300SE Climate Control - AUTO button

[Evan D]

1989 300SE - ac blows very cold, heat also emits nicely. Hi and lo buttons respond correctly. Issue is when the system is on auto the unit does not adjust the temperature. The cabin gets ice cold or extremely hot. Where to begin?? As always, any guidance is greatly appreciated.

[Sugar Bear]

If the temp adjusts correctly on other fan speeds my guess is the problem lies in the PBU (pushbutton control unit).

[Evan D]

Thanks Sugar Bear for the advice. But allow me to be clear, the temp does not "adjust" when using the other buttons, the system just pushes out both the heat and the ac. If you still think the PBU is the culprit, is there anyway to diagnose?

[Evan D]

Sugar Bear, I lost your contact info. Will you be at the New Hope Car Show on 8/9?

[Sugar Bear]

PM sent.

[Sugar Bear]

There probably is a way to diagnose the PBU but I don't know it. I did the try a "known good" substitute guess method. Yes, my guess is still the PBU.

Post back if you don't see the PM.

[Sugar Bear]

There is also a fan above the right kick panel that moves air across the cabin temp sensor. That fan may have stopped working.