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Engine Size: 350
Refrigerant Type: R134a
Ambient Temp: 80-90
Pressure Low: 45-55
Pressure High: 275-350
Country of Origin: United States
I'm in Cleveland and trying to help a college student in Alabama diagnose the POA system in his 71 Skylark over the phone. I've gone as far as I can without having the car in front of me, but it's a real puzzler. I thought I'd post the info here, if for nothing else than a test of my POA system knowledge.
The fellow is familiar with residential AC, has a good working knowledge of how a refrigeration loop works and has seemingly been quite thorough with his POA system. He seems to have made all the right moves as far as I can determine, but the system still doesn't work even marginally.
The system performance was poor with R12 (when he purchased the vehicle) before any repairs were made. Vent temps are/were in the 60-70 degree range with good airflow from the ducts. The heater core is currently bypassed for testing.
In trying to resolve the issue, he has replaced the TXV, receiver/dryer, replaced the compressor (pulley bearing failure), replaced the condenser, thoroughly flushed the remaining parts, adjusted the POA for R134a, correct oil and the system currently has ~48oz of R134a. (4-12oz cans)The original R12 charge was specified at 64oz.
The system was properly evacuated and steps were taken to minimize air ingestion via hoses, can changes, etc. I have to assume there is actually a bit less than 48oz of refrigerant in the system due to loss using cans and bleeding the manifold hoses.
Right now, the pressures are both high (45/+350 @1500rpm and 85 degrees)
Misting the condenser only drops the high side pressure about 25psi or so.
The evaporator tubes are both "beer can" cool but not cold, the POA and suction line "sweat" all the way back to the compressor. There is "quite a bit" of condensation dripping from the evaporator drain.
The discharge and liquid lines are very hot right up to the TXV. Almost untouchably so
-The pressures would tend to indicate poor condenser airflow or an overcharge of refrigerant. That doesn't seem to be the case.
-The high suction pressure could indicate a failed POA valve. Coupled with high discharge readings, I'd be reluctant to condemn the POA though.
- Excessive pressure on the high side could indicate a failed TXV, but the high suction pressure seems to contradict that.
-The condensation from the evap drain tells me that the evaporator is at least somewhat cool. The high system pressures explain why it's not colder, but I can't determine why both pressures are too high.
Any thoughts on what I'm missing here?
Edited: Sat July 31, 2010 at 1:24 AM by newton5
Ok after just finishing a 1972 GTO. I may be able to help.
First... If the system was marginal with R12 and is still marginal, I believe he has missed his largest problem. The heater core in this system is completely isolated from the air path when in A/C mode so that is not likely to be the issue. With all the new parts it would seem the other issues would have been covered as well.
Remember one other thing the low side is not connected to the suction line. It is connected to the evap outlet and should be controlled by the POA to stay at ~28-30 with original adjustment. The actual suction pressure may be very close to a vacuum which explains the sweat line back to the compressor.
There are one or two things I ran into with the 72 I just finished.
1. Air doors gaskets in the heater box. Check them to be sure they are all still there and the doors are functioning as designed.
2. Check the adjustment on the blend door which is controlled by a cable. This can be reached by removing the glovebox liner. Often when a heater core has been replaced the bushing for the blend door mechanism is left out or broken. There is a cam with a "pin" that controls the blend door. The slot in the cam is a little less than 3/8 inch in size the pin is about 3/16 if the bushing is missing the blend dorr will not seal. An acceptable replacement can be made from a short piece of 1/4 steel tubing. Not perfect but better than nothing. The door adjustment is not complete with the cable adjustment either as the cam is adjustable as well (just to confuse the issue). So remove the lower air ducts from the bottom of the dash. Look open the air door that will be visible and make sure the blend door completely seals the opening to the right. The heater core is behind this door. If not, remove the cable, loosen the three screws on the top of the air box and move the "cam plate" until the blend door is tight against the opening while pulling the cam/pin, the cable connects to, as far to the left as possible.
3. Check the door at the bottom of the air box which controls whether the air goes to the heater or defroster. It is pretty common to have the gasket missing at this door.
4. Recover the refrigerant. Take the expansion valve off and adjust the SH spring 1/2 turn clockwise. Yes this seems counter to what you may want but it will help. With 134a this system reacts like a "critical mass" type system. In other words it is very dependant on a very carefully controlled amount of refrigerant and expansion.
When the evap core pressures climb above 35ish the balance tube on the TXV will cause the valve to start to help close down the orifice. This then caused the pressure to go even higher and the cycle continues to get worse. By limiting the amount of refrigerant entering the core just a little you WILL gain cooling as the TXV will tend to balance at a larger opening overall.
5. charge with 2 lbs. of 134a to start. This system will use less 134a than you are expecting. Install a thermometer on the Cond. outlet and on the evap outlet. Put one in the center vent as well. Charge at a speed of around 800 to 1000 RPM to start. Add 1 ounce or so at a time until the cond outlet is ~ 10 degrees hotter than ambient. Ignore the sight glass it will get nearly clear then suddenly burst into foam when you are close. When you get to ~10 over ambient at the cond start watching the Evap outlet temp. If it is not below 50 degrees already add small amounts of refrigerant until it drops as far as it will go but at least under 50. If you take your time and add until it drops as far as it can there will come a point where the slightest amount will make it climb 3 to 8 degrees in an instant. You are now beyond optimum charge.
Anyhow...... This is where and how I would start.
I have to run. I will check back when I get home and see if there are any other things I can do to help.
Edited: Sat July 31, 2010 at 8:18 PM by 1stbscout
Disconnect the evaporator fan to reduce heat load on the system, low side should drop to the proper 28-30. If it doesn't, probably a bad POA. Also check for improper TXV installation.
I have had several GM TXV/POA cards over the years, and never found a single one of them to be "marginal" if they were maintained to OE specs.
I bought a 1971 Buick GS 455 in 1975, and that is awful close to that Skylark... My wife hated that car. It was "Too fast, too loud and too COLD!" In August.
The low side says the POA is not adjusted properly, or is just not working. I agree with 378, shut the blower down & see what the low side pressure looks like.
The high side says the condensor is not doing the job. My first question is replaced with what exactly? Did the replacement part match the OE part exactly? Or is it a chinese e-bay special?
Doing a conversion on a system that was not working properly in the first place makes things very complicated. Replacing a bunch of parts at the same time just makes it worse. There can be multiple issues, since the parts tend to be generic / offshore stuff for a car that old...
Get the POA issues settled first. It should not have a low side that high, and it should be stable.
Conversions are best left for places like Canada where they are mandated by law. Keeping an R12 car R12 saves a lot of headaches.
"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.
Have this problem with my own family members, can spend hours on the phone trying to give instructions, but the key problem is if one does not understand the system, how can they describe the problem. When I finally see the vehicle, nine times out of nine, the problem they described is not even the problem.
Beer can cold isn't exactly descriptive, and how can one really change from vacuuming to charging without changing hoses that lets air into the system, only a split second is required to do this. GM recommended using their charging station on these vehicles to charge strictly by weight, but with a great deal of experience could accurately charge without one.
My recollection is getting poor on this, but kind of remember only the vehicles equipped with ATC use the blend door, others used a manually operated coolant control valve. Recall one system that used an electrically controlled valve with ATC, was closed all the time until heat was called for. Would be helpful to have a shop manual on these cars, if the system was played for.
Can we assume this college person is of college age? If so, he wasn't even born when this car was made, so how would one know this system was marginal when it was new?
LOL, my major recollection of this era of GM vehicles, can also toss in Ford and Chrysler, they would rust out in the roadsalt area well before you had any problems with the AC or the rest of the mechanics of these cars. Was doing mostly body work back then and plenty of it.
Several have come on this board with R-134a conversion problems, Mitch, who is deceased now was going to tackle this problem living out in the dessert where rust is not a problem. Really can't help here, R-12 was dirt cheap and the only refrigerant ever used until the government got involved and created a whole new bunch of problems without any conclusive proof there was ever even a problem.
Thanks for the input guys.
I got involved with this long after the repairs were started, I'm just trying to pick up the pieces and understand just what the problem is.
After reading the replies, it makes sense that the POA has gone south. I can see how it could be causing both issues.
-The high Low Side pressure made me suspect the POA right off the bat, but I thought the excessive High Side pressure might also be causing that symptom.
-I had him disconnect the fan. The Low Side dropped about 5-6 psi
-As for the condenser, all indicators pointed to the original not doing it's job. The fan clutch is a new AC Delco unit and the condenser and radiator were clean but detergent flushed anyway. The correct shroud is in place. He says the fan is moving massive amounts of air and the water mist test seemed to agree.
The original condenser was replaced with one that was supposed to be a bit more efficient. According to a conversation with the supplier, it is the same tube and fin design as the old unit but has tighter fin spacing (more fins) to allow more heat dissipation. I'm not sure if I buy that claim or not, but there was no difference in the High Side pressure before or after the swap.
(I don't want to post an ebay link, but I believe it was item number 230357096817 if any one cares to look.)
I've rebuilt or repaired a lot of these POA systems and they've all been very good performers with R12 or R134a. I'm sure if I could see and feel what is going on, I could fix the darn thing. I'm squarely in the "keep an R12 system R12" camp, but I don't always have that control.
Nick, my digital thermometer is in Cleveland and the car in question is in Alabama. The leads are just a bit too short. "Beer Can cold" was used to give an approximation of the line temp. They're cool, but not as cold as expected.
When did Mitch pass away? I've read a lot of his posts in the archives but missed the news of his passing. Sad indeed.
the A-bodys from 1968 to 1972 basically all had the same system installed. They had a water control valve in 1969 only, previous to that none, after they changed the design of the heater box under the dash. The blend door redirects the air flow through the heater core or seals it completely if it is at either extreme. Obviously in the middle a little of both lol.
I believe there may have been some misunderstanding with my post. I did not say the original system was marginal... I said if he started with a marginal system and replaced the parts described and still had marginal performance there was something else wrong/missed.
GM used carbon dioxide in the original TXV. Many of the replacements have R12 or if you are lucky R134 in the bulb. If a TXV has been installed with R12 in the bulb the difference in the pressure/temp relationship between the 2 refrigerants will cause an interesting mix of issues.
Couple that with the A-6 compressor size and the equalizer connection to the POA and you have a whole list of new issues to boot.
From experience..... when the evap pressure climbs above 35 psi the extra pressure will close down the TXV and the cooling problems get worse quickly.
The fact the the POA and suction line is sweating tells me there is still a considerable amount of "cooling" left in the refrigerant leaving the evap. Sufficient amounts to allow the POA to become a "second" expansion device.
Without a way to check the pressure in the suction line it can be hard to diagnose a POA system.
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