A/C Pressure
Thread Starter
Instructor
Joined: Aug 2008
Posts: 104
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From: Warner Robins Georgia
A/C Pressure
What should the A/C pressure be on a 74 after the R134 conversion? The guy that I had work on my A/C said that the pressure was @ 50. Is that too high? I'm trying to figure out what is causing the compressor to squeal. The only difference with before it was putting out hot air (and not squealing) and now is that he put refridgerant in it.
Last edited by DJCorn; Aug 10, 2009 at 01:11 PM.
Former Vendor
Joined: Nov 2006
Posts: 2,482
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From: Carmichael ca
Somewhere between 30 and 40 on the low side, 200 to 250 on the high side. These are not carved in stone, as there are variables that effect the pressures, but this will get you in the ballpark.
Team Owner
Joined: Jan 2007
Posts: 53,648
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From: Southern Cal Ca
St. Jude Donor '22
50 sounds right for R-12.
I have seen 134 as low as 25 and still cool good.
I skimmed the other thread and wonder what is happening.
Seems if the compressor pushes 50lbs of 12 that 50lbs of 134
would be the same even though it might not cool right?
I am not sure the pressure is the issue.
I have seen 134 as low as 25 and still cool good.
I skimmed the other thread and wonder what is happening.
Seems if the compressor pushes 50lbs of 12 that 50lbs of 134
would be the same even though it might not cool right?
I am not sure the pressure is the issue.
Drifting
Joined: Jun 2005
Posts: 1,523
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From: grand prairie texas
There are a lot of variables on your conversion. If your outside air temp is up around 90 degrees and you have only the engine driven fan moving air across the condensor then 50 on the low side is about normal and around 250 on the high side should be normal. R134 runs higher pressures than the r12. The more air or the cooler your condensor is the lower your pressures will be. How old is your compressor? Did your mechanic flush out the system and old oil? Your old compressor may not be able to handle the higher pressures and you may have to relieve a little bit of refrigerant.
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
DJCorn
Assuming your A/C guy knows what he is doing, tighten the belt.
Pressure depends on temp., car, etc.
50 sounds high if at idle.
I am not running R134.
Since R12 was brought up:
R12 at idle about 30 low side will do it.
Assuming your A/C guy knows what he is doing, tighten the belt.
Pressure depends on temp., car, etc.
50 sounds high if at idle.
I am not running R134.
Since R12 was brought up:
R12 at idle about 30 low side will do it.
Race Director
Joined: Mar 2006
Posts: 14,112
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From: Florida
To even make a wild stab at this you have to supply a lot more info
Ambient temp
Inside vent temp, blower high
low side pressure @ 1500rpm
high side pressure # 1500 rpm
If it was 50 psi on the low side at idle that is way too high for either refrigerant.
On the high side , it would be way too low.
Ambient temp
Inside vent temp, blower high
low side pressure @ 1500rpm
high side pressure # 1500 rpm
If it was 50 psi on the low side at idle that is way too high for either refrigerant.
On the high side , it would be way too low.
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
Is it 50 without the compressor running or with stopped?
If it is when running, it would be 10lbs min high with 134.
Easy enough to release pressure.
R12, 50lbs running is @ the cutoff for testing.
Could be restriction between high and low pressures.
If it is when running, it would be 10lbs min high with 134.
Easy enough to release pressure.
R12, 50lbs running is @ the cutoff for testing.
Could be restriction between high and low pressures.
Burning Brakes
Joined: Jun 2008
Posts: 1,086
Likes: 1
From: Michigan
To add to what was said above, keep in mind the pressure is higher when the A/C is off than when it is running. 40-45 low side on an average summer day with A/C running seems about right.
If you converted to R134a keep in mind that the original R12 oriface tube is not the best choice for newer R134a systems. The oriface tube is what regulates the pressure between the high side and the low side.
Have your A/C guy take a look at the oriface tube for contamination on the screen or metal chips from the compressor failing. I would recommend replacing the original oriface tube with one of the new variable ones. The original oriface tube can be replaced from $0.88 to $4.50 depending on who you purchase it from. A new variable one would be $20-$30 but the cooling difference is worth it. You will loose your charge when the oriface tube is removed. Use a tool designed for this purpose (in and out) or you could break it off in the metal tube. The plastic is delicate and many decades old so it is brittle and breaks easily.
The squeeling sounds like your belt. Either the belt is loose or the compressor is laboring or failing putting a larger than expected strain on the belt.
If you converted to R134a keep in mind that the original R12 oriface tube is not the best choice for newer R134a systems. The oriface tube is what regulates the pressure between the high side and the low side.
Have your A/C guy take a look at the oriface tube for contamination on the screen or metal chips from the compressor failing. I would recommend replacing the original oriface tube with one of the new variable ones. The original oriface tube can be replaced from $0.88 to $4.50 depending on who you purchase it from. A new variable one would be $20-$30 but the cooling difference is worth it. You will loose your charge when the oriface tube is removed. Use a tool designed for this purpose (in and out) or you could break it off in the metal tube. The plastic is delicate and many decades old so it is brittle and breaks easily.
The squeeling sounds like your belt. Either the belt is loose or the compressor is laboring or failing putting a larger than expected strain on the belt.
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Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
To even make a wild stab at this you have to supply a lot more info
Ambient temp
Inside vent temp, blower high
low side pressure @ 1500rpm
high side pressure # 1500 rpm
If it was 50 psi on the low side at idle that is way too high for either refrigerant.
On the high side , it would be way too low.
Ambient temp
Inside vent temp, blower high
low side pressure @ 1500rpm
high side pressure # 1500 rpm
If it was 50 psi on the low side at idle that is way too high for either refrigerant.
On the high side , it would be way too low.
Just for starters do a search for a pressure/temp chart for both r12 and 134A you will see that the working pressure and temps are not that different. The idea here is to get the pressure down to a point were the temp will be just above freezing at the evaporator so the condensate will not freeze on the coil but produce the coldest temps possible, but keep in mind that you have to consider ambient temps in the car and at the condenser, there are many other variables to consider here also. Supply the needed info and someone here will offer a little better insight on your question.
Neal
Thread Starter
Instructor
Joined: Aug 2008
Posts: 104
Likes: 1
From: Warner Robins Georgia
I tightened the belt, then I replaced the belt. The sound seems to come from the clutch area, the pulley spins freely. He told me it was 50 at idle. I just had him order a new compressor, if this doesn't fix it then I'll take it to a shop. He is a coworker of my wifes and sells car parts on the side out of his Dad's shop or something. He agreed to come over and fix it just for the price or parts but its looking like I need to take it to a specialist. I just really don't want to fork over $1k for someone to fix it.
Last edited by DJCorn; Aug 12, 2009 at 09:04 AM.
Race Director
Joined: Apr 1999
Posts: 11,520
Likes: 13
From: CORVETTE 77 385 C.I. TEXAS
What should the A/C pressure be on a 74 after the R134 conversion? The guy that I had work on my A/C said that the pressure was @ 50. Is that too high? I'm trying to figure out what is causing the compressor to squeal. The only difference with before it was putting out hot air (and not squealing) and now is that he put refridgerant in it.
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He told me it was 50 at idle.
...
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He told me it was 50 at idle.
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Last edited by Ganey; Aug 12, 2009 at 09:20 AM.
Thread Starter
Instructor
Joined: Aug 2008
Posts: 104
Likes: 1
From: Warner Robins Georgia
I don't think it is a chirp, it is a loud, non-stopping, squeal. I can see on the front of the compressor that it is trying to lock up. So basically I have to take it to a shop for them to remove some of the freon and see if that works? I really don't want to drive down the road with that screaming sound coming out form under the hood.
Last edited by DJCorn; Aug 12, 2009 at 10:36 AM.
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
I don't think it is a chirp, it is a loud, non-stopping, squeal. I can see on the front of the compressor that it is trying to lock up. So basically I have to take it to a shop for them to remove some of the freon and see if that works? I really don't want to drive down the road with that screaming sound coming out form under the hood.
Neal
Thread Starter
Instructor
Joined: Aug 2008
Posts: 104
Likes: 1
From: Warner Robins Georgia
Okay, so he came back over with a new compressor and after he put oil in and spun it some, we reinstalled. Then we started adding freon. It ran great for like 10 minutes, blowing cold air, then it started making that obnoxious squeal (skreech) again. What could cause a brand new compressor to do this as well? The clutch area got really hot and started locking up. He also said that all of the sudden the pressure spiked on the low side. He kept releasing pressure to see if that was causing it but the skreech didn't stop. HELP!
Last edited by DJCorn; Aug 14, 2009 at 08:31 PM.
Team Owner
Joined: Jan 2007
Posts: 53,648
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From: Southern Cal Ca
St. Jude Donor '22
when you first put freon in, did the clutch cycle quickly or did it engage and not turn off?
I am thinking there is a blockage that gets pushed
or your pressure switch.
I am thinking there is a blockage that gets pushed
or your pressure switch.
Last edited by calwldlife; Aug 14, 2009 at 10:40 PM.
Race Director
Joined: Apr 1999
Posts: 11,520
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From: CORVETTE 77 385 C.I. TEXAS
5th Gear
Joined: Aug 2009
Posts: 5
Likes: 0
If you have converted to R134 you will need to change the pressure switch from an R12 to an R134 pressure switch. Its avaiable at any parts store. The pressure in 134 is highter than in R12 systems. Here is the formula when doing A/C charging and using 134.
Check the current outside Temp from a valid source. Low side should be 30 less. Outside temp makes a huge difference on pressures. On an average 50 is a good mark if it is really hot out like 95, fill it to 50 and let it set there. Start filling it while watching the pipes when they start sweating you are close. Only fill a little bit at a time as the pressure on the hisgh side and low side will adjust as freon is added.
Remember it will not get really cold until you start driving it and air starts hitting the condenser. A perfectly tuned harrison A/C unit will cycle on and off after running for 20 min and the pipes are sweating good.
Let me know if you need any more help.
NelsonComp
Check the current outside Temp from a valid source. Low side should be 30 less. Outside temp makes a huge difference on pressures. On an average 50 is a good mark if it is really hot out like 95, fill it to 50 and let it set there. Start filling it while watching the pipes when they start sweating you are close. Only fill a little bit at a time as the pressure on the hisgh side and low side will adjust as freon is added.
Remember it will not get really cold until you start driving it and air starts hitting the condenser. A perfectly tuned harrison A/C unit will cycle on and off after running for 20 min and the pipes are sweating good.
Let me know if you need any more help.
NelsonComp