In my car the blower motor speed is controlled by 4 valie resistor pack. I want to have fine tune speed control so I am planning to add a pwm fan speed controller ( Generic). My question is if I connect the resistor pack input to the pwm controller input and pwm output to the blower fan( without removing the resistor pack). My concern is if I connect both in parellal (resistor and pwm controller) will it work? Will that damage the pwm controller even if I use a diode at pwm output to prevent the voltage coming back to the controller?
You need to disconnect the resistor pack. If the stock control circuit calls for high speed on the fan, You will short 12 V to ground while the low side MOSFET is turned on in the PWM controller. You do not want two controls connected in parallel. If you put the two in series, you will only be able to make the motor slower than the stock control is calling for. It may be cumbersome to try to use two control methods in series.