Help me understand HVAC

[eric1514]

When my blender knob is set to dash and the temp knob is set to hot, hot air will come out of the dash vents like you would expect. When I turn the blender knob to dash/floor or to floor, again with the temp dial to hot, the A/C compressor will kick on and off briefly. The air will come out of the proper vents, and it feels hot. If I turn the knob back to dash the compressor will not cycle and the air will return to the dash. So, the blender doors seem to be working it's just the A/C wants to cycle when the blender doors are set to floor/dash or floor.

Why is it doing this? Is this normal or is this a symptom of something wrong?

 

[lfhoward]

Maybe the mode door actuator was replaced at some point and not calibrated afterwards. Maybe the computer thinks you are going past the defrost setting, or floor/defrost, even though the knob is on something different? The compressor will run when in any defrost mode.

 

[eric1514]

Maybe the mode door actuator was replaced at some point and not calibrated afterwards. Maybe the computer thinks you are going past the defrost setting, or floor/defrost, even though the knob is on something different? The compressor will run when in any defrost mode.

Reading through the FSM, the only calibration I see is the "A/C-Heater Control Calibration". It involves sweeping the temperature knob back and forth in a timed manner. I can try that.

The manual says

NOTE: Use this procedure to ensure that the A/Cheater
control is correctly calibrated to the blend
door actuator. If the calibration is not correct, the
heating-A/C system may experience a condition
where full heating or cooling capabilities cannot be
obtained.

I'll give it a try.

 

[eric1514]

Maybe the mode door actuator was replaced at some point and not calibrated afterwards. Maybe the computer thinks you are going past the defrost setting, or floor/defrost, even though the knob is on something different? The compressor will run when in any defrost mode.

I tried the calibration procedure, twice, and no difference in behavior. It's weird that the compressor will not run when the selector is set to dash (unless I turn the A/C on) but it will run when I select floor or floor/dash. I will try the calibration procedure again as it sounds kinda picky about time intervals, but I just might have to live with this. I think your right about the defrost mode.

 

[Turboman280]

It is normal and by design. The compressor will cycle when using heat because the air passing through the cold evaporator is dried in the process. His helps defrost the windshield.