The engine cooling fan is a variable speed fan. The ECM controls the fan speed by sending a pulse width modulated signal to the cooling fan control module. The cooling fan control module varies the voltage drop across the cooling fan motor in relation to the pulse width modulated signal.

Cooling fan speed is affected by many different conditions and can be adjusted from 10 percent to 90 percent duty cycle (PWM). A duty cycle of 90 percent is considered high fan speed. When multiple cooling fan speed requests are received, the ECM uses the highest cooling fan speed of all the requests.