Checking the electric fan with an ECU Peugeot & Citroen (PSA) on the car using diagnostic adapters.
For fans with an electronic control unit (ECU) forget such concepts as: first, second and third speed, even in spite of what is indicated in the diagnostic programs. In fans with an ECU there can be more than 100 speeds, since the speed is set proportionally to the pulse PWM signal on the control wire, which can have multiple meanings. By smoothly changing the PWM control signal, you can also smoothly change the fan speed, just as the volume control smoothly changes the volume of the sound. You do not say that you have 2 or 3 volumes in the radio. Such concept as the first, second, third speed can only be used for fans without an ECU, in which the speed is limited by powerful limiting resistors connected to the electric motor using a relay. Or multi-brush motors can be used instead of resistors, in which relays are also responsible for switching various brushes. Let’s say we have a fan without an ECU with one resistor and two relays. We can say about this fan that it has two speeds, each speed is responsible for a separate relay. For the first speed a relay is activated, that connects the electric motor with the resistor. For example, at the first speed the fan consumes 15 amperes with an impeller at 12 volts. When the second speed is activated, the second relay is turned on, which supplies power directly to the motor and it gives maximum speed, for example, 30 Amperes with an impeller at 12 Volts. If a fan without an ECU has 2 resistors and 3 relays, then we can say it has 3 speeds. But in no case is it correct to say that a fan with an ECU has 2 or 3 speeds. The fact that different speeds are indicated in diagnostic programs is ‘echoes of the past’ incorrectly interpreted by diagnosticians. If, for example, we run in the diagnostic program ‘Speed of the electric fan 2’ and connect the oscilloscope to the control wire then we will see that the control signal changes very smoothly and the fan speed also increases smoothly from minimum to maximum speed without any sharp changes.