The Jeep Grand Cherokee made its name by being just different enough to work for today, while being just "Jeep" enough to run like one. Simplicity of concept and design are two of any real Jeep's defining traits, and in that regard, this newer and larger version of the wildly successful Cherokee XJ was everything any Jeep ever was. However, new times demanded new types of sophistication to keep up with changing needs, and ever more complex executions for simple tasks like connecting the cooling sensor to the fan relay to the fan.
As metal gets hot, it loses its ability to efficiently conduct electricity; that's why powerful supercomputers use equally powerful cooling systems to keep the circuitry cold and functioning efficiently. The coolant temperature sensor contains a metal conductor wire. As the engine heats up, the wire's resistance increases, and less of the power that goes into one end of the wire comes out the other end. The sensor on this Jeep has two wires coming out; the positive wire goes directly to the No. 12 pin on the diagnostic port connector, and the ground wire connects to the ground from the intake air temperature sensor before going to the No. 8 pin on the connector.
Both wires from the temperature sensor's positive to the shared ground go back to dedicated terminals on the Jeep's engine management computer. The computer reads how much electricity the hot wire lost as electrons passed through the hot sensor wire, and the computer calculates the engine temperature from it. A certain resistance value in ohms corresponds to a certain temperature on a functioning sensor. The computer then uses this information to calibrate the engine parameters, and perform certain functions. One of these is sending a triggering signal to the fan relay, which goes out on a dedicated wire from the computer.
The positive voltage from the computer meets up with the armature magnet in the relay, where it combines with the ground to make an electromagnet. The relay ground goes through the ground in the main power distribution block, and from there to the battery terminal and chassis. The relay gets the fan motor power from the power distribution block, and uses an inline fuse to protect against overloads. Power goes out from the relay to the fan motor, mounted on the back of the radiator. The fan motor grounds directly to the chassis nearby, behind the headlight. that completes the circuit, and the fan turns on.
There's no direct connection between the coolant temperature sensor and the fan relay -- the sensor's data runs through the computer, and the computer turns the fan on. This is a bit different than on older Jeeps and other vehicles with electric fans, as many had simple temperature switches integrated into the sensor that sent power through the sensor and directly to the fan. For this reason, a malfunctioning temperature sensor can cause the fan to stay off, unless the computer turns the fan on all the time after detecting a bad sensor. This arrangement does make for pretty simple wiring if you want to run a manual fan switch -- for hard off-roading or towing -- though, as you need only run a manual switch in the cab connecting the motor power wire and positive battery terminal. Be sure to use another inline fuse in the power wires to and from the switch, since you're cutting the factory fuse out of the loop.
