Cooling Fan Control

The ECM varies the duty cycle of the PWM signal between 3-90 percent to vary cooling fan speed. The cooling fans are off at duty cycles between 3-9 percent. When duty cycle increases above 9 percent the cooling fans come on at minimum speed. The speed increases in proportion to increase in duty cycle, up to a maximum speed at a duty cycle of 90 percent. If duty cycle is less than 3 percent or more than 90 percent, the cooling fan ECU interprets the signal as an open or short circuit and runs the cooling fans at maximum speed to ensure the engine and transmission do not overheat.

The speed of the cooling fans varies between a minimum of 750 RPM at 6 volts and a maximum of 4000 RPM at nominal battery voltage. To reduce the noise from the cooling fans they are driven at slightly different speeds, except when running at minimum or maximum speeds. Stepped speed changes occur at 1500 RPM (right cooling fan) and 1450-2600 RPM (left cooling fan) to improve refinement.