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A Little Help With POA System, Please

69Sixpackbee on Sun April 13, 2008 1:14 PM User is offline

Year: 1972
Make: Chevrolet
Model: Pickup
Engine Size: 402
Refrigerant Type: 134A
Ambient Temp: 101
Pressure Low: 18
Pressure High: 240
Country of Origin: United States

Hi guys. I need a little advice....again.
I had installed the factory 4 Season system into my '72 Chevy truck a while back and now that the heat is on I could finish dialing in the charge. This system was entirely disassembled cleaned, condenser washed, new flappers and seals as well as a brand new heater control valve. It is the original POA/TXV system and is being fed by a Sanden SD5H14 pump. The system was flushed. It has a new rec/drier (of course), TXV and the POA was adjusted down to 26PSI by the great POA adjustment procedure on this forum before I installed it. I had initially pulled a good deep vacuum for about 1 hour and I left the gauges on overnite and it was still at 29" the next morning. No leaks. The next evening I charged it up to an arbitrary number and because it was so cool outside I was not going to get a good number anyway. I stopped at 2lbs-11oz. Although the outside temperature was 60 degrees the vent temps were @ 38 and the POA (half of it) and clear back to the compressor was frosted up so it looked like it was doing it's job and the gauge readings seemed to reflect this. I then quit and waited for a warmer day...and now it's here!

Fast forward to yesterday. The ambient was 101 but I was not getting as good of cooling as I had expected. I ran the truck around a while with the system running and I was getting abut 48 degrees from the center vent running in recirc mode. I parked it, opened the windows, set the idle to around 1600 and went for my gauges. I hooked them up and the reading I was getting was 240 high and between 18-20 on the low and the vent temps were in the mid to upper 40's. The aux fan kicked on and i had real good flow accross the condenser. I even squirted the condenser with water and at best the pressure dropped around 10PSI so the flow is good. It is a piccolo design and is quite large. What has me baffled is that I set the POA to 26 and now I am getting a gauge reading of 18-20 and the airflow out of the vents is hurricane-like so it is not frozen over nor is there any condensate dripping out after shutoff to indicate this. I put a bit more 134A into the system (4 oz) but the high side only went up a bit more and the vent temps stayed the same so I did'nt add any more. The original R12 with the A6 required 3lbs/4oz and I have around 3lbs in it now. You can see the flow through the sight glass on the rec/drier but not alot so it looks pretty full.

I did not pull the wire off of the blower to see what the lo side would read but if it is 18-20 now would it not be the same or lower?

Does the aforementioned sound like a mal adjusted POA? I was very careful when I adjusted it initially to 26PSI. I have several others that are all adjusted and ready to install if it not working right.

Thanx for the help!!

Edited: Sun April 13, 2008 at 1:16 PM by 69Sixpackbee

mk378 on Sun April 13, 2008 2:03 PM User is offline

Is your low side gauge connected before or after the POA valve? I think these systems have two low side ports, because...

You need to measure the pressure before the POA valve to evaluate its performance. Pressure before the valve will never go lower than the valve's set point even with no evaporator air flow. Pressure after the valve will be lower because the valve is doing its job and restricting the flow.

69Sixpackbee on Sun April 13, 2008 9:04 PM User is offline

Here' what I did today. I have two sets of gauges. One manifold for 134A and another for R12. I used the R12 set to adjust the POA initially and I attached that set to the port on the POA body. Well, today I put that same gauge on the same port and viola! It reads a steady 26PSI. I then attached the 134A manifold set to the ports (which are right at the compressor) and the 134A set reads the same 20PSI. So, I must have a bogus gauge even tho I always check/adjust to "zero".

I was driving around alot today and I got the vent temp to pretty well stay around 43 degrees. Hi or low was 43. Not "brisk" but cool none the less. Would taking the pressure down another one more PSI on the POA help or am I pushing the limit into freeze-up if I do?
I forgot to mention that the entire evaporator case has been insulated as well just to keep down on the absorbtion of underhood temperature. This is a big block with headers so every bit helps.

Here is what I have for a setup:

Edited: Sun April 13, 2008 at 9:05 PM by 69Sixpackbee

mk378 on Sun April 13, 2008 10:24 PM User is offline

The fitting on the side of the valve is measuring evaporator pressure. You could put a low side R-134a conversion adapter on there so you can measure with your other gauge. The ones like AMA sells that contain a valve core also have a feature where they will press down the stock valve core when coupled. This is not ideal for evacuating or charging but for just measuring the pressure it means you can install (or remove) the adapter with the system still charged and the stock valve still in place.

It does appear you might get a few more degrees by lowering the POA setting a couple of psi. Be sure the heater is completely shut off.

Also insulate all the low side lines under the hood for slightly better performance. No need to insulate any high side lines. With insulation in place you could try measuring the temperature of that line leaving the evaporator.

69Sixpackbee on Mon April 14, 2008 12:00 AM User is offline


Well, I did a little more tinkering. It was still about 85 degrees early this evening so it was time to hook the gauges up once again. I noticed the last coupla days that the temps were coming down REAL SLOWLY. I had popped the hood and noticed that the suction line clear back to the head of the compressor was fairly cold as was the back of the compressor. I was thinking that possibly I was slightly slugging the compressor. So, tonight I put my digital thermometer back into the center vent, set the fast idle, opened the windows & started pulling out refrigerant. The temps were coming down! I pulled out nearly 8oz and stopped. I stopped the compressor for a few minutes and the fan was still on "HI". I then reconnected the compressor and the vent temps came down SUPER FAST! I drove around the neighborhood for a few minurtes and I was getting 37.8-38.4 degrees out of the center vent regardless of the fan speed! I guess my biggest problem was too much charge

The fitting on the POA was still rock steady @ 26PSI but like before my 134 gauges were 18-20 on the low and the high side was right around 200. Granted, it was cooler and not in the sun but it was almost a 10-degree drop in vent temps within about 10 minutes of tinkering and the ambient was still around 84.

Still unsure of the necessary charge amount since this is a retrofit with not all factory parts and I tried to adhere to the ambient X 2.2 rule but it does not seem to apply here as has been eluded to that some need more and some need less. Mine needed far.

So what you are telling me is that the reading I am getting off of the POA is correct. 26PSI is about 30 degrees F And, the low side fitting that the 134A gauge set is hooked to is still possibly not reading correctly as well since 18-20 PSI is 20-22 degrees F. I am at about 700 ft. above sea level here and the relative humidity is at 36 percent. It's dry!

At the very least I am going to buy a replacement quality gauge and try it again.

I'll give it a few days of running time and chime back in. I looks markedly better than it was before to say the least!


Edited: Mon April 14, 2008 at 12:03 AM by 69Sixpackbee

bohica2xo on Mon April 14, 2008 2:11 AM User is offline

What engine RPM are you using for all of this testing?


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

NickD on Mon April 14, 2008 6:03 AM User is offline

With the POA came the GM charging station, with strong recommendations to charge by weight to factory specifications. The POA itself was never a perfect device in that it would bypass the compressor thus varying the high side pressure. So trying to charge by pressures never did work well.

To get around this, on a 80*F plus day for topping off, run the engine at 1,500 rpm and hand monitor the evaporator outlet, and the high side line input, idea was to slowing add charge until the pulsations in the high side line would tend to disappear and the outlet evaporator pressure would feel just as cold as the inlet and let it go at that. You seem to be doing incredibly well with R-134a, a ton better than others on this board that tried an R-134a conversion.

Recall a slow running blower with ATC keeping the vehicle at 65 inside with ambients at 125 using the above charging procedure and R-12, and also recall, you can go crazy reading gauges with this system.

Dougflas on Mon April 14, 2008 6:58 AM User is offline

Disconnect the blower motor. Run engine at 1200 to 1500 doors open. POA pressure should should be 26lbs. Plug fan motor in again. Fords had a POA system and also a low side port after the POA. They ran 20 lbs approx near the compressor. Perfectly normal. Looks like you're there.

69Sixpackbee on Mon April 14, 2008 11:10 AM User is offline

Originally posted by: bohica2xo
What engine RPM are you using for all of this testing?


About 1800.

69Sixpackbee on Mon April 14, 2008 11:37 AM User is offline

Originally posted by: NickD
With the POA came the GM charging station, with strong recommendations to charge by weight to factory specifications.

Now, if I only had one of those and knew the weight

Seriously, those are GREAT suggestions. Since this is a hodge-podge of parts it is hard to figure the initial weight so I am going by "put in a little and wait" method. Originally, this system called for 3lbs4oz of R12 with the original tube and fin serpentine condenser, larger rec/drier and the good ol' A6 but I started with 48 ozs. which obviously was too much. If I would have stuck with the "80%" rule then the amount I have in it now, which is about 40oz is darn close to the 41.6 number which is the magic arbitrary 80% amount. . Now I have the Sanden 5 cyl compressor, still very large piccolo condenser and a bit smaller rec/drier. I was unsure if the original Fridgidaire TXV was any good after 40+ years so I put in a new "universal" externally equalized TXV. All new HNBR "O" rings with a Nylog treatment throughout the entire system. I put in a trinary switch (with a relay of course) to handle the aux fan chore and I even fit a fast idle solenoid to the Q'jet to keep the idle steady. And as the image shows, I treated the evap. housing to a complete insulating blanket with a thin fiberglass underlayment which is topped by the reflective bubblewrap and glued on and wrapped with aluminum duct tape. It was alot of work but I wanted this thing to kick a** especially when I am tooling accross the southern United States this summer on vacation.

I do have a coupla jugs or R12 still but I wanted to see if I coud get this to work well on 134A since the compressor was setup with that originally and I don't have the A6 anyway. Plan "B" was to fit a V5/V7 compressor and tool it up for that and lastly would be the A6 which would be the most costly because I would have to get all the pass. side bracketry (ever priced those for a BB Chevy lately?) and hoses.
So, if this Sanden works well for the task at hand then I will be happy...and so will my wallet Granted, the Sanden is a bit over 8cu/in where the A6 was over 12 but in the smaller cabin like my pickup I feel it should suffice. Besides, the cab is insulated from top to bottom and even the door insides so it is not even comparible to the job GM did back in '72.

I was thinking of droppin another 1PSI off of the POA but I might be asking for trouble, no?

Thanx again!


Edited: Mon April 14, 2008 at 11:39 AM by 69Sixpackbee

bohica2xo on Mon April 14, 2008 11:59 AM User is offline

Probably should have installed a sightglass in that system when you put it together. Hard to say what your drive ratios look like, but that sanden needs to see 3200 rpm for full output, so make sure you are getting there. You are never going to see A6 performance, that missing 4 cid and all... 12/8=1.5

You only need two ports to test that system. The low side on the POA, and the high side on the compressor discharge. Forget about using the port on the suction side of the compressor. The suction port could go all the way into vacuum under some conditions, it just does not matter.

Low pressure gauge calibrations can be off a couple of PSI, and you have no way to tell. It is possible that your POA is set a bit higher than you think it is. The average gauge is +/- 2% of the full scale reading, so even within spec you could be set at 28 psi.

You can slip a thermocouple in with the TXV sensing bulb & have a look at the evaporator discharge temperature. It may not be anywhere near 32f.

Finally, have you calibrated the thermometer you are checking the vent temps with? I have seen some shirt pocket units that were 8 degrees off. Calibrate it in an ice/water mixture in a thermos for about 5 minutes.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

69Sixpackbee on Mon April 14, 2008 12:45 PM User is offline

Thanx for the advice, Bohica! There is a sight glass on top of the receiver/drier. The instrument I use is calibrated every 12 months so it is good. I also have a Fluke IR handheld that I use to check superheat with. I can spot the beam on the front of the suction line and wrap the thermocouple around any other line and it will display the difference. I will use it to check the evap discharge temperature as suggested. That shoud be a good indicator of where my POA pressure-to-temp. setting is. I also am going to drill a small hole into the evap. housing so I can check the core temp of the evaporator with a K probe.

Now I know not to be concerned with the low side port of of the compressor. This is my very first "attempt" with a POA system so I am learning alot!

Thanx again for everyone's help!


steve325is on Mon April 14, 2008 12:45 PM User is offline


I know Tim doesn't like links to outside sources, but at least this isn't e-bay. I saw this on the local Craigslist, and I remembered that there was someone on this board looking for BBC A6 brackets, but I couldn't remember who. I'm not even sure if this is what you would need or not.

Craigslist ad for BBC stuff

I do not know this person, nor do I know if the stuff is still available.


69Sixpackbee on Mon April 14, 2008 12:49 PM User is offline

Thanx, Steve, but those brackets are for '73 up BB Chevies.

BTW, the A2 pulley diameter on my compressor is 125MM (4.92") and I believe the crank pulley is 7" so it is about a 1.4:1 ratio so it would be 3200 at around 2300 RPM of crankshaft speed. Just about crusing engine speed which is good for that but kinda falls short at low/idle speeds for sure. The Sanden "H" series, from what I am gathering, are good for 6K RPM continuous. I either need a smaller pulley on the compressor or a larger crank pulley. I do not know if I can get a smaller dia. pulley for the Sanden, can I? I could possibly adapt a larger crank pulley to the snout of the big block. I will do some hunting.


Edited: Mon April 14, 2008 at 1:01 PM by 69Sixpackbee

bohica2xo on Mon April 14, 2008 1:27 PM User is offline


Have a look at this Performance Curve.

Based on that data, you should spin your engine up to 2800 & collect some pressures & temps. Tape the gauge set to the windshield & go for a cruise in low gear...

Since you have a sightglass, you only need to charge a couple of ounces past clearing the sightglass. Liquid to the TXV is all you need, and excess charge just reduces condensor capacity.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Edited: Mon April 14, 2008 at 1:28 PM by bohica2xo

69Sixpackbee on Mon April 14, 2008 4:49 PM User is offline

Originally posted by: bohica2xo

Since you have a sightglass, you only need to charge a couple of ounces past clearing the sightglass.

I had the sightglass on the rec/drier fairly clear before but it is cooling much better with the liquid visibly passing through. Not like you would see with an R12 charge, obviously.
I have read numerous posts in the procedures and tricks secion that states charge up to clearing the sight glass and even some I saw elsewhere that said otherwise. I guess I am going to shoot for the best temps and monitor my superheat readings and evap. discharge temperature. From what I have read 26PSI seems to be good with a POA and 134A. I don't want to start pushing the limits of freeze-up.
Boy, this has been a learning curve but also fun at the same time!

Thanx to all of the knowledgeable folks who pitch in!


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