1991 BMW 735iL Radiator Fan Clutch Replacement Cost

Vehicles which drive the engine cooling fans directly from the engine need a means of slowing the fan down when full engine speed is not needed from the cooling fan. This is achieved by using a cooling fan clutch, which allows the fan to slow down or speed up depending on the required speed of the fan compared to engine speed. This allows the engine to maintain appropriate operating temperatures, and reduces parasitic power draw from the fan running at full speed.

In older models, the fan clutch is controlled completely by the temperature of the engine versus the engine speed. As the clutch heats up, friction builds, and this causes the fan to rotate closer to the speed of the engine. When the clutch cools, the engine fan slows down because of a reduction in friction. This can waver, however, as the engine speed may increase or decrease suddenly, and the state of the clutch will determine how fast or slow the fan will speed up or slow down. The goal is to prevent the fan from running unless it is needed, providing more of the engine's power for operation of the vehicle. Electronically controlled fans work in the same manner, but they are actuated by a control module instead of mechanical actuation.

The first indication of a defective fan clutch is a healthy cooling system that runs below or above the normal temperature range. This is due to the fan running too fast or too slow. Also, a seized cooling fan will cause a loud roaring fan noise from under the hood every time the accelerator is pressed, and the engine will feel as if it lacks power. In this situation it would be very difficult to turn the engine cooling fan by hand, of course when the vehicle is off. Finally, if the clutch fails the fan will either turn very slowly, and not respond well to engine speed.