My Honda civic 1999 used a power transistor to achieve variable speed control of the AC blower motor. This is a wasteful technique since significant power is wasted across the bipolar transistor.
I wanted an efficient means to vary the speed and also as the groundwork for my upcoming car pc automation system. This system will manage the AC and other non safety critical systems in the car.
The screenshot of the honda civic manual I highlighted the wires I needed to isolate to make the pwm controller drive the motor. Basically I removed all other wires and relays. My car ventilation system I modified heavily for my hot climate; the air only blows from the dash vents and it's in permanent recirculation mode. I gutted out all the motors for vent control since I never had use for those. I streamlined my ventilation system to suit my climate.
Step 1: The Pwm Controller.
This 15amp pwm controller I got from Amazon.com. I used an identical one for my home water pump which is directly supervised by my home automation system.
This controller uses a potentiometer to vary 0 to 5v to a control pin. My home automation system can produce that same output so it's very easy for me to control this via software.
For now I will use the potentiometer to manually control the car AC blower motor.
To allow fast changeout of a blown pwm controller I used 2 pin nema 15r male and female plugs. I have a spare pwm controller stored for such an event.
The wires for the motor I attached a male plug.
Step 2: Installing the Pwm Controller.
I removed the bipolar power transistor and used cardboard and silicone to seal off its location.
I used plastic ties to secure the pwm to the AC blower intake to provide cooling. Luckily the pwm controller does not get hot even with the 200 watt motor running.
Step 3: Wiring Harness for the Potentiometer.
I used a length of cat5e cable to get the wires extended for the potentiometer. The 3pin jst connector is on the opposite end to the potentiometer.
Step 4: Installing the Potentiometer.
I drilled the console and installed the potentiometer. The oem controller for AC and blower I gutted out. The future automation pc will replace it all. For now manual controls and the pwm controller works perfectly with the AC blower.
I have noticed that the blower motor is much quieter and pushes more air. It also consumes significantly less power now so my alternator works less to help provide the cold air I enjoy so much. This mod is definitely worth a try for pre-2000 vehicles.