964 CCU fan knob failing to switch speeds
 

Hey everyone, I have been searching high and low for a solution for this to no avail.
I had my CCU fan speed on 4, and when I parked the car and wanted to switch off the a/c, I moved the knob back to 0.

Nothing happens, and I can't switch fan speeds anymore, I am literally stuck with a full blast fan, which is really annoying because sometimes I want the windows down and not hearing the fan howling.

Has anyone encountered this before?

The blower "final stage" amplifier may have given up the ghost. It is placed right aft of the fuse box in the front luggage compartment. You could check the input signals coming from the CCU with a oscilloscope, if the PWM signals are there, the final stage is probably shorted. (the internal transistors is permanently conducting) This will make the fans go on full speed. See "5" on pic.
Cheers,
Tore

https://cimg9.ibsrv.net/gimg/rennlis...fa3cf4da5f.jpg

Is that fixable? The transistors replaceable?

Just a thank you for your contributions to this overly-engineered part of our cars! :-)

I've a servo I need to investigate on mine and using your site and documents to do so.

The final stage is difficult to repair without modifying it. The main switching transistors are nailed to the heat sink. Most of these are damaged beyond repair due to corrosion.
Cheers,
Tore

Dear ToreB,

Can't give enough thanks.

Forgive my ignorance, but what is the fix for this? I have to get the whole amplifier (labelled at #5) replaced?

The complete final stage must be changed if defective. I would start with the investigations i described above first.
Cheers,
Tore