1991 tt z32. With the car running, and the air con./heater temp. Set to 40, then auto button pressed on, the ac compressor kicks on. Is this normal operation? Press the econ. Button and compressor turns off. Seems strange to have the compressor turn on for heating. It blows hot air. No issues there.
Could it be the Suction Throttle Valve on the a/c evaporator unit malfunctioning? The a/c system had a blown high pressure valve, replaced it and regassed it yesterday. Set a/c to coldest setting and pressed auto, compressor kicks on, cold air blows from bottom ducts, ambient air blows from centre duct.
Probably a bad sensor. If you can find someone with full Conzult or Nissan Consult, it would be easier to diagnose.
Compressor engaging when auto button is pressed is normal operation. It helps defog the windows with cold or hot air blowing as well as maintaining your preset temperature. Cold air at your feet at lowest temperature set and auto button pressed sounds like one of those two sensors at the foot well ducting possibly faulty or disconnected. A good place to start diagnosing anyway. I don't think the suction throttle valve is the problem.
As "Jimmeyz" says , it's normal and helps keeps your windows from fogging up. ---- Auto climate just does what you're meant to do with old separate systems manually.