Automotive Air Conditioning Information Forum

Hi my name is Clint and I'm working to get the factory air working on my 1970 Chevrolet Nova. I am a mechanical engineer with a background on the theory of refrigeration cycles, but this is my first time diving into the practical implementation of automotive AC. I stumbled on this forum while researching POA valves a few years ago and I hope sharing my experience here will establish two-way learning. Thank you for any help/advice you can offer as I work through this!

The Car and System

My car is a factory AC 1970 Nova. I have swapped the engine for a modern LS engine, but I've kept the factory AC "suitcase" on the firewall, retaining the evaporator and all the factory controls. Here is what the car looks like inside and out:









The original system uses a thermostatic expansion valve (TXV) and pilot operated absolute suction throttling valve (POA STV). I plan to keep these components and the original evaporator and controls, but I will be connecting them to fresh barrier hoses, a modern parallel-flow condenser, new drier, and a Denso 10S17 compressor which I've tucked down low in the OEM location on the LS engine. I will also be converting to R134a. The system uses a two-speed electric fan for radiator and condenser airflow.

Theory of Operation

The operation of the system will, as originally designed, run the blower and compressor full time any time the AC or defroster is selected. The original HVAC controls have mechanical/cable duct controls to route air through the evaporator and/or heater core depending on whether the system is in AC, blend or defrost mode and incorporates a vacuum valve to open and close fresh or recirculated air. It also had a vacuum-controlled valve to shut off flow to the heater core when in AC mode.

The refrigeration cycle basically works as follows:
1 - Vapor refrigerant at low pressure is compressed to a high pressure by the compressor which leads to an increase in temperature above ambient.
2 - Vapor refrigerant at high temperature and high pressure then delivered to the condenser in front of the car where ambient air blowing across the condenser removes energy from the refrigerant and it condenses into a liquid. This heat transfer is possible because the refrigerant is at a higher temperature than ambient.
3 - High pressure liquid refrigerant passes through a drier which traps any moisture and prevents it from circulating in the system.
4 - High pressure liquid refrigerant passes through a small restriction at the TXV which causes the liquid to drop to a low pressure and low temperature (see more explanation on the TXV valve below)
5 - Low pressure low temperature liquid refrigerant evaporates inside of the evaporator by sucking heat from the air passing across the evaporator into the cabin. This heat transfer is possible because the temperature of the refrigerant is lower than the ambient air passing through the evaporator into the cabin.
6. Low pressure refrigerant passes through the POA valve and back to the compressor. Under some conditions the POA valve restricts the refrigerant to maintain a minimum temperature in the evaporator (see more explanation on the POA valve below).

When the system is being used for defrost, the goal for the evaporator is to condense water out of the cabin air so dry air can be heated by the heater core and directed at the window to defrost.

The TXV valve is located at the inlet of the evaporator. It is a flow regulating device which meters the amount of refrigerant delivered to the evaporator based on superheat. It also creates the pressure drop that causes the refrigerant to evaporate. I spoke with 4 Seasons about their replacement TXV valve pn 38622 for this application and found it is calibrated to maintain a superheat of 10.5F for R134a. The TXV operates based on a reference pressure from the evaporator and an exit temperature measured by a bulb. The evaporator pressure sets the evaporation point aka saturation temperature of the refrigerant. The superheat is the difference between the saturation temperature and the exit temperature of the evaporator = essentially it's how much beyond boiled you make the refrigerant to ensure no liquid refrigerant makes it back to the compresser.

The POA valve is located at the exit of the evaporator. It is a pressure regulating device which ensures the evaporator pressure is always above a minimum setpoint. Since the evaporator pressure sets the evaporation point aka saturation temperature of the refrigerant, regulating the minimum evaporation pressure also regulates the minimum evaporator temperature. The POA is calibrated from the factory at about 30psig. For R12 freon this corresponds to a saturation temperature of 32F - the freezing point of water. By keeping the evaporator at or above the freezing point of water, the POA ensures any condensate from the cabin air does not freeze on the outside of the evaporator. This is especially important at low ambient temperatures or low blower speeds where the system is able to outpace demand and might otherwise just ice up the evaporator. Unfortunately 30psig corresponds to a saturation temperature of about 34.6F for R134a so if the POA is not adjusted, the evaporator will not perform as well. To get to 32F saturation temperature for R134a, the POA would need to be adjusted to 27.75psig.

The Plan

Compressor Electrical:
I plan to use the original system to power the AC compressor full time when the AC is running, however I will add a binary cutoff switch for over pressure / under charge cutoff at the drier.

Fan Electrical:
I will add a relay to ensure the engine fan is running any time the compressor is powered. Some people use a trinary switch for this function, but after looking at a number of trinary switches it does not appear they would run the fan at normal operating pressures. My fan will either be commanded hi/low speed by the LS engine controller OR overriden to high speed by a relay when the AC compressor is running.

Duct controls:
I have lubricated all the cables that operate the doors and flaps which direct air through either the heat core (heating/blend), evaporator (AC/blend) or both (defrost mode). There is also a vacuum valve that controls fresh or recirculated air by operating doors at the cowl and in the kick panel areas. I have not yet tested this valve or the flaps, but I will need to hook it up and make sure it and all the flaps are working properly

Heater control:
The original system uses a vacuum-controlled valve to shut off coolant flow to the heater core when the system is in AC or vent mode. However this was just a simple shutoff valve. The LS engine waterpump relies on full time flow from it's heater ports, so I will have to find a new directional control valve that will either direct coolant through the heater core or recirculate it back to the water pump.

Blower control:
The original system uses a set of resisters to achieve 3 blower speeds (low/med/high). I tested these and they are functioning properly. The system also uses a dedicated relay for the high current demand of high speed only. This relay had malfunctioned, so I replaced it and confirmed high speed now works. The blower itself is original and seems to be functioning okay. I wish there were a way to test how much air flow it's putting out - I am not sure if it's blowing as hard as it should. It certainly doesn't blow as hard as any of my modern cars, but I don't know if it's just normal for a system designed 50 years ago or if the performance has degraded over that time. Any suggestions on how to test the blower performance?

Ducting:
I have repaired a few holes in the fiberglass on the evaporator suitcase. I will look for and repair leak paths in the ducting to be sure all the air from the blower is directed where it needs to go.

Condenser/Drier
I have installed a large aftermarket parallel flow condenser tightly in front of the radiator and a new drier located in the factory location on top of the fenderwell. This should improve capacity to compensate for potential performance loss due to converting from R12 to R134a. It is a better heat exchanger than the original tube & fin design and much larger.

Compressor:
I am using a Denso 10S17 because it is the factory compressor attached to this LS engine and therefore has a good mounting and tensioner setup. I am hopeful it will provide adequate performance for this system. I am using a set of Docs Blocks adapters to go from the ports on the compressor to SAE o-ring fittings.

Hoses and o-rings:
I will fabricate custom barrier hoses using beadlock hose fittings. I will borrow a Mastercool tool to crimp the ends onto barrier hose. All the fittings will use new o-rings, including at the POA valve.

POA valve:
I will test and recalibrate the POA valve to run at a suitable pressure for R134a. As noted above if I want the saturation temperature to be 32F, the POA should be adjusted to 27.75psig, however I have seen people on here adjusting it to lower pressures for example 26psig which corresponds to a saturation temp of about 30F (a couple degrees below freezing). I do not want the evaporator to ice up if I use this for defrost or at low blower speeds - does anyone have suggestions on where to set the POA valve?

TXV valve:
I will replace my valve with a new one from 4 Seasons pn 38622 which is calibrated for R134a

Flushing:
I will flush out the POA valve and the evaporator using an over-the-counter aerosol flush. Since the POA valve is easily removed I will bench flush it and coat the inside with oil (fill and dump out) so make sure it has lubrication at startup. I will have to flush the evaporator on the car, so my plan is to spray flush into the upper outlet fitting and attach a clear hose to the inlet so I can direct the flush into a bucket. I will then have to blow out the evaporator with compressed air from the outlet side because it will otherwise trap flush in the bottom.

Charging:
A friend of mine does refrigeration work and will help me with charging the system, however I am not sure what method should be used to establish the right amount of R134a for this hybrid system. All the volumes in the system will be different from original due to a larger condensor and different hose routings, so I don't know if the original R12 charge amount would be a good starting point. I may have to charge using the sight glass and/or approximate pressures. If anyone has a link or can suggest an appropriate method for charging please let me know!

Oil:
I could use suggestions on what oil and how much to use. I was thinking about using Ester100 because of dual compatibility, but the Denso compressor was originally used with PAG46. I am also not sure how much oil or refrigerant to use since I have created a hybrid system

Thank you for any help/support as I embark on this journey! I will begin sharing photos and video soon detailing the process.

Good plan, PF condenser and retain the TXV and POA that everyone wants to toss in the trash for cycling crap. Would be better to let the stock PCM handle compressor control. With the stock pressure sensor installed It'll handle the fans, under/over pressure, idle compensation, RPM cutout, etc. Factory compressor and drive ratio the idle compensation should be dead-on.

Any suggestions on how to test the blower performance?

I like to check max current draw and voltage at fan on high. 15-20A would be max, general rule of .1v per connection in the path.

PS, remember seeing you and this car on ls1tech. Been a long time good to see it completed.

POA valves.
Be gentle with the schrader valve on the oil return line at the POA body. It is not a standard schrader - it is a special low cracking pressure part & they are getting hard to find. It is used as a check valve to let the oil in the bottom of the evaporator bypass the POA when the suction line pressure is below 15 psi & return to the compressor inlet.

Adjustment.
The 26 psi adjustment vs 27.5 is largely academic.
The refrigerant may be at a pressure that coincides with a 30f gas temperature, but you still have the conductivity of the metal the evaporator is made of, and any air flow over the evaporator. Also remember that the average pressure gauge is +/- 2% of full scale accuracy, so different readings for gauge set to gauge set can be more than 1 psi.

Defrosting.
Generally you are running the heater in winter. If you are in a location where the dew point is high enough to cause fogging inside the windshield around 60f ambient, then you might be running the refrigeration loop to remove humidity while running the heater as well. This condition would be called tempering, and there should be enough heat to prevent icing.

The 4 port heater valve is an improvement on any system, because it offers better isolation of the heater core. There are plenty of them to choose from. If you find an electric one you like, a vacuum switch & a relay will make it work with your OEM A/C system.

Use the PAG 46 oil the compressor calls for, the POA should be fine with that.

Obviously you already know you can't use the PCM to run the compressor, because it needs the feedback from the 3 wire pressure sensor - and it wants to cycle the compressor based on that information. Run the compressor from the OEM 1970 circuit.

The TXV's are rarely an issue. Superheat is not something that matters as much when there is a POA downstream. If it floods the evaporator a little at low heat load, the low pressure on the suction side of the POA takes care of things. Not uncommon to see frost on the exit of a POA under light loads. Keep in mind the OEM suction hose made a wide, horizontal sweep to the back of the compressor. I would avoid a short, downhill run to the new compressor.

Cabin fan airflow.
The percieved airflow can depend on several things. If you have not had the suitcase out of the car yet, there are seals inside that need attention. Air bypassing the evaporator & heater core, flap seals etc. all can cause less than adequate flow. The OEM systems always had plenty of volume when they were new. Fan motors can wear out of course. Current draw can go up if the bearings are dragging, or if it has shorted turns. They are pretty tough however.

Flushing the evaporator in place can be done, but it is easier with it out. Besides the previously mentioned seals...

You can charge the system with the help of the sight glass. Just because you have a larger parallel flow condenser, don't assume it needs more refrigerant. The PF condensers often take less refrigerant - they have more area, and expose more refrigerant to transfer surfaces - with less volume.

bohica2xo wrote:Be gentle with the schrader valve on the oil return line at the POA body.

I was thinking to just leave that valve in place. I worry about damaging it while trying to remove and reinstall. Is there any reason to remove it for flush or test? If not, I will leave it undisturbed.

bohica2xo wrote:The 26 psi adjustment vs 27.5 is largely academic.

This makes sense. I understand that having the refrigerant at 30F does not necessarily mean water will freeze on the evaporator, but I was curious because it seems the original setpoint from the factory corresponded with 32F. Maybe they were being more conservative than needed?

bohica2xo wrote:The 4 port heater valve is an improvement on any system, because it offers better isolation of the heater core. There are plenty of them to choose from. If you find an electric one you like, a vacuum switch & a relay will make it work with your OEM A/C system.

Are you aware of a vacuum operated bypass valves that would be suitable? I believe the valve is normally closed and then opened by vacuum to allow flow to the heater core.

bohica2xo wrote:Use the PAG 46 oil the compressor calls for, the POA should be fine with that.

Thank you. How would I figure out how much is needed?

bohica2xo wrote:Obviously you already know you can't use the PCM to run the compressor, because it needs the feedback from the 3 wire pressure sensor - and it wants to cycle the compressor based on that information. Run the compressor from the OEM 1970 circuit.

I agree and this is my plan. Also I believe my PCM and operating system (GM E38 '07+ Silverado) would require a body control module (BCM) for the AC request, which I do not have. I would prefer to use the original controls which are very simple and setup to run the compressor continuously.

bohica2xo wrote:Keep in mind the OEM suction hose made a wide, horizontal sweep to the back of the compressor. I would avoid a short, downhill run to the new compressor.

This might be an issue for me. My compressor is located low on the passenger side, almost directly in front of and below the evaporator. The most obvious hose routing is to have it trunk directly down to the compressor. What is the concern with this kind of routing? Is the concern potentially delivering liquid refrigerant to the compressor? I will share a photo shortly of the mockup for this hose.

bohica2xo wrote: If you have not had the suitcase out of the car yet, there are seals inside that need attention.

I did open up the evaporator suitecase about two years ago. There were a few cracks in the fiberglass and one area I needed to notch for clearance with the engine. I re-fiberglassed those areas and resealed the case as I put it back together. I replaced the seals at the front of the evaporator, the seal around the heater outlet, and the duckbill valve at the bottom of the suitcase that allows condensate to go out. Under the dash there were some cracks in the ducting also which I taped over with duct tape as a temporary repair. Eventually I will take them off and do fiberglass repair. I have not yet replaced the seals between each individual duct under the dash and there is probably some leaking going on there which I should check for.

bohica2xo wrote:Flushing the evaporator in place can be done, but it is easier with it out.

Taking the suitcase off would be pretty difficult at this point so if I can flush without removal, that is my preference. Is the way I outlined it above appropriate or should I be thinking about another method?

bohica2xo wrote:You can charge the system with the help of the sight glass. Just because you have a larger parallel flow condenser, don't assume it needs more refrigerant. The PF condensers often take less refrigerant.

Thank you for explaining! Do you suggest adding refrigerant until the bubbles clear or should I be targetting specific operating pressures or is there another approach?

Suction Hose

I have been sorting out fittings and hose ends to plumb the system. One place where a standard fitting poses a problem is at the exit of the POA valve. This joint is a #12 o-ring, however the standard size now used for the suction line to the compressor is #10. I needed a 180 degree #12 to #10 "step down" fitting which was much harder to find than I thought it would be. First I ordered one from Nostalgic Auto Air part number G135. I was hoping I could flip this fitting down so the hose would exit underneath the POA valve, but unfortunately it interfered with the hood hinge spring. So here is what it looks like aiming in the factory orientation



I didn't particularly like the aim of that fitting or how the barrier hose would be routed along the top of the evaporator. I found another option from Original Air aka Classic Auto Air part number G13-407 which mimics the original "shepard's hook" fitting, but is sized with a #10 hose outlet.



I picked up a prepackaged assortment of #6, #8 and #10 beadlock fittings and hose from ebay. #10 is used for the low pressure vapor suction line from the evaporator to the compressor. #8 is used for the high pressure vapor line from the compressor to the condenser. #6 is used for the high pressure liquid line from the condenser to the drier and back to the TXV valve on the evaporator.

I cut the #10 hose for the suction side. I can't think of a better way to route it than to just elephant-trunk it directly down to the compressor. bohica2xo expressed concern about having it route down like this and I'm interested in any suggestions if I should be routing it differently.



I have been debating whether or not to add a low side service port near the compressor. The engineer in me wants to have a test point to understand how the POA is operating, but I think this is not necessary since POA issues should still be detectable at the POA low side service port.

Leaving the check valve alone is a great plan. Too often people get carried away, and assume it is a regular tire valve...

You can flush the POA with a light oil & plenty of dry air. You will be flowing through it recalibrating anyway.

Since docs blocks has been out of business for a while, I believe this is a long term project. If you calibrate & pop test the POA with some of the oil you intend to use & seal it up in a plastic bag it should be fine. Especially if you use some DEC PAG 46.

DEC PAG explanation https://www.autoacforum.com/messageview ... adid=17571

The bottom photo with the long J pipe should be just fine. A little superheat insurance before the drop to the compressor. That J tube is mostly horizontal & being bae aluminum it will transfer heat better than several feet of hose across the top of an engine bay.

Thank you bohica2xo! Can't tell you how much I appreciate the input.

Yes this was a long time in the planning. I bought the docs adapters when I first got the compressor because I was aware they were going out of business and they were liquidating supply. However at this point my timeline is within a few weeks. I want to have the system running at least for the tail end of the summer. We tend to have some hot days in September where I live. I am thinking I might leave the heater core disconnected for now and tackle the bypass valve before winter, but otherwise I want the system fully operational in the next few weeks.

I appreciate the input on the J-pipe adding some superheat. That makes sense and it's kind of where I thought you were going with the concern on the short, downward hose. The J-pipe has about 7" of exposed aluminum plus any heat that gets picked up in the POA and the evaporator exit - hopefully all that will keep the compressor fed with superheated vapor only.

I also checked my 4 Seasons replacement TXV and it is actually already setup for R134a, so the 10.5F superheat setpoin is applicable to R134a. The TXV is part number 38622.

History Detour

I have had my car for 20 years now. Back in 1998 when I first got it, the original engine and AC system were fully connected, although I don't know if the system was functional. Here is what things looked like way back then:



Shortly after I got the car, I took the whole front end off it it and rebuilt and cleaned up most of what I could get my hands on. It was at that time that I first took apart the AC, including taking off the POA valve and putting it in a ziplock bag - the same bag it's been stored in for the last twenty years. At that time I was particularly obsessed with the idea that everything should either be painted or polished. That lead me to the idea to polish the tubing on the evaporator. I don't think I even knew what the evaporator was at that time - I was only 15!



It took me about two years to get the car running again.



I wanted to get the AC working back then but life happened and I did very little work on the car until after college. The evaporator was taped off with duct tape at the TXV inlet and the outlet fitting, as were the small oil feed tube and the reference connection for the TXV. The car has been driven a lot since then in many different and dirty environments. Given this history, it will be extra important to flush the evaporator.

For heater valves on GM stuff I use a vacuum unit, like the AC Delco P/N 155536 It was used on G vans, and P vans from 1990 to 1996.

There is a 4 port valve on the 03-04 CTS as well, it is vacuum with a solenoid valve.


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POA Valve Testing

The POA has followed me in storage for the last twenty years, always kept in a ziplock bag in a storage bin of parts for the Nova. It's been moved dozens of times and it's sort of a miracle I haven't lost it. For a long time I was dragging around all the AC-related parts including the big heavy A6 compressor, tube & fin condenser, drier, and hoses which I did not know whether or not I would need, but some years ago I started getting rid of the parts after I realized I likely would not use them. I've been really anxious to see if the old gal still has life in here and I finally got the chance this weekend to test it out.

The first step was removing the shrader valve from the original service port.





Then I installed an adapter fitting to convert to old thread-style R12 fitting to an R134a style coupler fitting which has it's own shrader valve. I found this one at NAPA. It has an o-ring inside that seals against the face of the old R12 fitting and also has sealant on the threads. Hopefully it does not cause a leak.



This fitting allowed me to connect a set of R134a gauges so I could see if the POA valve would open and regulate at the set pressure.



Before testing, I got some paper towels to catch any spray from old oil that might come out of the POA exit. Then I supplied pressure at the evaporator inlet (about 80psi from my air compressor). I was able to confirm the valve opened and maintained an evaporator pressure of just under 30psi. I was really happy to discover it still functioned despite being almost 50 years old and spending the last 20 years in storage!



I wish I could have found a video explaining this because it was pretty tough locating information on POA testing and calibration. There are some great threads right here on the autoacforum but they are archived and difficult to search, plus some photos are now broken links. So I decided to make my own video that will hopefully help others in my situation. Here it is!

https://www.youtube.com/watch?v=PhWW7hD9-Yw

Some time this week I will try to adjust the valve down to a lower pressure suitable for R134a. I think I will target 26psig as suggested. I should note that I am located near sea level at just about 200ft elevation.