1972 Air Conditioning

[Michael C.]

I was hoping to get some pointers as I try to get my 72 A/C to work properly. Here is the current status:

My system was working fine with a previous retrofit to R-134a.

The A/C stopped blowing cold.

My POA valve was apparently stuck as I was getting the same psi readings on both the hi and the lo side, so I replaced it with a "restored" original POA valve which I tested before installing with compressed air (I assume is it set up for R-12). I input 75 psi and the output was 28 psi. I installed that valve, evacuated and charged the system.

Performance test results:

No Cooling in interior.
Running R-134a -- I put in 2.6 lbs. per specs.
Ambient Air Temp= 80 deg. F.
Engine RPM= 2,000
Inside temp at vent= 80 deg. F.
Pressure Readings= Lo = 30 psi Hi = 144 psi
POA valve is icing up at outlet end. Compressor hose -- Lo fitting is cold and icing at the compressor fitting.
When running, the Sight glass shows bubbles. When the engine is shut off, sight glass foams then clears within 30 sec.

My thoughts are that my compressor may be worn out, but I am hoping it's not something so drastic.

Any ideas would be appreciated.

Thanks,

Mike

[Tom L.]

Re: 1972 Air Conditioning

Is the suction line cold, or at least cool at the outlet of the evaporator? Your pressures seem low even in cool conditions. The high side should be MUCH higher if under load and not moving down the road. The low side is also a bit low. The fact that your sight glass is not clear and the pressures indicate a low charge.

Coincidentally I charged my A/C today on my '72. the sight glass is a tool, it should be clear (or you should see very few bubbles to be on the safe side). Another indicator that the suction line should be "Beer can cold" if you're in the ball park in terms of the charge. High side pressures should be quite high, closer to 200+ (numbers in the refrigeration world, where I live, are way over the top). Hope this helps. Or other will chime in. Have fun!!!

[Larry M.]

Re: 1972 Air Conditioning

Mike:

I see a number of issues going on with your air conditioning system, and will try to address each one.

First thing I notice is that with the compressor running and the suction lines getting cold, there appears to be no heat transferred by the evaporator. You do not tell us what your blower setting is, but it should be on HIGH and the outside air vent door should be closed (i.e. total cabin air recirculation). Please confirm. But again, there appears to not be any heat transfer in the evaporator, since the cabin cooled air is the same temperature as the outside air (80 F).

I am unsure about the 1972 factory air cars, but the C2 cars had a vacuum operated shutoff valve in the hot water going to the heater. But I think they did away with this for the 71-72 cars.............only to bring it back in the later C3 years. So, in my opinion, you either have an interior ac/heater box door that is not working or you have a failure of this hot water valve (if you have one). Your "cooled and conditioned" air is either not going thru the evaporator core OR it is being both heated and cooled together resulting in no overall cooling.

Please confirm if you have this heater water shutoff valve, and if so, is it working to stop hot water flow to the heater box. Also check your cables or vacuum hoses to make sure the interior doors in the ac/heater box are moving as they should. The factory service manual has an excellent section on HVAC that you should read and study the diagrams.

Now some of the other things I see that do not seem correct:

1. If you truly added the 2.6 lbs of R134a to the system, you should not see any bubbles in the drier sight glass.......unless you have either air or moisture still in the system. However, both of these tend to provide high compressor discharge pressures and not lower pressures. But this issue still needs to be addressed.

2. Your 30 psig suction pressure looks fine, but having icing on the POA and a very cold compressor inlet connection tells me you have liquid refrigerant getting out of the evaporator core. It should be gas that is heated slightly above saturation conditions (this is called superheat). Your expansion valve is designed to provide abut 10-15 F superheat and if things are too cold, this TXV valve should close off and slow down or almost stop the refrigerant flow to the condenser. Again, this low temperature you are seeing with the icing, tells me the evaporator core is not transferring heat and something is wrong in the heat/ac box.

3. Your high side pressure should be around 180 - 220 F at 80 F ambient temperatures, and the air from the cabin ducts should be around 40 F for a properly operating factory system. The lower pressure you show could be a compressor issue or it could be you have other issues going on. I think it is the latter. Having R134a refrigerant in the system should show even higher compressor discharge pressure.

4. It is possible you have a low charge................but you would need to be down to 1 lb or less of refrigerant to give the symptoms and pressure numbers you are showing. And then, your POA valve (evaporator outlet) would be very warm and not cold or icing. And there would be LOTS of bubbles in the drier sight glass.

5. Having issues with the TXV valve are possible, but the charts I have indicate a stuck open valve will have HIGH pressure readings for both the low and high pressure sides, and a TXV valve stuck closed will have LOW pressure readings for both the low and high side pressures. Again, your readings do not indicate either of these.

6. A compressor malfunction or failure will show HIGH low-pressure readings, and LOW high-side readings. Again, not what you indicate.

I am assuming that your low pressure side reading are obtained at the POA valve............confirm that this is the case and that you are not getting them from another place.

To help you any more, I need for you to run your air conditioning system again. RPM 2000, blower on high speed, no outside air, only recirculated air. Watch when the compressor starts and observe how the low-side and high-side pressure change and their final values running at steady state. The use an IR temperature gun and thermometer and give us the following:

Outside air entering condenser
Cabin duct temperatures at right and left ducts
Temperature of compressor outer casing
Temperature of condenser inlet at the condenser
Temperature of the drier outer case
Temperature of the evaporator inlet metal tubing
Temperature of evaporator outlet/POA inlet at the first uninsulated opening.
Temperature of the POA outlet to the compressor

Also if bubbles are still visible in the drier sight glass??

I personally believe your issue is with the heater/ac box or the heater inlet water valve............but providing the additional information will allow a better troubleshooting of your system. I also think you have some air in the refrigerant that needs to be corrected.

Let us know what you find.

Larry

[Bruce C.]

Re: 1972 Air Conditioning

Add another half pound of Freon and then performance test again.Sounds like system is slightly low charged.

[Michael C.]

Re: 1972 Air Conditioning

Llarry,

Thanks for your thorough analysis.

After reading Bruce's post, I think he may be correct in that I may have an undercharge, as I forgot to mention that I did have a schrader valve stick open on me and lost some R-134a before I could get it closed.

Before I put more R-134a in, here are the temperatures I got with an IR gun. I am skeptical of the accuracy since I got 41 deg. on the POA outlet when it was visibly icing.
I ran the system for 10 minutes and got the below specs:

Pressure at Static State (engine off)= Lo 78 psi, Hi 78 psi
Pressure at startup= Lo 25 psi, Hi 122 psi
Pressure at 3 minutes= Lo 29 psi, Hi 140 psi
Pressure at 5 minutes= Lo 29 psi, Hi 145 psi
Pressure at 10 minutes= Lo 29 psi, Hi 162 psi

Outside air entering condenser= 75 deg
Cabin duct temperatures at right and left ducts= 75 deg
Temperature of compressor outer casing= 145 deg.
Temperature of condenser inlet at the condenser= 105 deg
Temperature of the drier outer case = 85 deg
Temperature of the evaporator inlet metal tubing = 127 deg
Temperature of evaporator outlet/POA inlet at the first uninsulated opening. = 132 deg
Temperature of the POA outlet to the compressor= 41 deg

Also if bubbles are still visible in the drier sight glass??= Yes, lots of foam.

Maybe I just need to add more refrigerant?

I can do that this evening - if you think it might help.

Best,

Mike

Mike:

I see a number of issues going on with your air conditioning system, and will try to address each one.

First thing I notice is that with the compressor running and the suction lines getting cold, there appears to be no heat transferred by the evaporator. You do not tell us what your blower setting is, but it should be on HIGH and the outside air vent door should be closed (i.e. total cabin air recirculation). Please confirm. But again, there appears to not be any heat transfer in the evaporator, since the cabin cooled air is the same temperature as the outside air (80 F).

I am unsure about the 1972 factory air cars, but the C2 cars had a vacuum operated shutoff valve in the hot water going to the heater. But I think they did away with this for the 71-72 cars.............only to bring it back in the later C3 years. So, in my opinion, you either have an interior ac/heater box door that is not working or you have a failure of this hot water valve (if you have one). Your "cooled and conditioned" air is either not going thru the evaporator core OR it is being both heated and cooled together resulting in no overall cooling.

Please confirm if you have this heater water shutoff valve, and if so, is it working to stop hot water flow to the heater box. Also check your cables or vacuum hoses to make sure the interior doors in the ac/heater box are moving as they should. The factory service manual has an excellent section on HVAC that you should read and study the diagrams.

Now some of the other things I see that do not seem correct:

1. If you truly added the 2.6 lbs of R134a to the system, you should not see any bubbles in the drier sight glass.......unless you have either air or moisture still in the system. However, both of these tend to provide high compressor discharge pressures and not lower pressures. But this issue still needs to be addressed.

2. Your 30 psig suction pressure looks fine, but having icing on the POA and a very cold compressor inlet connection tells me you have liquid refrigerant getting out of the evaporator core. It should be gas that is heated slightly above saturation conditions (this is called superheat). Your expansion valve is designed to provide abut 10-15 F superheat and if things are too cold, this TXV valve should close off and slow down or almost stop the refrigerant flow to the condenser. Again, this low temperature you are seeing with the icing, tells me the evaporator core is not transferring heat and something is wrong in the heat/ac box.

3. Your high side pressure should be around 180 - 220 F at 80 F ambient temperatures, and the air from the cabin ducts should be around 40 F for a properly operating factory system. The lower pressure you show could be a compressor issue or it could be you have other issues going on. I think it is the latter. Having R134a refrigerant in the system should show even higher compressor discharge pressure.

4. It is possible you have a low charge................but you would need to be down to 1 lb or less of refrigerant to give the symptoms and pressure numbers you are showing. And then, your POA valve (evaporator outlet) would be very warm and not cold or icing. And there would be LOTS of bubbles in the drier sight glass.

5. Having issues with the TXV valve are possible, but the charts I have indicate a stuck open valve will have HIGH pressure readings for both the low and high pressure sides, and a TXV valve stuck closed will have LOW pressure readings for both the low and high side pressures. Again, your readings do not indicate either of these.

6. A compressor malfunction or failure will show HIGH low-pressure readings, and LOW high-side readings. Again, not what you indicate.

I am assuming that your low pressure side reading are obtained at the POA valve............confirm that this is the case and that you are not getting them from another place.

To help you any more, I need for you to run your air conditioning system again. RPM 2000, blower on high speed, no outside air, only recirculated air. Watch when the compressor starts and observe how the low-side and high-side pressure change and their final values running at steady state. The use an IR temperature gun and thermometer and give us the following:

Outside air entering condenser
Cabin duct temperatures at right and left ducts
Temperature of compressor outer casing
Temperature of condenser inlet at the condenser
Temperature of the drier outer case
Temperature of the evaporator inlet metal tubing
Temperature of evaporator outlet/POA inlet at the first uninsulated opening.
Temperature of the POA outlet to the compressor

Also if bubbles are still visible in the drier sight glass??

I personally believe your issue is with the heater/ac box or the heater inlet water valve............but providing the additional information will allow a better troubleshooting of your system. I also think you have some air in the refrigerant that needs to be corrected.

Let us know what you find.

Larry

[Larry M.]

Re: 1972 Air Conditioning

Mike:

I do not think you are very low on refrigerant, for if you were and all else was okay, you should have evaporator outlet temperatures of maybe 50-65 F. This is called superheat and shows that the refrigerant has vaporized and cooled and is then further heating up by exchanging (removing) heat with the cabin air across the evaporator coil. But since your evaporator outlet refrigerant temperature is low (41 F) it tells me low refrigerant is not the issue. But it is easy to try. Go ahead and add 1/2 a can...............recheck your temperatures and cooling...........and if needed add the remaining 1/2 of can.

Watch the slight glass for bubbles...........they should stop when the refrigerant is at the correct charge. If they do not, water or air is a problem. This is one of your problems. But I do not think it is your only problem.

Appears POA is working as designed.

I like the temp of the compressor outer casing. The condenser inlet temperature is also consistent with the high side compressor pressure.........although this compressor pressure and the condenser inlet temperature should both be much higher. Like 220 psig pressure and maybe 160-180 F condenser inlet temp.

Your drier casing temperature is a bit lower than expected, but does show your condenser is doing a good job and is also providing good sub cooling of the refrigerant.

Your evaporator inlet and outlet temperatures are way out and should be much, much colder than what you show. Evaporator outlet should be around 34-40 F. Unsure for the evaporator inlet, but I would guess 40-60 F..............and this is a guess on my part at this time. But if the refrigerant from the drier is 85 F and the evaporator inlet is perhaps 32 F, I would expect to see a temperature somewhere between the two, and really on the lower side of this.

Please confirm blower speed and no outside air makeup..........only recirculated air in cabin.

Also tell me about the heater hot water valve..........or that your car does not have this valve.

My original advice still stands at this point.................but go ahead and add the refrigerant and see what happens. If discharge pressure goes above 250 psig stop adding additional refrigerant.

Larry

[Michael C.]

Re: 1972 Air Conditioning

Larry,

Yes I should have confirmed this earlier: that MAX AC was on, the fan was indeed set to Hi and blowing Hi, windows closed, and, NO heater hot water shutoff valve is on this car. I believe they eliminated that valve for 1972 according to the 70-72 Judging Manual. I also tried the Temp thumb wheel to see if I could hear the cable-operated duct door operating, and it appears by auditory evidence (clunk) that the door is operable.

Going to try the refrig. top off now, and I will let you know what happens!

Thanks,

Mike

Mike:

I do not think you are very low on refrigerant, for if you were and all else was okay, you should have evaporator outlet temperatures of maybe 50-65 F. This is called superheat and shows that the refrigerant has vaporized and cooled and is then further heating up by exchanging (removing) heat with the cabin air across the evaporator coil. But since your evaporator outlet refrigerant temperature is low (41 F) it tells me low refrigerant is not the issue. But it is easy to try. Go ahead and add 1/2 a can...............recheck your temperatures and cooling...........and if needed add the remaining 1/2 of can.

Watch the slight glass for bubbles...........they should stop when the refrigerant is at the correct charge. If they do not, water or air is a problem. This is one of your problems. But I do not think it is your only problem.

Appears POA is working as designed.

I like the temp of the compressor outer casing. The condenser inlet temperature is also consistent with the high side compressor pressure.........although this compressor pressure and the condenser inlet temperature should both be much higher. Like 220 psig pressure and maybe 160-180 F condenser inlet temp.

Your drier casing temperature is a bit lower than expected, but does show your condenser is doing a good job and is also providing good sub cooling of the refrigerant.

Your evaporator inlet and outlet temperatures are way out and should be much, much colder than what you show. Evaporator outlet should be around 34-40 F. Unsure for the evaporator inlet, but I would guess 40-60 F..............and this is a guess on my part at this time. But if the refrigerant from the drier is 85 F and the evaporator inlet is perhaps 32 F, I would expect to see a temperature somewhere between the two, and really on the lower side of this.

Please confirm blower speed and no outside air makeup..........only recirculated air in cabin.

Also tell me about the heater hot water valve..........or that your car does not have this valve.

My original advice still stands at this point.................but go ahead and add the refrigerant and see what happens. If discharge pressure goes above 250 psig stop adding additional refrigerant.

Larry

[Michael C.]

Re: 1972 Air Conditioning

Larry and Bruce,

I was way lower on refrigerant than I thought...

I went out and added R-134a to my system. I added about 1 and 1/4 cans before I got a clear sight glass. Now it is truly clear. Outside air temp: 76 degrees. Inside air temp at vent: 52 deg. Lo Side = 40 psi with engine at 2,000 RPM. Hi Side = 220 psi. I stopped charging with R-134 when the Hi side pressure reached 240 psi. POA inlet tube is "beer can" cold.

Much better results. Any other thoughts would be appreciated.


Thanks for your help.

[Larry M.]

Re: 1972 Air Conditioning

I am glad to hear things are improving for you. But you are still not there. The low side pressure is higher than desired and you high side pressure is also marginally high for the outside temperature.

If sight glass is crystal clear, then you are probably okay about no air or moisture in the system. But now we are faced with trying to get the low side pressure down to 28-30 psig to improve cooling. Duct temperatures should be about 40F.

A HVAC guide I have for cars shows if you have both higher than normal low-side and high-side pressures the system is likely overcharged......but it also indicates air may still be present. It is also possible that POA valve is acting up..............although it seemed good a few days ago.

I would do two things at this point:

1. I would slowly vent refrigerant out of the system until bubbles just start appearing in the slight glass. This should be at a point of being just a little bit low on refrigerant, but still probably acceptable for cooling.....certainly for testing at this point. You can always add 1/4 or 1/3 can later as a "top off". If you don't like what you get/see, I would take the system down to zero pressure, pull a strong vacuum to remove any inerts (water/air) and recharge with the recommended 75% of factory charge for R12, or .75 X 3lbs 3 oz or 40 oz (basically 3, 12-oz cans of R134a + a bit extra. If you have a large container of refrigerant, you can use a weigh scale.......although accuracy will not be great. This way, you know all moisture/air is gone and you know the charge size is per spec.

2. I would install a manual shutoff valve in the hot water hose to the heater (heater inlet). And keep valve closed during summer months......only open in the winter for heating and occasional coolant circulation for corrosion prevention/protection in the heater core.

If the POA valve is/was acting up, it may clear up by itself with time. When I had a sticking problem with mine, I added (very slowly) pressurized oil into the system thru the POA connection. It appeared to free up or "jar" the POA back to normal and the suction pressure dropped to 30 psig and all was fine. But if you do it wrong or too fast, you could slug the compressor with liquid oil. That is bad...............

I am not certain I can add much more to what I have already said. But I will continue to try and help if needed.

Larry

[Michael C.]

Re: 1972 Air Conditioning

Larry,

Thanks you have been a lifesaver! I appreciate your knowledge and expertise.

Best Warm Regards,

Mike

[Michael C.]

Re: 1972 Air Conditioning

Larry,

One last question... Isn't the R-134a system supposed to show higher pressures than the r-12? I have read that pressures in the 45-55 psi region are normal...or is all that moot because I am still using the OEM type Expansion and POA valves?

Thanks,

Mike

[Larry M.]

Re: 1972 Air Conditioning

Larry,

One last question... Isn't the R-134a system supposed to show higher pressures than the r-12? I have read that pressures in the 45-55 psi region are normal...or is all that moot because I am still using the OEM type Expansion and POA valves?

Thanks,

Mike

Mike:

R134a will show higher condenser pressures (high side pressure) than R12. For 80F outside temperature, the factory service manual for your car with R12 refrigerant shows this to be 175-195 psig. Compressor RPM at 2000. Switching to R134a will increase this pressure probably 20-30 psig .........or 195 -225 psig.

But on the evaporator side (low side pressure) R134a pressure will actually be LOWER than R12 refrigerant. This is due to the vapor pressure vs temperature curves of R12 and R134a. R134a pressure is higher than R12 at temperatures of 60F and above. It is the same at 60F. And R134a pressure is less than R12 pressure below 60F.

So for R12 system, the factory wants a evaporator pressure (low side pressure) of approx 30 psig. This corresponds to 32 F temperature inside the evaporator core........and if you try to go lower in temperature the core will form ice and plug. To achieve the same cooling in the evaporator using R134a the pressure should be 27 psig. This is why the factory set the POA valve at 30 psig. And knowledgeable auto ac techs will adjust the POA valve or STV valve (for 1963-66 cars) to the lower 27 psig value for R134a. No one knew the POA could be adjusted like this until a guy named Mitch on the AUTOACFORUM discovered it. Mitch was the forum Moderator and a super ac tech.

The factory service manual for your car (with R12 refrigerant) shows 38-47 F air temperatures at the cabin ducts for outside air temperatures of 70F up to 120F. Obviously the lower the outside air temperature, the lower the duct air temperature is in the service manual tables.

At idle speed (700 RPM) or when first starting the ac system for a hot car, the low-side pressures will be in the range you mentioned (45-55 psig). But they will/should come down to the lower pressure (30 psig) with time. A good POA valve will not let the pressure drop below 30 psig (to prevent evaporator freeze-up) and will slowly close to maintain this 30 psig pressure. This will occur when the cabin is cool and the outside air is rather cool and humidity is low. But if the POA valve "sticks" closed or in a restricted opening, it will cause the evaporator pressure (low-side pressure) to go up..............which will effect how efficiently the refrigerant can remove heat and cool the cabin. At 50 psig low side pressure, the best the refrigerant can do is cool the cabin air to 55F. That is why you need to get the evaporator pressure down to 30 psig (R12) or 27 psig (R134a) to get the evaporator temperatures down to around 32F.

Above is a bit simplified version of what actually happens. The expansion valve (TXV) controls the amount of refrigerant being admitted into the evaporator core for cooling. It has a thermal sensor bulb on the outlet from the evaporator which tries to give the maximum cooling yet prevent LIQUID refrigerant from leaving the evaporator and getting into the compressor. So you have the TXV and the POA both working to control your cabin temperature to the best and coolest temperature without freezing up the evaporator core and without damaging the A6 compressor from liquid.

Larry

[Michael C.]

Re: 1972 Air Conditioning

So as you suggested, Larry, I bled off some R-134a and now my Lo side is reading 30 psi. High side went up to 259 psi. Outside air: 85 deg. Inside Air at vent: 57 deg. Some bubbles in the sight glass.

I think I should probably evacuate and put a vacuum on the system for a few hours, then try recharging. Any additional thoughts?

Thanks,

Mike

[Larry M.]

Re: 1972 Air Conditioning

Mike:

Yes, I would do this. Just be sure to not get any air into the system when charging...........especially when adding the first can of refrigerant, and also when changing cans of refrigerant.

I have a special technique that I use when having to use the small cans. We can talk about it if you feel the need. Phone 985-785-6142. But if you are very careful, you should be able to do this by yourself without any issues.

As a test, I would remove the heater hose going into the heater and plug it. Do this at the engine. I would install a small plugged heater hose at the engine to cap this supply off. Then test the car in this configuration. I know you are probably reluctant to install the non-factory shut-off valve, so this test will achieve the same thing. If it helps, you know what you need to do. If no effect, reinstall the hose and be happy. But I believe there is an issue inside your heater/ac box that is effecting good cooling. A LOT of people have done this and are now believers. One just today on the Corvette Forum C2 Section.

Larry

PS: With the higher outside temps (85 F and above), put a box fan in front of your grill to improve air flow across the condenser and radiator and keep the high side pressures down to the minimum.

[Michael C.]

Re: 1972 Air Conditioning

Larry,

I did the heater hose test that you suggested: Plugged the Intake Manifold nipple on the engine, and plugged the end of the removed hose. I tried this first to see how much the heater core was messing with my interior temperature readings when the A/C was on.

I also put a box fan in the front of the grille, and one behind the car to pull more air through (and also pull the exhaust gasses outside of my garage!).

Results were similar to before the test: Ambient Temp.= 86 deg. F. Inside Temp. = 56 deg. F. @ 2,000 RPM on MAX A/C at RH vent.

This would seem to indicate that the heater core is not affecting the temperature that much, if at all.

Also, I am getting 30 deg. F. cooler air inside the cockpit. Isn't that about as much as can be expected while keeping the system OEM and with running R-134a?

I could evacuate and try to recharge again, but it seems like a lot of effort for a few degrees of cooling.

Best,

Mike