Hey georges 928S, There have been a few times when Miss Purdy (84 Model) will occasionally hesitate to make the transition from heat to no-heat while moving the temperature select lever.
Initially, I had suspected that it might be a defective hot water vacuum shut-off valve or one of the control solenoids. But when I had the dash and the console out for a dash replacement, I checked the integrity of all my vacuum modules, the bank of vacuum control solenoids, all the flaps, doors and pivot points and the interior/exterior thermistor circuits. Then as long as everything was scattered out on the garage floor, I disassembled and cleaned the slide lever contacts inside the console mounted Heat/AC control unit. Strangely- everything was found to be fine?
My only assumption is that the wear and tear on a 20+ year old ventilation system sometimes needs a little time to think things over as you adjust the Heat/AC to your liking. I have discovered (with my car anyway) by cycling the temperature control and function levers very gradually - that my system operates fine (except for when it sometimes doesn’t and I must slowly repeat the command).
Good Luck, Michael
