Aux fan doesn't always work

I recently got a 94 xj country and the aux fan doesn't come on when its suppose to. Had to pullover on the highway the other day because it was running way too hot. Couldn't pull over till I was just before red line. Anyone have any ideas on why its not working? Radiator and the cap and the thermostat have been replaced. The relay is good also. This is my first jeep but I'm mechanically inclined.

If you have AC, does the fan engage when you turn the A/C on? Could be a bad sensor, ECM, relay, etc.

When it happened I put the AC on and it didn't work. I've checked the relay and its good but the fans working now. My guess is that its a wiring issue