Automotive Air Conditioning Information Forum (Archives)

custom 68 on Sat May 15, 2010 5:07 PM

ok all I am not sure what I have...
First I am working on a 68 chevy pickup with a 2002 motor and compressor setup. a friend of mine ended up giving me a "POA Update" kit he was not going to use. Me not knowing anything about AC thought an update this must be great. Well I think I threw away my original POA that was probably working but thought this must be better.
anyway, I looked thru this thing I have and it is a straight thru shot so I bought and cut hoses and put this all together charged it and it seemed to work some, now I am revamping the system and after finding this forum I realized I probably need an Orifice somewhwere, yes I ran this without one so now what?
I thought I had everything I needed but now I dont know? The system seemed to cool? So can you tell me what I have and more importantly what I need.
thanks for helping an non AC person out
Dave

custom 68 on Sat May 15, 2010 5:14 PM

here is what I have

custom 68 on Sat May 15, 2010 5:28 PM

custom 68 on Sun May 16, 2010 12:16 PM

alright I am still reading and found a lot of information from Mitch and Big V.
Last year this seems to work ok but since it was apart I thought I could make it better long story short it seems like my system should have the original (pressure changed) POA, I cant find mine so I guess to the salvage yard I will go to see if I am lucky. What is strange the "update" I have as you can see from the photos above is a straight thru design, since I keep seeing to adjust the pressure this one would have no pressure restriction so that is what is strange.
I don't remember what my pressure values were last year but I will be sure to monitor them now and record them.

Now all that being said I assume this is all still true even though I am using a newer compressor. Any input on this would be great. I just want to get this together correct and work the best it can. I am going on the hot rod power tour the first of june and will want it then...
thanks
Dave

EDIT:
What years used the POA valve? I know there are 3 different ones based on info from this site.

Edited: Sun May 16, 2010 at 12:21 PM by custom 68

barraza on Mon May 17, 2010 11:36 AM

I have a similar system on a 72 PU. The POA is nothing more than a pressure regulator, no different than the regulator hanging off the end of your shop compressor. Its function is to keep the evaporator at a pressure that will provide optimum cooling without freezing the core. A stuck open POA will allow the pressure to go low enough to turn the evap into a block of ice(I've been there). A stuck closed POA will keep the evap pressure so high that you will get no cooling(been there also). Those POA "updates" were popular when people started changing to 134 from r12. Not exactly sure of the reason, but I assume it was because GM manuals have always said POA valves were non-adjustable. 134 needed a lower evaporator pressure to provide temps as low as r12. There is nothing inside the tube, because the only functioning part is a pressure sensor. Instead of regulating the evap pressure with a pressure regulator like the POA does, it controls the pressure by turning the compressor clutch off and on. The update works, but it is seen as not nearly as elegant a solution as the POA valve. The update also has the problem of compressor clutch wear, the old style compressor was never designed to be turned off and on every minute or so.

Since you have a new style compressor, I would guess it will work fine being cycled. Someone else will have to answer that question for sure, but if it will work ok, there really isn't a reason to go looking for a POA valve. Especially since it has become quite rare to find closed systems in the junkyard (around my parts anyway, YMMV)

NOS POA's are over $400 now, replacements $150-$200, POA eliminator are about $100. Original POA's can be rebuilt by

http://www.classicautoair.com/GM_OEM_Parts_Valves.html

All those prices seen high for a pretty simple part.

Take the above with a grain of salt, I'm not a AC tech, just a shade tree type with old cars.

custom 68 on Mon May 17, 2010 3:29 PM

Thanks for the input. I may just put this back together again and give it a shot. It did work before but after reading on here it seems like I may have made a mistake by not utilizing my POA valve. I believe the compressor I am using is a HU6 which is a variable vain pump I assume all this and think it does not cycle it just allows some bypass. I just wanted to understand this better before I charged it again.
I am stumped on what the pressures should be on the high and low side nor do I know how much freon to put in. I have read I can still use the site glass even with the 134A. Initially I thought this did not work.
I will give this a shot once it warms up here again. Any other input would be appreciated as I am working on a collection of AC stuff hoping to get it to work together.
Dave

mk378 on Mon May 17, 2010 5:04 PM

A variable compressor does the same thing as a POA valve. So you can just use the straight pipe "POA eliminator." Do not wire up the pressure switch on the POA eliminator, let compressor run constantly as was stock with the POA system.

The TXV serves as the orifice. So basically you had it right the first time but may need to tweak the charge amount. Variable compressors are hard to charge by pressure. I think the best way to approach it is first know the factory spec (for R-12)-- don't ever use more than that. Charge R-134a until it gets good and cold but don't go much past that point.

custom 68 on Mon May 17, 2010 5:11 PM

thanks that makes sense. So I am not too bad off after all.
I have changed to a newer condensor also new hoses but the same evaporator so as far as the charge goes I didnt know how correct the "old" values for amounts would be. This is quite a project. I don't think I evacuated it long enough anyway so that is probably part of my problem.
again thanks
Dave

bohica2xo on Wed May 19, 2010 12:36 AM

The 2002 Corvette came with a V7 compressor. This is a variable displacement compressor, with a control valve set for 134a. It is a 7 piston swash plate compressor, and would be the right compressor to use for a POA system that no longer has a POA installed. It has close to the same capacity as the A6 compressor that vehicle had OEM.


I do not believe GM had a variable displacement vane type pump in any 2002 domestic model.


B.


-------------------------
"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 Wed May 19, 2010 7:17 AM

In the original GM POA system, compressor ran all the time, A-6 has an oil sump in it when refrigerant passage was very low so it wouldn't seize. You have to know what you are doing and understand the old and new systems before making changes. Or you will get smoke.

custom 68 on Wed May 19, 2010 10:32 AM

Thanks guys for the latest info. Thanks for the clarification on the variable compressor, I don't want to give mis information and steer sombody wrong. I dont know much about these only what I understood.

Nick, do you have any suggestions on a direction for me? I dont want to smoke the system as they say in electrical terms "dont let the smoke out". Anyway I am hoping to find a way to use the combination I have. I should be able to find another POA valve if that should be replaced to get rid of the "eliminator" or as I was told an upgrade. I started putting this back together last night and had hopes to charge it maybe over the weekend depending on the temps outside.
Most of the thoughts on here are to stay away from the "orifice" type systems so I was trying to go this direction. Last year things seemed to work but since I had things somewhat apart due to other changes I wanted to get it as right as possible.
Again thanks all.
Dave

custom 68 on Wed May 19, 2010 11:43 AM

One last thing, I believe this is an HT6 compressor, do I just add oil to the suction port on this? I dont see any drain or other place.

bohica2xo on Wed May 19, 2010 12:01 PM

The HT6 is a fixed displacement compressor. This means that you will need to set the system up as a CCTXV type, and use the eliminator. The switch on the eliminator tube is the low pressure cycling switch for this job.

Personally, I think the V7 would be a much better choice - but what you have will work. The HT6's are known for a failure called a "belly leak". Use dye in your system when you put it together, and keep an eye out for leaks. Do a search here for belly leaker and you will find lots of pics...

Hopefully you have upgraded the condensor on the system. The condensor from the donor vehicle would be a good thing to use.

B.

-------------------------
"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.

custom 68 on Wed May 19, 2010 12:26 PM

Bohcia, you are a wealth of knowledge I wish I had found this site years ago. I had already did the search for the HT6 and found the leak info you mentioned. Yes the condensor has been replaced with an aftermarket one. The doner one had some problems.
So to swap to this new V7 this would work better with what I have and would not need to use the orifice type. Is the TXV I currently have in place not going to work the same as some thing else. I don't know all the terminology so I may have things confused.
I still have the valve just not the POA valve.
With my conversion I don't have much room and the compressor is mounted in the stock location on the motor and it puts it inside my frame rails so unless I can get this V7 upgrade with the same mount points I may not get it to fit without changing pully systems or other things.
Again thanks all
Dave

bohica2xo on Wed May 19, 2010 1:46 PM

Dave:

The Corvette came with a V7. Trucks came with HT6's. I believe the mounts are different, but never tried to swap one - perhaps someone that has both units on a bench or shelf someplace can tell you for sure.

The HT6 can be used. Keep the TXV, and the eliminator in place. Use the low pressure switch to cycle the compressor clutch to prevent freezing of the evaporator. No need to install an orifice tube. CCTXV operation. It is somewhat better than an OT, and will probably cool a pickup cab just fine.

What sort of climate do you plan to run this in?

B.

-------------------------
"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.

custom 68 on Wed May 19, 2010 2:20 PM

Quote
Originally posted by: bohica2xo
Dave:



The Corvette came with a V7. Trucks came with HT6's. I believe the mounts are different, but never tried to swap one - perhaps someone that has both units on a bench or shelf someplace can tell you for sure.



The HT6 can be used. Keep the TXV, and the eliminator in place. Use the low pressure switch to cycle the compressor clutch to prevent freezing of the evaporator. No need to install an orifice tube. CCTXV operation. It is somewhat better than an OT, and will probably cool a pickup cab just fine.



What sort of climate do you plan to run this in?



B.
B,
I looked back thru my post and see I did not clarify what motor I was working with but you hit it on the head with your above info. The motor is indeed a gen 3 5.3 liter motor from a Silverado. I am using the truck accessories and pulley system. The setup fits nicely in my truck so I have left it along for now. I can upgrade to the Corvette accessories but that will be a future project if necessary. I will watch for the "belly" leak but maybe this will work for a few years.
I live in Missouri so we have our share of heat and humidity. I drive it quite a bit but it is more of my "play" truck than anything. I will be leaving to go on the Hot Rod Power tour in a couple weeks this will take us from Newton Iowa to Mobile Alabama. I am sure there will be a range of temperatures along the way.
The setup worked ok last year but I don't think I evacuated it long enough or took as much care as I should have. I am being overly cautious on this hookup.
As far as the charging I am at a bit of a loss on what to shoot for. The condenser is aftermarket the evaporator is the original for the 68 chevy truck the hoses are new and I have the funky no POA using a TXV so the reference books for pressures and amounts are probably no use. I did get told to shoot for a high pressure around 2.2-2.4 times the ambient temp, will the low pressure go down as the evaporator starts to freeze? The TXV has a pressure port and a temperature port.
Dave

bohica2xo on Wed May 19, 2010 11:42 PM

Dave:

The HT6 will get the job done. Maybe someday a set of corvette parts will fall into your lap... If not, the sponsor stocks a quality NEW replacement for the HT6 when the time comes.

Make sure the condensor fan is working properly, and that you have a fan shroud installed. The condensor should be coupled to the radiator so that the air can't bypass it on the way to the radiator.

Is the replacement condensor a parallel flow type? Is it roughly the same size as the condensor in the donor truck?

B.

-------------------------
"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 Thu May 20, 2010 11:01 AM

I like the V5/V7 concept, simple and straight forward, compressor feeds the accumulator, to a fixed orifice, to the evaporator, to the condenser, and back to the compressor.

Only sensor is a high side thermistor, typically feeds the PCM to tell it to cut the compressor off if the pressure is outside of a 40-415 psi window, but I suppose a guy could design a window comparator to do that so the compressor doesn't run if it's cold outside or if the pressures get out of hand. Maybe something like that is on the market already made, or perhaps a dual function real contact switch like Toyota and Honda use in their TXV systems. Just don't want the compressor to run outside of that pressure window so this contact opens out of range and can be put in series with the clutch relay coil so it doesn't energize if out of range.

For the climate control, just a switch to apply a constant 12 V to the clutch relay whenever you want AC, these compressors will run all the time, but on cooler days, barely loading the engine. The V5/V7 get away from that constant cycling that tears the hell out of the clutch and compressor with at zero to engine speed acceleration in an instant. Even better than a TXV system, they still cycle, but not as much as a FOT system.

Notice with mine, vent temps are rather high on cooler days, but comfortable and you don't need 30*F air on a 72*F day, but man, does that thing kick butt when its over a 100 outside.

Would look into this system, far less complicated than a POA and works just as well, even better with a good condenser.

custom 68 on Thu May 20, 2010 3:41 PM

great info guys, I will keep my eye out for one laying around Maybe I can find the entire engine with all the accessories.
I will give this a shot this weekend as it is supposed to be pretty warm here. I will look tonight to see that is is a parallel flow condensor, it is a new aftermarket one so I would assume it would be but I will check, it is simular in size to the one off the Silverado.
I will keep you all posted on my progress.

bohica2xo on Thu May 20, 2010 9:00 PM

If the condensor is a replacement part for the Silverado, then you have a system capacity similar to the OEM Silverado.

Add the listed oil charge for the Silverado to the system. If you add oil to the suction port on the compressor, turn it by hand to be sure it is not full of oil on startup. The suction line should not be full of oil. Close things up, and evacuate.

With the engine off, charge the system with 80% of the charge into the evacuated system. This is done by charging liquid into the HIGH side port. Get as much of the charge in as you can this way. Close the high side valve on the gauge set. This valve will remain closed through the balance of the charging process. Set the cabin fan to high speed, and close the doors.

Start the engine, and let it idle. The system should be cycling, this is ok. The sightglass will be foamy. Add refrigerant, and observe the sightglass & cycling. The cycles should become longer as the sightglass becomes more clear. At idle the high side pressure should be someplace between 1.6 to 1.8 times ambient temperature. The vents should be getting cold. Add refrigerant as vapor, through the LOW side. Add a small amount at a time, and watch the system for a minute or two before adding again.

Once it is cooling, and cycling you can check / adjust the cycling switch. It should drop the compressor out about 26 to 28 psi, and restart the compressor when the low side reaches 32 to 35 psi.

Now, with the doors open and the fan on highest speed - adjust the engine speed to 1500 rpm. Let it run for 5 minutes, and check the pressures. It may stop cycling with a full load test like this. If it is still cycling and the pressures are below 2.0 times ambient, you may need to add some refrigerant. Check the sightglass too.

You will reach a point where you see little or no high side pressure change when adding refrigerant. This means the system is beginning to store excess refrigerant in the receiver / dryer. Add an ounce or two past this point and you are done. The receiver has some extra capacity, and the ounce or two of stored refrigerant makes up for tiny leaks of a period of years.

If you find a condition where adding a small quantity of refrigerant spikes the high side up over 2.4 times ambient, you are overfilled.

Once you have a full charge, and good cooling - record the quantity of refrigerant you have installed for future reference.

B.

-------------------------
"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.

custom 68 on Fri May 21, 2010 9:09 AM

B
that is a great writeup and explanation. I will be doing the charge this weekend if I find no problems during evacuation. The outside temps are going to be 85 this weekend and the humidity will also be pretty high.
I will print your info for reference that is good stuff.
I will report back on Monday or maybe even Sunday if I get the chance.
Dave

custom 68 on Mon May 24, 2010 1:51 PM

ok to report in.
I evacuated and charged this on Saturday. The ambient temps were 85 I ended up with low pressure at 25 psi and high pressure at 225 and cooling in the cab at 48 degrees. That evening it cooled down and I ran it again this time the compressor cycled as I could not pull enough cold off the evaporator. Went for a drive on Sunday about the same temps, on high fan speed I was at 47-48 degrees. I know you said to cycles this around the pressure I am sitting at. I believe this is due to freezing of the evaporator? I hated to mess with this anymore since it seems to be working fine. I assumed if the pressure on the low side was higher my vent temps would be higher.
Now the kicker is I only put in about 22 oz of R134. Since this is a mis match of parts I had no idea on how much it would take but the donor Silverado has a capacity of 32 oz. I will compare my condenser to the truck one as mine is aftermarket. The "tubes" running horizontal are very flat "tubes" I assume they don't hold much capacity even thou this is large. I don't have a site glass on this new drier,
I can add more but assume things are working so why change it unless you see an apparent problem. my pressure switch is adjustable.
Again thanks
Dave

bohica2xo on Mon May 24, 2010 3:03 PM

You are close, perhaps right on the money with the charge volume. Sounds like you have a serpentine replacement condensor.

The 48 degree discharge air may be due to an air leak in the mixing door, allowing re-heating of the cooled air with the heater core. Or, you may simply have a gauge error & higher evaporator pressures than you think.

There is a lot of testing you could do, but how well does it cool the cab?

You could put the gauges on, and tape them to the windshield. Go for a drive @ 45 mph & see what the pressures look like while driving.

You could adjust the cycling switch a bit lower, and see if you get ice buildup on the evaporator. If you have a thermocouple, you could test the discharge temp by checking the eliminator tube temperature near the switch. Compare the temp to a P/T chart & see what the pressure should look like.

You could bypass the heater core to see if the vent temps drop.

You could obsess with it a very long time. If it cycles at those pressures, has enough oil, and cools the cab well - you can leave it as is if you like.

B.

-------------------------
"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.

custom 68 on Fri May 28, 2010 12:46 PM

sorry for the delay on a reply, things have gotten busy. I have not made any more changes and all seems well. It also sounds like you know me too well, as I cant leave well enough alone that was why I figured I better quite while I was ahead.
The cab seems to cool pretty good these old trucks did not have much for insulation on the sides and top I have added quote a bit to the floor and the air is cold. when driving around town and at slower speeds the vent temps will get down to 42 or so, I dont think I have any freezing and have not had a chance to check the POA temp but it is very cold to the touch but not frozen. I am driving this today I will continue to monitor and hope for the best. I will be leaving for a 2000 mile road trip like mentioned before so I will take my gages and freon incase things get weird on it.
Again this site has been great I hope I can add to this in the future.
Thanks
Dave