A Jeep Cherokee uses a silicone-filled expansion spring type of fan clutch for the cooling function of the motor. This type of engine cooling fan has been in service on most rear-wheel-drive vehicles for a long time. It was designed to allow the fan to operate only when the temperature of the engine requires extra cooling. Its ability to turn on and off helps fuel economy and requires less horsepower to operate.
When the car is started and the engine is cold, the fan clutch is released and is freewheeling. It remains freewheeling as long as the temperature remains under 210 degrees F. When the vehicle is stopped with the engine running and there is no air moving through the radiator, the temperature will begin to rise and the fan clutch will kick in and draw more air through the radiator, thus cooling the engine down. As soon as the temperature drops and the fan clutch cools off, it will disengage and will no longer draw horsepower from the engine. The fan clutch also releases when the car is under acceleration to allow for more horsepower.
To diagnose the clutch fan, lift the hood of the vehicle and with the engine off spin the fan a bit with your hand. You are checking to see if the fan is locked up where it will not spin freely. The fan will still cool the engine but will draw excessive power and reduce fuel economy. Eventually, a locked up fan will come loose on the inside and will need to be replaced.
If the fan spun with some resistance, start the engine and allow it to warm up. Observe the fan speed and amount of air expelled when cold. Watch the temperature of the engine on the gauge. As the temperature rises above the normal mark, the fan should pick up speed and expel much more air. If the fan blade is turning so slow that the individual blades can be seen as the engine warms up, the fan clutch is bad and needs to be replaced.
If the blades are turning fast but no air is expelled, watch the temperature; when it reaches a little above normal, the fan should come on and a definite airflow increase should be felt. If you cannot feel an airflow increase, the fan is bad. If it does come on and the temperature increases steadily, then the thermostat or radiator is at fault. Feel the top radiator hose. It should be hot to the touch. If it is not, the thermostat is stuck and not opening. If it is hot, the problem is with the radiator. Replace the thermostat or radiator as indicated, then retest the fan.
