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.
While the engine is running, the ECM adjusts the speed of the cooling fans in response to inputs from: The thermostat monitoring sensor, for engine cooling. The fans come on at minimum speed if the coolant temperature goes above 194 F (90 C), and progressively increase to a maximum speed at a coolant temperature of 216 F (102 C).
The A/C system, via the instrument pack and the CAN bus, for refrigerant system cooling.
The Electronic Automatic Transmission (EAT) ECU, via the CAN bus, for transmission cooling.
If there is a conflict between requested cooling fan speeds from the different inputs, the ECM adopts the highest requested speed.
As part of the power down routine, when the ignition is turned off, if the ambient air temperature is more than 59 F (15 C) the ECM samples the coolant temperature using the input from the Engine Coolant Temperature (ECT) sensor. If the coolant temperature is more than 223 F (106 C), the ECM signals for the cooling fans to come on. The speed of the cooling fans is in proportion to the coolant temperature, from minimum speed at less than 223 F (106 C) to maximum speed at 239 F (115 C). The cooling fans are turned off after 5 minutes or when the coolant temperature decreases to 223 F (106 C), whichever occurs first.