A/C system pressures
05-06-2004, 10:47 AM
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A/C system pressures
This past month or so, I've rebuilt my A/C all but the condensor...which does not appear to be obstructed or bad at all. I installed a NEW A6 compressor, new hose manifold, NEW POA valve and dessicant, and NEW evaporator.
Which low side fitting do you use to check pressures and charge system? THere is certainly only one HIGH side valve but there is a low side one in the line and one at the POA. And they give different readings.
Originally the system was charged with the amount recommended on the compressor. It blew ice cubes! For a couple of days anyway but on a hot afternoon, the pressure release valve popped and sprayed a bunch of oil, messing up my under hood :mad area. We charged it up again, only to have same thing happen yesterday.
High side pressures are 275 and low side run 35. Good? This is the current reading info taken from the low side tap at the POA valve. If I read from the low side tap in the line up top, it reads about 60. Don't actually know the amount of freon used but that's the pressure currently. And it does not blow any where NEAR as cold as it did right after rebuilding the system and charging it up. So far (this time) it has not blown the popoff valve yet and it's been hot out. But I want it colder!
What am I missing? :cuss :flag
[Modified by SanDiegoPaul, 7:50 AM 5/6/2004]
Which low side fitting do you use to check pressures and charge system? THere is certainly only one HIGH side valve but there is a low side one in the line and one at the POA. And they give different readings.
Originally the system was charged with the amount recommended on the compressor. It blew ice cubes! For a couple of days anyway but on a hot afternoon, the pressure release valve popped and sprayed a bunch of oil, messing up my under hood :mad area. We charged it up again, only to have same thing happen yesterday.
High side pressures are 275 and low side run 35. Good? This is the current reading info taken from the low side tap at the POA valve. If I read from the low side tap in the line up top, it reads about 60. Don't actually know the amount of freon used but that's the pressure currently. And it does not blow any where NEAR as cold as it did right after rebuilding the system and charging it up. So far (this time) it has not blown the popoff valve yet and it's been hot out. But I want it colder!
What am I missing? :cuss :flag
[Modified by SanDiegoPaul, 7:50 AM 5/6/2004]
05-06-2004, 11:37 AM
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Re: A/C system pressures (SanDiegoPaul)
This is from the GM shop manual for 1971 with POA valve:
Low pressure gauge connects to the POA, high pressure to the back of the compressor schrader valve on the hose assembly. You can connect the low hose to the back of the compressor also, the readings should be identical.
From the pressures you indicate, they are WAY high. The low side should be 29.5. The POA adjusts the pressure for altitude, but you're in San Diego, so 29-30 psi on low side. High side should be 145psi at 70 degrees airflow across the condenser. Pressure will increase as outside temp. increases, but your high side pressure corresponds to about 130 degrees. I know SD has been hot lately, but not that hot.
From your pressures it appears you have an overcharged system, an obstruction, a clogged filter, or a failed high pressure valve. The quickest check would be to inspect the sight glass, then vent freon (illegal) in small quantities while monitoring the pressures. (Sorry for the length)
Low pressure gauge connects to the POA, high pressure to the back of the compressor schrader valve on the hose assembly. You can connect the low hose to the back of the compressor also, the readings should be identical.
From the pressures you indicate, they are WAY high. The low side should be 29.5. The POA adjusts the pressure for altitude, but you're in San Diego, so 29-30 psi on low side. High side should be 145psi at 70 degrees airflow across the condenser. Pressure will increase as outside temp. increases, but your high side pressure corresponds to about 130 degrees. I know SD has been hot lately, but not that hot.
From your pressures it appears you have an overcharged system, an obstruction, a clogged filter, or a failed high pressure valve. The quickest check would be to inspect the sight glass, then vent freon (illegal) in small quantities while monitoring the pressures. (Sorry for the length)
05-06-2004, 02:00 PM
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Re: A/C system pressures (SanDiegoPaul)
yeah, what 'Missileman' said. Even with R134a, you should be close to the numbers for R-12 listed for 1971. Was the system evacuated properly? Even a little moisture will mess up the restrictor/TCV.
05-06-2004, 02:44 PM
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Re: A/C system pressures (SanDiegoPaul)
Your pressures are way high, as already posted. The low side is measured from the fitting near the compressor. The POA fitting should always be at 29.5 PSI for R12. With R134a you want to see if you have an adjustable POA and if so turn the adjustment 1/4 turn CCW. This lowered my vent temps 10F (I have R134a).
The pressures you see will vary depending on temp. On a 70F day I'd like to see about 20 PSI low side, 150 high. 90F day high side of 200-220PSI is ok. Add 10% for R134a.
If you see normal high side at first but after running it for awhile you start seeing very high pressures (300PSI), you have insufficient airflow over the condensor.
The pressure release valve on the A6 compressor doesn't open until 400PSI. Needless to say, if you see pressures that high something is seriously wrong! Either the system is way overcharged or you don't have airflow over the condensor.
With the selector on MAX AC and after running it for awhile you should see vent temps in the mid to high 40's on a 70-80F day. Maybe low 50's on a hot day.
You don't say if you're using R12 or R134a. With R134a you need to use less refrigerant than the sticker says for R12. After some testing on my 71 I settled on 3 12oz cans which is 70% of the R12 charge.
The pressures you see will vary depending on temp. On a 70F day I'd like to see about 20 PSI low side, 150 high. 90F day high side of 200-220PSI is ok. Add 10% for R134a.
If you see normal high side at first but after running it for awhile you start seeing very high pressures (300PSI), you have insufficient airflow over the condensor.
The pressure release valve on the A6 compressor doesn't open until 400PSI. Needless to say, if you see pressures that high something is seriously wrong! Either the system is way overcharged or you don't have airflow over the condensor.
With the selector on MAX AC and after running it for awhile you should see vent temps in the mid to high 40's on a 70-80F day. Maybe low 50's on a hot day.
You don't say if you're using R12 or R134a. With R134a you need to use less refrigerant than the sticker says for R12. After some testing on my 71 I settled on 3 12oz cans which is 70% of the R12 charge.
05-06-2004, 03:51 PM
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Re: A/C system pressures (Missileman)
From your pressures it appears you have an overcharged system, an obstruction, a clogged filter, or a failed high pressure valve. The quickest check would be to inspect the sight glass, then vent freon (illegal) in small quantities while monitoring the pressures. (Sorry for the length)
There IS NO sightglass! The pressures were taken on a 101 degree day BTW on the tarmac out behind the shop. The condensor does not appear to be obstructed, and it is the only part of the system that is not new!
:cuss
There IS NO sightglass! The pressures were taken on a 101 degree day BTW on the tarmac out behind the shop. The condensor does not appear to be obstructed, and it is the only part of the system that is not new!
:cuss
05-06-2004, 03:53 PM
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Re: A/C system pressures (PureSilver)
yeah, what 'Missileman' said. Even with R134a, you should be close to the numbers for R-12 listed for 1971. Was the system evacuated properly? Even a little moisture will mess up the restrictor/TCV.
It was evacuated to 25-28 inches of vac. and held vacuum for 20 min before charging. I watched him do that. It is charged with R12 BTW, not 134.
[Modified by SanDiegoPaul, 12:53 PM 5/6/2004]
It was evacuated to 25-28 inches of vac. and held vacuum for 20 min before charging. I watched him do that. It is charged with R12 BTW, not 134.
[Modified by SanDiegoPaul, 12:53 PM 5/6/2004]
05-06-2004, 04:00 PM
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Re: A/C system pressures (SanDiegoPaul)
There should be a sightglass on the dryer. In my experience it is pretty useless though.
How is the airflow over the condensor? What fan are you running? If stock, are you sure the fan-clutch is good? Are all the shrouds and seals in place?
As for parts being new, that doesn't mean they're good. When I replaced my POA valve it blew hot air. After some diagnostics it turned out the brand new POA valve was dead. Replaced it again and cold air.
How is the airflow over the condensor? What fan are you running? If stock, are you sure the fan-clutch is good? Are all the shrouds and seals in place?
As for parts being new, that doesn't mean they're good. When I replaced my POA valve it blew hot air. After some diagnostics it turned out the brand new POA valve was dead. Replaced it again and cold air.
05-06-2004, 04:25 PM
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Re: A/C system pressures (zwede)
Thanks
I just read my '76 Motor's manual (yea I have one!) and it shows that there is indeed a sightglass but alas why bother? They could have made it useful but no.
Airflow is awesome. The fan is OEM but the fan clutch works fine. In fact it surprises me very well how much air it pulls at fast idle. I installed a throttle kicker when I put the system together, and it brings her up to a good fast idle when the A/C is on. The fan clutch pulls so much air, you'd think the car was doing 45MPH.
Seals are all good and intact. Only thing is that 12" wide or so space above the radiator where the OEM air cleaner horn went. Using an aftermarket air cleaner, that area may be letting air pass. Can't beleive that's a problem though since all the other seals are good and in place.
I'm tempted to warranty that POA myself. It seems like that is the only item in question. There is no obstruction in front of the condensor - and in fact when the engine is at fast idle, putting your hand in front of it is like putting your hand directly in front of the fan. :yesnod: Lots of airflow.
I just read my '76 Motor's manual (yea I have one!) and it shows that there is indeed a sightglass but alas why bother? They could have made it useful but no.
Airflow is awesome. The fan is OEM but the fan clutch works fine. In fact it surprises me very well how much air it pulls at fast idle. I installed a throttle kicker when I put the system together, and it brings her up to a good fast idle when the A/C is on. The fan clutch pulls so much air, you'd think the car was doing 45MPH.
Seals are all good and intact. Only thing is that 12" wide or so space above the radiator where the OEM air cleaner horn went. Using an aftermarket air cleaner, that area may be letting air pass. Can't beleive that's a problem though since all the other seals are good and in place.
I'm tempted to warranty that POA myself. It seems like that is the only item in question. There is no obstruction in front of the condensor - and in fact when the engine is at fast idle, putting your hand in front of it is like putting your hand directly in front of the fan. :yesnod: Lots of airflow.
