Thanks Umski!
This is superb information.
In combination with the post by Gatos "How to Replace the Heater Resistor" this is all great info on accessing, repair and the design of this for the phase 2 cars. Page 5 of the "How to Replace the Heater Resistor" also has images of the unit used in the phase 1 cars and the wiring diagram from Haynes.
From the info in Haynes the fan switch in the phase 1 cars (non climate control at least but maybe includes them too) just switches the electrical path to going through all 3 resistors (setting 1), 2 resistors (setting 2), 1 resistor (setting 3) or none (setting 4). I suppose one could replace this unit if needed with a setup that offered the same resistance per fan switch setting. Or any other series of steps too of your own choosing!
TBC if a later phase 1 car with climate control uses the same resistor block as shown in the Haynes manual and on page 5 of the "How to Replace the Heater Resistor" thread. Or if it uses the one from the phase 2 cars. I'll update my posts with any further info I get as I'm looking into this at the moment.
Gatos info that "later 1999 cars to 2004" use the unit from the phase 2 cars may well tie up with Model Year 2000 of the phase 1 cars (from August 1999) and the option of climate control - which I think was only on the later phase 1 cars as well. Indeed earlier phase 1 cars had air con as an option and non standard too!
Some posts on the design and fixes for the phase 2 cars on this forum suggests the fan switch / knob is moving a potentiometer. It's that output which would be the input you refer to as:
"There is a smaller transistor (2SD1407) which the ECC drives using a PWM signal (I think but in my haste I didn't verify this)".
Potentiometer output signal is then the input to the smaller transistor here rather than a PWM? I can see it could be either option (potentiometer output or PWM) and indeed one or the other (or both system types could be the same signal) for if the car has climate control or not too.
Are the cold-hot and vent position knobs also driving potentiometers on the phase 2 heater control units with and without climate control too? Or just for those with climate control? I suppose the phase 1 models with climate control could be similar too with potentiometers rather than mechanical connection to the cold-hot and vent position (phase 1 non climate control have mechanical connections controlled by the knobs). A few different ways to make these HVAC units I suppose too!
Thanks
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