Electric fan not working when overheating but WORKS with AC on
 

If idling for extended periods of time without the AC on, my 1996 xj classic will start to get hot and climb to 220+ (I always turn the AC on at 220 to kick the aux fan on and cool it back down) As long as the AC is on, the fan comes on and it doesn't overheat. It's only when the AC is off that it won't kick on when it should at 215-218.

Why would the aux fan not be coming on when the temp is over 215 like it should?

I replaced the coolant temp sensor (on thermostat housing), aux fan itself, aux fan relay and yet still it does not kick on when it gets hot without the AC on. I have my code reader on live data watching the temp get to 220 and still no fan..

I cannot think of anything else to check. Any help would be greatly appreciated!!

Isn't the temp sensor for the aux fan on the driver side of the radiator? It is on my 88. You can just jump the temp sensor and the fan will run anytime the ignition is on.

I have a 96 as well, and run real-time data whenever the vehicle is on (obd2 scanner permanently wired in). My aux fan comes on at 221F. Is it possible that you are never reaching that before turning it on manually via AC? (I sometimes kick the AC on, like you, to knock the temperature down).

Possibly, but on my 97 & 99 the aux fan comes on when the temp goes slightly above 210 and keeps it at or below 210. I never let it get above 220 because I worry about the head gasket at any temp over that.

The temp sensor on the open cooling systems is located on the thermostat housing, as far as I am aware, there is no temp sensor directly on the radiator itself. I am guessing your 88 has the closed system correct?