The 1988 Jeep Cherokee, Comanche, and Wagoneer 4.0L engine series use an auxiliary fan. The auxiliary fan is controlled by a relay mounted on the left inner fender panel. When coolant temperature is more than 190°F/88°C, the radiator coolant temperature switch closes allowing current from the ignition switch to flow through the fan relay to ground activating the relay. When relay is activated, battery voltage is supplied to the fan causing it to operate. When coolant temperature is below 190°F/88°C, the radiator coolant temperature switch opens preventing the relay from being grounded and electric cooling fan from being energized.
When the A/C (if equipped) is turned on, the Electronic Control Unit (ECU) grounds the A/C relay coil allowing current to flow through it. This activates the A/C relay which then supplies current to the A/C clutch, fan diode assembly and cooling fan relay. The cooling fan relay is activated and the fan operates. Whenever the A/C is used, regardless of engine coolant temperature, the auxiliary electric cooling fan operates.
Find more information about 1988 Jeep Cherokee Engine Cooling Fan Circuit and Wiring Diagram here – http://jeepgarage.free.fr/OWN/engine%20cooling%20fan.pdf – free download PDF file from jeepgarage.free.fr.