Random thought/question: Automotive HVAC systems

Aug 3, 2017
I'm not sure what brought up this thought, however it's been on my mind and I'm curious as to whether any member of this forum can shed some light on the subject. After buying my 2nd Grand Cherokee, I noticed that the automatic A/C control panel had what looked like a sunlight sensor built into it. I've seen this sensor in other Mopar vehicles, but never thought much of it. I've owned my share of vehicles equipped with one form of auto A/C or another, yet my GC seems to do MUCH better at maintaining not only the cabin temperature as a whole, but also managing zone temps when the significant other and I have our temps set differently.

After some digging, it turns out said sensor isn't a solar sensor at all, it's a dual infrared sensor designed to detect the heat radiation of the front seat passengers, plus the surrounding areas. All the systems I knew of previously consisted of a small fan mounted in front of a thermistor which took a sample of cabin air temperature and used that to calculate the proper adjustments (sometimes in conjunction with other sensors).

Maybe I'm missing something, but is the infrared system not leaps and bounds better than the ambient system? Say in a particular car, the thermistor is mounted slightly below a dash vent. Would it not just sample a disproportionate amount of cold air that's naturally sinking, vs. actually knowing what temperature the majority of the cabin and/or the passengers display? Granted, maintaining the temperature of an automotive cabin is basically impossible without massive investment in thermal glass and insane amounts of insulation, however while they're in motion the modern systems are actually quite capable of providing a comfortable environment.

Case study: I've made the ~2.5 hour journey to a buddie's cabin several times already this summer, in temps varying from 75-98 degrees. In my Volt, setting the climate to "comfort" mode, selecting 70 degrees, and leaving all other settings on AUTO results in a both A: too much fan speed and B: far too cold of an output temperature to be tolerable for that length of drive. I constantly find myself adjusting either the temp or fan speed trying to find the right combination. Automatic just doesn't work. Looking at the schematics, the Volt system doesn't use an interior temperature sensor at all, it seems to rely on a combination of sun load sensor inputs and a "windshield temperature sensor/in-car humidity sensor" mounted to the front glass.

In contrast, my Jeep has absolutely no problem keeping the cabin at the perfect temperature, all settings being the same. 70 degrees in both zones, auto fan and auto mode selection. I can literally watch the display change when it goes from vent to bi-lev, recirc to fresh, and back again while it does it's job maintaining the selected temperature. I can't help but think this is directly related to the system being used. The Jeep uses no sun load sensor, no cabin temp sensor, no humidity sensor. It simply relies on radiant heat readings from the object it's sensors are focused on. In this case, that's me. By forming a picture of my body's PERCEIVED temperature of the cabin, instead of the temperature of the cabin itself, it has a marked advantage in maintaining a truly comfortable environment.

Since I've noticed this phenomenon, I've played with several auto A/C units in various vehicles at work while test driving, and I have to say none of them seem to work as well as the system in my Jeep. I also have a 2013 Chrysler 200 that a buddy and I bought for a flip, and it uses the IR sensor technique in it's system and again, it's quite good.

Anyone else care to chime in with their experiences?
My only experience with auto climate control in cars is the Dodge/Jeep/Chrysler system and I HATE it, mostly for the fact setting it to say 72 even when it’s in full manual control results in it blowing air that feels significantly cooler or warmer than 72F, almost like it’s trying to get the cabin to that temp and maintain that temp. Even once the cabin has pretty much equalized in temp I’d notice a change in air temp coming from the vents, just blow whatever temperature air I tell you to blow!

My Ram does not have that system and it’s great. Adjust the dial and the air blows 1 consistent temperature.
That's exactly what it's doing...just like the thermostat in your house does.
It drives me nuts, it’s always right as I hit peak comfort it adjusts the temperature!

One thing I forgot… dual zone climate is a must, but sadly it’s coupled with auto climate… I’m always cold and my wife is always hot (😉), in the van I’ll have my side set to like 75-78 in the summer once the cabin is cooled down and she will have hers set as cold as it will go.
Over on alldata, the writeup for a GM auto system says, in part:

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators automatically adjust to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:

- Monitors the following sensors:

- Ambient air temperature sensor

- Ambient light/sunload sensor

- Duct air temperature sensors

- Evaporator temperature sensor

- Windshield temperature and inside moisture sensor

- Regulate the blower motor speed

- Position the air temperature actuators

- Position the mode door actuator

- Position the recirculation actuator

- Request A/C operation

- Control of the A/C compressor

I know in my car I can program the minimum blower motor speed to be high/med/low. Med is the OEM default and it's too high for my taste, but I've tried low and it's too low for warmer weather. The '03 TL-S auto climate did things better, but the blower motor speed went so low it would burn up the transistor pack (insufficient airflow) if I didn't change the cabin air filter every 6 months.

Try reprogramming the minimum blower motor speed in your Volt.
Only car I had with auto climate control was my Ford Taurues. It worked great and set it and forget it.
My 2005 Buick Century, Canadian built did quite well for me but thr girlfriend I had at the time would complain about 1 degree
Volvo p2 had the best auto climate control I’ve known. Very intuitive and stayed out of the way!
My 2010 F-150 doesn't have the automatic AC but it does have a solar sensor on the very front of the dash and just below the windshield. But it IS an IR sensor and it does monitor the strength of the sunlight falling on the windshield and the car and it does not monitor the front seat passengers' body temperatures. That signal is used to control the AC system but not the temperature setting directly. I'm pretty sure that my old 2000 Mitsubishi Diamonte had something similar and I expect that most modern mid to high level cars also have one. If nothing else, to help reduce the amount of AC usage when it's not necessary and to very slightly increase the gas mileage and to very slightly reduce emmissions.
One thing I hate about the setup on the Volt is...if I set the temp to 75F during the day when it's 80F outside, it runs the AC.

If I drive it later in the evening, the outdoor temp may have dropped to 65F, and now it runs the electric heat to maintain 75F, so I gotta lower the temp to keep it from using the electric heat.

If I could tell it "maintain the temp inside the car anywhere from 60F to 78F and use the minimum amount of energy needed" that would do what I want.

I've also seen the Volt turn on the electric heat even if it's 85F outside, if you turn the temp up quickly (say, going from 70F to 75F, it might turn on the electric heat when it's 85F out..)