1990 BMW 735i 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.

Vehicles with a seized fan clutch can be operated indefinitely, however, the engine will lack power, fuel economy will greatly diminish, and the engine will run cooler than its optimal operating temperature. This can eventually cause other issues, like higher levels of carbon buildup or catalytic converter failure, but in general the vehicle will operate. A fan clutch that fails to engage should be replaced immediately, as this will result in the vehicle overheating when not in motion, or when at moderate to high engine loads.

Median failure rates of fan a sample set of domestic and foreign vehicles shows fan clutch failure is most typical around 140,000 miles. This is no guarantee, but repair data has shown this is the most likely range for failure. Most vehicles which use fan clutches are older, and may not be in operation any longer, but those that are will likely have this component replaced at some point during its useful life.

Diagnosing a fan clutch which does not function properly is different for mechanically actuated clutches verses electronically actuated clutches. Mechanically actuated clutches are diagnosed by their inability to maintain proper fan speed during operation, and the amount of tension required to turn the fan while the engine is stopped. This is the same for an electrically actuated fan clutch, except the fan clutch switch and relay will need to be tested for proper operation before declaring the clutch faulty. If the fan clutch switch or fan clutch relay is not activating the fan clutch, the fan clutch can seem as though it requires replacement, when, in fact, it is not receiving power.

Replacing the radiator fan clutch requires that the fan be disconnected from the fan clutch, and this means the fan will need to be locked or held in position to loosen all fasteners. Once the fasteners for the fan are removed, the fan clutch will either be removed with the fan drive shaft, or pulled from the drive shaft if it is not replaced as an assembly. For electronically actuated fan clutches, the fan clutch will be disconnected from the electrical connector as well, and the connector will be cleaned with electrical parts cleaner before reassembly. Assembly is the reverse order of disassembly, and the repair must be verified by warming the engine and checking the operation of the radiator cooling fan and clutch assembly.

We recommend checking the operation of the coolant temperature sensor prior to replacing the fan clutch assembly. If the engine coolant temperature (ECT) sensor is faulty, it may be reading too low or too high, and the fan clutch may be operating properly according the actual temperature of the engine. This check could be done by comparing an infrared reading of the thermostat housing temperature, and comparing that temperature to the engine temperature gauge.

Never touch a fan while the engine is running. Especially if the fan clutch is permanently seized, the fan will be spinning at the same speed of the engine, with the full power of the engine behind it. This can easily cut or maim a hand or arm. Also, never wear loose clothing while leaning over the engine compartment. If loose clothing becomes trapped in the fan, the fan will pull the clothing and the wearer of that clothing into the fan.

This is not a difficult repair or diagnosis, but for electronic fans a complete understanding of the fan circuit and controller is necessary to identify the cause of failure. If the clutch must be removed from the fan drive shaft, a specialty puller will need to be used, but this is normally not the case. Great precaution should be taken while working around the engine fan, and the battery should be disconnected to render the engine incapable of starting.