It should be two wires to a plug that reaches pretty close to the top of the fan: Red with White trace (hot) and Black (gnd). Both run thru the loom to the relay panel on the left fender behind the airbox. The relay is the forward-most relay.
The radiator switch is on the lower left side of the radiator (looking forward from the cab). The wires are a Yellow from the ignition switch that is hot (12 volts) in the RUN or START position, 0 otherwise. The other side of the switch is Gray with a White trace which goes to the Fan Diode Assembly (2 steering diodes). The FDA has a Green with White trace lead that goes to the fan relay coil and an Orange lead to the A/C clutch relay.
Same as i)
Three cases here. a. Ignition on, engine cold: same as i) b. Ignition on, A/C on, any temp: Terminal #2 (relay coil) will be battery voltage - about .6 volts (1 diode drop) and Terminal #1 will be at battery voltage. c. Ignition on, A/C in any position, temp above (about) 210 F: same as b.
Certainly. (Now I bet you want to know where ).
This is from the Factory Service Manual, but I'm not sure you can get that anymore. I find the Haynes manuals to have better wiring diagrams than Chiltons.
Test procedure: unplug the radiator temp sensor. Turn the ignition on and short across the temp sensor plug. Fan should run with the key on. Ignition on, turn the heater control the the defroster position and the fan should run. The diode assy could easily be the problem although a bad relay would be more likely, especially if the original fan seized. In the same vein, there is a fuse wire ( Green) on the power distribution center (on the other fender) that would likely be blown if the old fan seized.
If it gets too grim, email me and I'll scan the FSM schematics for you. The MJ is the same as your XJ under the hood.