BMW M4 Radiator Fan Relay Replacement Cost

The radiator fan relay is an electromagnetic device that switches the fan on and off when it receives a signal from a computer module in the vehicle.

Like many of the electrical components on a vehicle, the radiator fan (or cooling fan) requires 12 volts of power to operate. Since the fan is commanded by a small computer called the Engine Control Module (or the Powertrain Control Module) that emits a signal at a much lower voltage, a relay is tasked with actually switching the fan on and off. When the relay receives the command (by way of an electrical impulse in the control circuit) from the Engine Control Module, an electromagnetic field is created to move a switch inside the relay. That switch closes or connects the circuit for the radiator fan and turns it on. On older vehicles, the radiator fan relay is triggered by a thermostatically controlled sensor rather than by a computer.

The primary symptom of a bad radiator fan relay is an inoperative fan. But since most drivers are not paying attention to if and when the radiator fan is running, the symptom more likely to be noticed is an engine that is running hotter than normal or is overheating. A temperature gauge on the dashboard may read high or a warning lamp may come on. Engine coolant could be seen dripping underneath the vehicle or in the engine compartment, and steam could come from underneath the hood. A slight clicking sound coming from under the hood is not unusual when the relay kicks in normally, but if the sound is louder than normal, the relay might be failing.

A vehicle should not be driven if the engine cooling system is not in proper working order. An engine that overheats can become severely damaged if driven for very long. If the radiator fan relay is bad, the fan will not be able to assist in lowering the temperature of the engine coolant in the radiator and the engine will overheat, especially if the vehicle is left at idle. Have the vehicle inspected and repaired as soon as possible.

Relays, like fuses, can wear out and fail over time. There is no replacement schedule, and some relays will last for many years. Still, it is not uncommon - in fact a fan relay is one of the most common cooling system failures - and a bad relay is at the top of the list of culprits should the engine begin to overheat.

A technician will check the engine coolant level to make sure that overheating is not happening because of a loss of coolant. Next, the technician will check for a blown fuse. If the fluid is full and the fuse is intact, the technician will remove the relay and use a multimeter to make sure that the relay is getting voltage. The relay will also be checked with the multimeter. It may also be tested by triggering it with another power source to see if it switches properly. Sometimes the technician will start the engine first and warm it up to observe the temperature gauge and check the fan operation to verify the condition. If the fan does not kick in when the engine temperature rises to a specific level, the relay is checked.

Replacing the relay is simple: like a fuse, it is pulled out of the fuse box (its usual location) and replaced with a new one. Diagnosis is the more challenging part of the repair.

If the engine overheats while driving, the vehicle should be pulled over and the engine turned off as soon as is safely possible. If the vehicle needs to be driven to a safe stopping point, proceed at a low speed and avoid stop and go traffic where airflow through the radiator is diminished.

An engine in a late model vehicle is designed to run somewhere between 195 and 220 degrees. But the fluid in the cooling system can rise well in excess of 220 degrees when the engine overheats. This produces steam that will cause severe injury from scalding if the system is opened when hot and under pressure. Burns are possible as the engine components are also at a high temperature. Extreme caution should be exercised when working on a hot engine, especially one that is overheated. If steam is seen coming from under the hood, the engine should be allowed to cool for at least a half hour before the hood is opened, and the radiator cap should not be removed until the engine is cool enough to touch with a bare hand.

Anyone who can change the fuse in a vehicle can change a relay if it is located in the fuse box. On the other hand, accurate diagnosis of the cooling system is not only challenging and potentially hazardous, it is essential for proper engine function. Misdiagnosis that leads to an engine overheating is not just an inconvenience, it is a mistake that can lead to thousands of dollars in engine repair or replacement.