Coolant and Heating - Dash blower PWM upgrade
Changing the blower control to PWM
The stock dash blower motor usually needs replacement after 30 years. Typically, higher rated fuses end up fitted due the fuse blowing every time the fan is used. All indicative of a tired motor. Replacing the fan involves pulling the dash out so why not ditch the Frankenstein resistors and upgrade to a dedicated motor controller?
First a word about blower speed control. I'll keep things simple:
The blower motor always runs flat out. The only thing that stops it doing so at other switch settings, is by using resistors in circuit. These two resistors (for speeds 1 and 2, merely reduce power to the motor by effectively soaking it up. With no resistor in the circuit (speed 3), the motor runs flat out which is why a current carrying relay is employed when you switch to top speed - there are some amps there at high speed and a relay is needed to carry that power.
It works but its not the most efficient way to control a motor, even if relatively simple. The resistors generate heat and its not unknown for them to melt plastic parts in close proximity. Worse, as the motor ages, it requires considerably more amps to get the motor going and this is why you can experience a blown fuse, and in the case of pre-85 models, the very same fuse that also supplies the wipers, so both cease to function. An old motor, with worn bearings can pull a whopping 30amps on startup when it should be averaging around half of that and around 10ams flat out.
With a PWM DC motor speed controller, things are a little more linear, with speeds from practically zero to full speed and any settings in between in a very smooth fashion.
PWM or Pulse Width Modulation essentially means that rather than using resistors, power to the motor is being switched on and off at high speed and changes in the gap between on and off states via the control knob (potentiometer) is what controls its speed. For example, if its on more than off then the speed will be higher etc etc. This explains it perfectly: https://www.youtube.com/watch?v=Lf7JJAAZxEU
So why change to a dedicated controller?
Well, apart from it being more efficient and more controllable, its actually cheaper than, or no more than replacing damaged resistors and if you're planning on replacing the fan, thats an ideal time to consider upgrading the control circuit too. On a stock system that is working fine, its best to leave things as they are unless you're fitting a brand new fan - not one that is 30 years old.
Wiring it up is simple. The speed controller (rated at a minimum of 20amp) just needs a 12v supply and two wires (positive and negative) to the motor - and thats it.
Most of these controllers come with a cheap soldered-on blade fuse on the board or a glass fuse but if the unit is protected by a fuse at the fusebox, there some redundancy built in.
With a new motor, its not obvious which terminal is positive so you'll need to work this out first. It won't actually harm the motor being run in reverse, but it won't be operating correctly either. If you connect the motor to a 12v battery, you'll find that it kicks out considerably more air when the motor is running in the correct direction so you can mark up the positive terminal then. Try it both ways to make sure - it will be obvious when it is wired correctly.
In the video below I mention the above plus some suggestions on how to mount the controller and keep things looking stock.