zero2005

just wondering if anyone has any info on how the HVAC for our cars work.
i get the basic theory on how AC works (phase change). and i'm assuming that the heater works by passing the coolant from the motor after its been heated into the heater core.

but i'd like to know more like how it "controls" the temperature - how does it achieve the "warm" temperature? does it modulate the AC compressor (on/off) to achieve a warmer temperature (from cold)? does it control the flow of the coolant through the heater core?

how do the controls work? what info does it pass to the ECU - if anything? does it run mostly standalone or does it rely heavily on the ECU?

for some reason i'm really interested in this. i guess its because i like to relate car stuff to aircraft stuff lol.

Bmitchell373

The heat side of the control is fixed based on how "hot" you turn the dial. It has a valve and will limit flow if its not turned to full hot. The a/c side has a thermostat in the evaporator and will cycle the compressor.

zero2005

ah alright, so the AC is not only controlled by cycling the compressor, but it also limits the flow of gas through the expansion? interesting.

does the blower(fan), blow air through both the heater core and the evap core simultaneously (as if they are in series?)? now i really wanna take my dash apart just to look at it and poke it. lol.

i'm assuming that it doesnt "mix" air (eg, pass cold air from evap core through heater core or vice versa)? it just controls how hot or cold the core is to get the temp i select?

sorry for the odd Q's, its easier for me to relate it to the AC system of a KC-10. not it makes alot of sense for me to do so lol.

Bmitchell373

It dosent control flow of the refrigerant. It has a fixed expansion valve but that just part of the system. I do believe the evaporator and heater core are in series, but i do not know the specifics on our cars as to when its on fresh air or reticulation.

Bmitchell373

I havent checked on my car yet, but my old ranger and my moms old expedition. when you had it on mix (dash and floor) it would kick on the a/c, even with it set to full hot. Id assume the mustangs do it as well. To me it would have to heat the cooled air. The only reason i can see for this would be for the air dryer.

zero2005

ah... hmm... if thats the case then i could only assume that the heater core and expansion core are definately series then - probably with the expansion core first...

interesting... i wonder if i could make up a completely stand-alone temperature control system lol. that would be interesting. guess i'll have to get a good look at it when i take my dash apart. planning on a completely custom (fiberglass?) dashboard eventually

Bmitchell373

That would be cool, or go aftermarket hvac

zero2005

yea, like that touch screen one they have for the new 2011 mustangs. i havent seen anything that would fit our cars though. guess it wouldnt matter if i made my own dash, but even still, seems like 95% of people are content with whatever the stock HVAC controls are.

cliffyk

No pun intended, right?

Good explanations from BMitchell373, one point of clarification however; it is the airflow through the heater core that is variably blocked or allowed as the cabin temperature control is adjusted.

The system used vacuum motors to control flapper valves that direct the airflow. Here is the HVAC system description document from the 2003 shop manual. It has some great diagrams showing the relative positions of the evaporator and heater core, and how the air flows depending on system settings.

Bmitchell373

actually no haha

i know it has the flapper valves for directing airflow to the different dash vents, but do they also "bypass" the heater core for when the a/c is on and vise-versa?