Most engine driven fans on vehicles now incorporate a fan clutch. The clutch is actually a drive coupling that is filled with a viscous fluid that will allow various engine fan speeds in relation to the engine speed.
Understand that the engine fan only needs to rotate at some speeds and at some temperatures. As an example, on hot days in heavy stop and go city traffic, the fan clutch should engage the engine fan, assuring that it is turning at full force.
However, once the vehicle is moving at highway speeds, the engine fan is really not required. The ram air being created by the forward motion of the vehicle provides adequate air flow over the radiator and condenser to maintain proper cooling. Therefore, the fan clutch disengages the fan so that it no longer turns with the engine. Some refer to this as free wheeling.
The biggest problem with the fan clutches are that the fluid that drives the clutch and engages the engine fan leaks. When that fluid has leaked out, the engine fan may never become properly engaged.
For auto A/C systems, that means the A/C condenser is not being cooled properly. The sole function of the condenser is to remove the heat that the refrigerant absorbed from the hot air passing over the evaporator. Once the heat is removed in the condenser, the vapor refrigerant is condensed back to a liquid before leaving the condenser.
When the fan clutch is not engaged when it should be, like in stop and go traffic where heat loads are high, the condenser can not properly cool the refrigerant. In that case, the A/C system is returning liquid and vapor back to the evaporator, and cooling performance is seriously reduced.
Inspect fan clutches frequently and assure that they are engaging when the vehicle is under a heat load and engine fan motion is required to maintain proper cooling.