altarchsa

Recently I replaced the right front fan on the HVAC system and I thought things were working fine. But now, with a little cool weather, I find that I have no control over the heat temperature. If the control is moved off the blue dot, the temperature goes to full heat and stays there.

Same result when on AC. When AC is on and I move the control off the blue dot, heat comes on full-hot, I assume without turning off the AC. I hadn't noticed this problem until now because I either ran the AC full-on at blue dot or turned it off due to its lack of ability to get the temperature really cold in our HOT weather.

I checked the mixing dampers and their servos on both sides when I had the system opened up for the blower replacement, and again yesterday. The are working to completely open and close the dampers, but there seems to be no modulation.

It seems as if the CCU is not sending a signal to adjust the dampers to control the temperature???????

I purchased a used CCU last Spring to replace the small buttons on mine. I wound up just switching the face plate off of the unit onto my CCU. Could that be causing a problem????????? (Maybe if the unit is from a newer car?) Could it be a dirty filter on the sampling air intake in the faceplate, or a malfunctioning CCU fan????????

AS always, any help you can offer will be greatly appreciated.

Rocket Rob

What model CCU do you have? There was a wiring change in 1991. The early cars need to use an early CCU. You can make wiring change yourself to allow use of the late 964 or 993 CCU.

Rocket Rob

More thoughts - Do you have Adrian's book? It lists the CCU differences, respective part numbers and the wiring update required.

altarchsa

I'll have to check part numbers. I have e feeling the new CCU, from which I used the face plate on my unit, is off a later model car.

Maybe it would be smarter to swap the face plates back to the original units, leave ly old one in place and just swap the small buttons out?