Coolant and Heating - Dash blower PWM upgrade

Changing the blower control to PWM

(Covkid)

There are some strong reasons why you might want to convert your blower fan to PWM. Principally it is more efficient that the resistors VW fitted and since the stock dash blower motor usually needs replacement after 30 years, why not take the opportunity to ditch the Frankenstein resistors altogether and have any fan speed you desire.

Warning: You should only carry out this mod with a new, or relatively new blower fan to avoid stressing the new controller. An old blower fan may draw far more amps than the controller is rated for so fit a new fan.

Video: https://www.youtube.com/watch?v=3GzJ8N1Rqp8

First a word about blower speed control - and I'll keep things simple:

When you operate the speed control, you're not increasing the speed - the blower motor always runs flat out. The only thing that slows the blower down on other switch settings, is by using two whopping wirewound resistors in the circuit, one for speed 1, and another for Speed 2. These two different value resistors merely reduce power to the motor by soaking up the excess power in heat. 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 ofcourse, but its not the most efficient way to control a motor, even if simple on paper. The resistors generate considerable 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. This is why you should not attempt this with an old blower fan.

With a PWM DC motor speed controller and new blower motor, 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.

So why change to a dedicated controller and where can I buy one?

Well, apart from it being more efficient and more controllable, a PWM controller is 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. See https://www.youtube.com/watch?v=Lf7JJAAZxEU for why you should use PMW to control blower.

Ebay is awash with them but you're looking for one rated at least 20amps, and with the control knob on a flylead for ease of fitting. You should be able to find one for under £10.

Wiring it up is simple. The speed controller 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 above I mention the above plus some suggestions on how to mount the controller and keep things looking stock.

Footnote from forum threads:

It depends how your new fan was packaged when you got it but you only need two wires from fan which should BOTH go to controller ONLY. The earth (negative) must go to controller negative. The positive at fan should go to positive at controller. On a stock system the only wire on blower switch that actually matters here is the yellow one - this is the flat-out speed and ignores the resistors.

Heres how the stock system works so you can understand what needs to be done:

Firstly, go grab your Haynes manual and turn to 12.38 - (wiring diagrams) - its towards the back. See V2 bottom left? thats your blower motor.

The wire below it is the earth (brown) trhats going to earth crown. Just above the motor there are two other wires that connect to it, but further up there are three that come out of E9 (the fresh air/heater blower switch) - the three wires for the three different speeds. By default the motor runs full pelt so they added Dr Frankenstein resistors (N23) to slow it down for the two other speeds.

Basically these are the three speeds and the wires are coloured thus: 1) ge (stands for yellow and motor would be flat out) 2) ge/sw (yellow and black - motor at medium speed)- power now going through one resistor 3) ws (white - motor at slowest speed) - power now going through TWO resistors

So, the only wire you need (if you leave resistors where they are, is the solid yellow (flat-out) one. Thats positive and should go to motor-positive on controller. Ignore yellow/black.

The brown that normally goes left, under the dash to earth crown needs moving and should now go to motor-negative on controller. The other terminals (plus % minus) on controller are for your 12v feed. The one you found from ignition should go to battery-positive on controller and the negative you can just ground to chassis. My video helps you translate chinese symbols. If your new fan has extra wires, you can ignore them.

Subnote:

For anyone else fitting this mod, DO NOT fit it to an old fan motor. Only do this with a new one. Old motors draw considerable amps at startup which might damage the controller. Therefore this mod needs new fan, new controller - period. If you've not long fitted a new fan and want to go PWM, you can do it with dah in situ but you have to discover which brown wire at earth crown serves the blower and the only sure way to find out is turn on blower and disconnect earths one at a time until blower stops - you've found the right wire. As mentioned above, that wire should go to motor-negative connection on controller instead. If you don't do that, controller will have no effect on blower speed at all. The controller MUST have the blower to itself - if that makes sense.