Alright this will be my disclaimer: I do not accept responsibility for anything as a result of this mod or any Check Engine Lights.
At least 18 gauge wire.
Switch (2 or 3 pole depending on which option you do)
30 minutes, if that.
With that being said, this mod will allow you to control the electric fan by way of a two pole switch
The fan gets power through a relay in the PDC under the hood. From the relay are four wires, they are Green, Green w/ red stripe, Blue w/ white strip, and Dark Blue w/ pink stripe.
According to my Chilton and Haynes book, the green w/ red stripe was direct 12v, the green went to the fan, and the blue w/ white stripe went to a ignition activated 12v source. The dark blue w/ pink stripe went to the PCM.
When the PCM sees that its getting hot, or the a/c is selected, it connects the dark blue w/ pink stripe to the ground activating the fan. There is no power going to or from the PCM through this wire. All it does is connect the ground side of the relay to the PCM. Internally, the PCM handles the rest.
So now, what i did was use a wire splice my red wire into the dark blue w/ pink stripe wire going into the PCM via the blue wire splice i installed. (It is pin 31 on 91-95 and for 97-01, it is pin c2 on connector C3.)
I then ran the green wire from the switch to a grounded location. Shown below via a wire splice.
i spliced the red wire for the switch into the DK blue and pink wire as shown below.
Both wires went through the hood latch grommet, along with my amp power and other wires.
Backside view of switch.
The fan will come on whenever the ignition is in the ON position.
** THE DARK BLUE W/ PINK STRIPE APPEARS TO BE THE WIRE FOR THE FAN ON ALL 88-01 XJ'S **
** CEL'S SEEM TO BE HIT OR MISS AS FAR AS THE 97-01 **
3 different FSM's, showed that it always uses a dark blue w/ pink stripe. After some reading, it shows that for 96-01, the wire is indeed a dark blue w/ pink stripe. It goes into the pcm at connector C3 and is pin c2 for 96-01. (Make sure that it is a dark blue wire w/ pink stripe). This will work for 91-01 Cherokees.
For 88-91, it shows that it is not controlled by the computer, but just by the radiator pressure switch and/or a/c. It is always grounded, so you would have to put 12V to the orange/black or gray/ black wire going into the Fan Cooling Relay. Again, this is only for 88-91 Cherokees.
Now, you can turn the fan on and off via the switch and the computer still has control, as long as the switch is OFF.
I installed on 4/15/12. Tested successful the next day sitting in traffic waiting to pick up my little brother from school and this mod helped bring my temp down while waiting in line. I turned it off once i got into open traffic with good airflow.
Edit as of 6-29-12: I have added a 12 volt diode from Radioshack just for the heck of it (No problems before or after). If you add a diode make sure you have the cathode and anode ends installed correctly.
Question presented to me by Dukie564 as to weather or the fan could be turned on manually (like first set up), run normally ( ECU controlled), and lastly be completely off by the same switch (For uses like water crossings when the fan would never operate). So after thinking long and hard on this i came up with an idea using a switch like this
. So attached are how i would wire up this "option" with this switch.
Any questions just PM me and i'll get back with you!