91 models) is located on the inner driver's side fender, just in front of the washer fluid bottle. * (For '91 & newer see note below).
You can tap into your fuse box for a 12 volt source ** (you will have a choice of hot all the time, or ignition on, so you need to decide which you want to use), and run a wire from here to your switch. Then from your switch, you can run a wire to the Fan Relay. The Orange wire (pin 2) is the relay control wire (some models may be Yellow, but you won't have both), this is the one you want to tap into with the wire from your switch. There is enough slack here to easily use a press on wiretap, or a crimp on splice tap.
This will not affect the automatic functions of the fan in any way, but will allow you to turn it on manually.
(More specifics on tapping into the fuse box on request.)
* The fan circuit on later model XJs (after '91) is wired much differently than the early ones. The fan relay is energized by the ECU when the Coolant Temperature Sensor reaches the predetermined temperature. For these models it is necessary to install a relay with your manual switch to turn on the fan.
** I have wired mine hot all the time for two reasons:
1) Under real hot conditions, I can leave it on if I am shutting the engine off for a short period to help keep things cool.
2) It reminds me to turn it off (when hear it running after the engine is off). Otherwise, it would be on every time the engine was running, ...forever (not that it would hurt the engine, but it would wear the fan out prematurely), I won't forget it this way.
I have put together a page on how I installed a rocker switch panel over the coin holder on the 5 speed console (it should work on models with autos, but I haven't tried it). This is not a detailed procedure, however there are good photos that will give you the basic idea of what you need to do to install one yourself.
Three Position Rocker Switch Panel.
Care and Feeding of the closed Cooling System
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