I probably currently have a similar problem like Sam G in this post:
... but unfortunately in the moment there seems to be a little issue, adding a reply on this discussion.
I tested all the vacuum and resistance, ordered a new used transducer from Russ Austin which helped a bit, but not finally. When I'm driving everything is working perfectly. When the engine is in idle at a stop light etc. the AC switches to warm. Quite uncomfortable on a hot summer day. What I didn't tested before was the voltage from the alternator and/or voltage regulator. If the engine drops down to under 500 rpm the voltage drops from 14 to 13 Volts. And then the voltage to the transducer of course even get 1 Volt less and switches to warm.
In Tims book he doesn't write anything about it.
I don't think it's normal, right? So do I have to buy another voltage regulator? Or do I have another problem? Perhaps the alternator is not good anymore? It seems to be the original one from 1964.
a one volt difference at the battery shouldn't create a one volt drop at the transducer, the control panel circuit should compensate for voltage fluctuations. My 66 drops to 12.7v with the AC and lights on but the climate control doesn't change temperature with the same circuit board used as the 64.