What is the principle of the three wires in a BMW fan?
2 Answers
BMW fan has three wires: one is for high speed, one is for low speed, and the other is the negative wire. Both the high and low speed wires have no power. The power control is achieved when the temperature controller reaches 92-98 degrees, and the temperature signal is transmitted to the computer board to directly control the fan speed. Additional information: The working principle of BMW's electric fan: The electric fan in front of the condenser serves an auxiliary cooling function. The fan and its power output stage are designed as an integrated unit. Therefore, the speed of the electric fan is controlled by the coolant temperature at the radiator outlet and the pressure of the air conditioning refrigerant, and it gradually decreases as the driving speed increases.
I've previously researched BMW's design for this. The three wires are actually for intelligent speed control of the electric fan. The thickest one is the main power line, directly connected to the battery positive terminal; the slightly thinner one connects to the vehicle ground, serving as the current return path; the most crucial is the third signal wire, which links to the engine control unit (ECU). The ECU sends pulse commands through this signal wire based on temperature data from the coolant temperature sensor. This signal acts like a rapidly switching on/off mechanism – the longer the "on" duration ratio, the higher the voltage to the fan and thus the faster its rotation speed. When you hear the fan whirring loudly during summer traffic jams, that's the ECU automatically upshifting the fan speed. Conversely, it downshifts to save power when coolant temperatures are low. This is far smarter than older two-wire systems that could only run at full speed, providing both engine protection and noise reduction. However, be aware that oxidized wire connectors can cause signal instability, potentially making the fan inexplicably spin at crazy high speeds.
