Temperature Control in Cars - Heater core and evaporator

[nevinjohn]

When we set the temperature on the dial, are we setting the set point for the thermostat to engage and disengage the compressor clutch, or does the 'blend door' comes into play?

I have an old car, (Ford) and the AC was working well. Few days back, after starting from a 30 min drive the AC vents started blowing hot air. The temperature dial was set to the lowest (18 deg C) and I could hear the compressor was engaging, but for some reason it was just blowing hot air.. Then I started searching many forums and found out about 'blend door actuators'. This gives the flexibility of having different temperature setpoint for driver and passenger vent respectively.

From my understanding, for this to work the heater core and the evaporator needs to work simultaneously. Heater core is usually flooded with engine coolant, but doesn't that impact the AC efficiency? We are having a potential heat source (the heater core) and the evaporator (AC) side by side. Now, coming back to my question, what does the temperature dial do? Does it actuate the blend door actuator, or the thermostat set point? Since there is no VFD, I believe refrigerant flow rate cannot be varied, so is it the combination of both?

Any car techies out here?
Thanks!
Nevin

[charlie patt]

There is multiple ways of controlling ac signal on vehicles varying from sensor ice stats sunlight sensors even suction pressure blend motors mainly on modern cars with digital climate not the older stuff, the idea of blend motors is to make the system a better dehumidifier and give gradual control the best way to check if it a heater issue is leave car in the sun but the engine cold then fire up the car if you get stone cold air of in the beginning and then warms up as the engine gets hotter the hot water is bleeding into the system when a vehicle is hot it will always override the ac due to duty
As far as control goes most manual systems run a stat and a ice sensor when on part or full heat the ac stays in until coil ices thus then the ice sensor cuts in the heater knob on manual ac purely controls the heater control valve the heater is normally pre heater thus air is dried on blend units normally digital or twin compartments this is controlled manually or via ac climate are control ECU
If all running well but the ac vents are warm check your suction pipe for sweating if suction pipe is sweating well but no cold air possible heat overriding ac