Automotive Air Conditioning Information Forum

Hi

I've been looking at a couple of universal aftermarket AC systems: Old Air Hurricane and Restomod Air Cyclone. Both of these are compact systems with expansion valves. I have noticed both also have thermostat switches. I am no AC mechanic, but I have pulled a few air boxes apart in my time. I've not seen an OEM system with a TXV and a thermostat switch in the evaporator before. I understand it is to stop the evaporator from freezing. How do OEM systems avoid this, or why do the aftermarket systems need it when the OEM systems don't seem to? Or have I just never noticed it in an OEM system and it is common?


Thanks!

Oem systems use what is in effect a thermostat. There is a thermistor probe either in air or fins on the outlet face of the evaporator. Computer reads the temperature and decides when to cycle.

Agree.
Those thermostatic control switches work well, and a TXV system is better than an OT.
The switch is more than an outright freeze-up preventer, it allows you to dial in a more moderate setting if the ambient isn't a full ripping 100°F.

An OEM system with an OT is at this point prolly the most common system out there, and the cycle switch is screwed into the accumulator which is on the engine side of the firewall.

Your aftermarket setups work just fine. Have installed several of them. Have no fear.

BTW as far as the Old Air Hurricane unit, I installed one in a vintage pickup not long ago.
Something to note is there are 4 outlets on the top of the unit. I only was able to fashion 3 vents for the dash. At first we blocked off the 4th outlet but learned from the book "how to air condition your hotrod" by Norman Davis (also known as the legendary Ice-N-Tropics here on this forum), to not do that, as it would cause turbulence. We wondered why the output air flow was less than it should have been. We have been doing mobile air for many years here but had not run into this situation. Problem was solved by running 2 outlets into a WYE and routing that to the 3rd dash vent.
If you have not done mobile AC, his book would provide an enormous amount of help to you in so many regards, especially in matters such as insulation, etc.

Thanks very much, I will pick that book up.

That explains why I haven't seen these thermostat switch units or probes in the OEM systems I've pulled apart. I would guess they have a thermistor in there somewhere, just without the obviousness of a capillary tube snaking out of the evaporator. It seems that these 2 aftermarket units are both using fixed temperature thermostat switch units teed straight into the clutch wire. The Restomod unit's manual says they are achieving temperature blend by running the compressor all the time (except full heat), and then mixing more or less water into the heater core. Obviously it would be better to just run the compressor less, and that is what an OEM would be doing with a thermistor and clutch control.

Miles B wrote: ↑Sat Oct 08, 2022 7:49 pm Obviously it would be better to just run the compressor less, and that is what an OEM would be doing with a thermistor and clutch control.

Yes, but it's not the complete picture. As said in part, a thermistor used in MVAC that is mounted directly on the evaporator and has less resistance at higher temperatures, signals the PCM to engage the compressor as long as the temperature of the evaporator remains at ~33°F or higher. Conversely, below ~33°F, the thermistor signals the PCM to disengage the compressor to prevent the evaporator from freezing up and blocking airflow.

Pressures also play a role in compressor cycling, with a binary (or trinary) switch being used to sense pressures on the high side of a TXV system. A CCOT system uses a cycling switch mounted on the accumulator to sense pressures on the low side of the system.

The Old Air units I have installed (and that is several, including the Hurricane and more traditional styled under dash units) are not fixed thermostats, but variable. You may want to call them and ask directly. I believe they have both types available , however. I personally dislike the idea of mixing in a heat source to control cabin temp, it just makes things more complicated, especially when it is very simple to merely cycle the comp.

In fact, I have brand new in the box here an Old Air variable thermostat. The mfg number on it is A10-6490-000. Cuts out at 24° and cuts in at 45°
Old Air PN is 24-0318.

I am working on an underdash unit from another mfg for a customer where the mfg has installed a variable thermostat that CO at 24° and CI at 65°.
Such a high CI is ridiculous and the cabin warms up too much between cycles, so I am replacing it with the Old Air switch which in my opinion is a better design.

You will find the Old Air ppl responsive to phone calls. Ask for John, but if he is not available any of the techs can help.

tbirdtbird wrote: ↑Sat Oct 08, 2022 9:29 pm The Old Air units I have installed (and that is several, including the Hurricane and more traditional styled under dash units) are not fixed thermostats, but variable. You may want to call them and ask directly. I believe they have both types available , however. I personally dislike the idea of mixing in a heat source to control cabin temp, it just makes things more complicated, especially when it is very simple to merely cycle the comp

I agree on the heat mixing. It just doesn't seem like a great idea. Question on the Old Air.. the thermostat is variable, but does the driver use it to adjust temperature? I was under the impression that it is buried under the dash and adjusted only to get the evap working as close to freezing as possible.

I am favoring the Old Air system. The Restomod is a slightly more compact setup but I don't like the water mixing, and their controls are just a bit fancy for me. I did note their installation instructions don't have a warning about running good antifreeze to stop the heater core from splitting. That tells me they might always be leaking a little coolant through it.

JohnHere wrote: ↑Sat Oct 08, 2022 8:51 pm Yes, but it's not the complete picture. As said in part, a thermistor used in MVAC that is mounted directly on the evaporator and has less resistance at higher temperatures, signals the PCM to engage the compressor as long as the temperature of the evaporator remains at ~33°F or higher. Conversely, below ~33°F, the thermistor signals the PCM to disengage the compressor to prevent the evaporator from freezing up and blocking airflow.

Pressures also play a role in compressor cycling, with a binary (or trinary) switch being used to sense pressures on the high side of a TXV system. A CCOT system uses a cycling switch mounted on the accumulator to sense pressures on the low side of the system.

Understood. I would wager they also hold the evap at a slightly higher temperature for demisting. More control equals a nicer experience in a newer OEM system. I don't think the aftermarket units have captured all of this, but they sure work a lot better than no AC at all!

" the thermostat is variable, but does the driver use it to adjust temperature? "
Correct. But be sure to ask for the variable thermostat I think they have started to install the fixed thermostats, which baffles me if I am correct.
Call and ask for John