A/C questions
Thread Starter
Pro
Joined: Jun 2010
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From: CA
A/C questions
ok just finished converting my old 1988 corvette R12 system to a newer R-134a system. i removed the old R12 (per government regualtions 
) Vacumed down the system. removed the old dryer and installed a new one, took off the compressor to clean it up and blow it out with compressed air, ran flush through the removable A/C lines and installed the retrofit R-134a nipples and Tag.... I ran another Deep vacume to 29.7 lbs or so and charged the system with 3 12oz cans of R-134a coolant (i pored the oil into the dryer and compressor before reinstalling). I put a meat thermometer gauge in the main vent in the cabin and i get 58*F.
my question is when i check the low side A/C port with a gauge i get "weird" reading. when i turn the ac to max ac and check the gauge i get about 30psi which will hold constant untill the engine fan kicks on and then my psi drops to 25psi. is this normal?
also when i hold the engine at 2,000 rpms and check the low side gauge it reads a little less than 25psi,,,, is this normal?
should i add more coolant to get the psi up? i already put in three 12oz cans of r-134a which would be about 85% of the R-12 volume.
Please let me know!!!!!!!!!!!!!!!!!!!!!
) Vacumed down the system. removed the old dryer and installed a new one, took off the compressor to clean it up and blow it out with compressed air, ran flush through the removable A/C lines and installed the retrofit R-134a nipples and Tag.... I ran another Deep vacume to 29.7 lbs or so and charged the system with 3 12oz cans of R-134a coolant (i pored the oil into the dryer and compressor before reinstalling). I put a meat thermometer gauge in the main vent in the cabin and i get 58*F.my question is when i check the low side A/C port with a gauge i get "weird" reading. when i turn the ac to max ac and check the gauge i get about 30psi which will hold constant untill the engine fan kicks on and then my psi drops to 25psi. is this normal?
also when i hold the engine at 2,000 rpms and check the low side gauge it reads a little less than 25psi,,,, is this normal?
should i add more coolant to get the psi up? i already put in three 12oz cans of r-134a which would be about 85% of the R-12 volume.
Please let me know!!!!!!!!!!!!!!!!!!!!!
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
You're good.
The pressures are a reflection of the heats thats being absorbed. As the fan comes on heat is removed so pressure drops...as rpm rise heat is being transfered to the condensor for removal, so pressure drops.
85 to 90% of what the r-12 charge is what works for 134a. You can add maybe an oz at a time to see, but the pos or neg will be so slow to show itself, its hard to know if you helped or hurt. The system will give better cooling with increased airflow to the front end...it'll get colder when its moving.
Did you install an orifice for 134a? or a variable orifice? That helps.
The pressures are a reflection of the heats thats being absorbed. As the fan comes on heat is removed so pressure drops...as rpm rise heat is being transfered to the condensor for removal, so pressure drops.
85 to 90% of what the r-12 charge is what works for 134a. You can add maybe an oz at a time to see, but the pos or neg will be so slow to show itself, its hard to know if you helped or hurt. The system will give better cooling with increased airflow to the front end...it'll get colder when its moving.
Did you install an orifice for 134a? or a variable orifice? That helps.
Thread Starter
Pro
Joined: Jun 2010
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From: CA
i bought a new orifice tube but i never put it in.... kinda forgot to do that... where does the orifice tube go? can i put it in without depressuring the system? does it matter that much that i didnt put a new orifice tube in?
Le Mans Master
Joined: Dec 2000
Posts: 7,839
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From: San Diego, Ca
36 ozs is the full R12 charge. Ignore the Shop Manual (corrected by Service Bulletin) and use the weight on the Blower Housing which should indicate 2.25 pounds.
R134 at 30 psi is about 36 degrees; with superheat probably 42 degrees and inefficiency gives you the vent temp you're seeing. Make sure there isn't an air stream below the ECM which would indicate leaking ductwork or below the Evaporator Case, especially if someone swapped it out, indicating a bad seal.
Because R134 freezes at a lower pressure than R12 (28 psi vs 30 psi), adjust the Low Pressure Cutout to 22.5 psi. Get it on Max and pull the Blower Motor Harness to drop system pressure and note where the cutout is. The Low Pressure Switch has an adjustment screw between the terminals. Turn it counterclockwise to lower the threshold (1/2 turn at a time) or clockwise to raise it, or simply buy a new switch - and in my experience, they're all set for R134 meaning if you're still using R12, it needs the threshold raised.
Take pressures noting the temp of the air at the Condensor (usually 10 degrees above ambient), Main Fan on (disconnect the fan switch), engine steady at 1200 to 1500 rpms. If the compressor is cycling, momentarily jumper the Low Pressure Switch, though it will probably ice up while your checking pressures. Compare to Temp/Pressure Chart in your Manual which will be close enough.
R134 at 30 psi is about 36 degrees; with superheat probably 42 degrees and inefficiency gives you the vent temp you're seeing. Make sure there isn't an air stream below the ECM which would indicate leaking ductwork or below the Evaporator Case, especially if someone swapped it out, indicating a bad seal.
Because R134 freezes at a lower pressure than R12 (28 psi vs 30 psi), adjust the Low Pressure Cutout to 22.5 psi. Get it on Max and pull the Blower Motor Harness to drop system pressure and note where the cutout is. The Low Pressure Switch has an adjustment screw between the terminals. Turn it counterclockwise to lower the threshold (1/2 turn at a time) or clockwise to raise it, or simply buy a new switch - and in my experience, they're all set for R134 meaning if you're still using R12, it needs the threshold raised.
Take pressures noting the temp of the air at the Condensor (usually 10 degrees above ambient), Main Fan on (disconnect the fan switch), engine steady at 1200 to 1500 rpms. If the compressor is cycling, momentarily jumper the Low Pressure Switch, though it will probably ice up while your checking pressures. Compare to Temp/Pressure Chart in your Manual which will be close enough.
Thread Starter
Pro
Joined: Jun 2010
Posts: 535
Likes: 0
From: CA
36 ozs is the full R12 charge. Ignore the Shop Manual (corrected by Service Bulletin) and use the weight on the Blower Housing which should indicate 2.25 pounds.
R134 at 30 psi is about 36 degrees; with superheat probably 42 degrees and inefficiency gives you the vent temp you're seeing. Make sure there isn't an air stream below the ECM which would indicate leaking ductwork or below the Evaporator Case, especially if someone swapped it out, indicating a bad seal.
Because R134 freezes at a lower pressure than R12 (28 psi vs 30 psi), adjust the Low Pressure Cutout to 22.5 psi. Get it on Max and pull the Blower Motor Harness to drop system pressure and note where the cutout is. The Low Pressure Switch has an adjustment screw between the terminals. Turn it counterclockwise to lower the threshold (1/2 turn at a time) or clockwise to raise it, or simply buy a new switch - and in my experience, they're all set for R134 meaning if you're still using R12, it needs the threshold raised.
Take pressures noting the temp of the air at the Condensor (usually 10 degrees above ambient), Main Fan on (disconnect the fan switch), engine steady at 1200 to 1500 rpms. If the compressor is cycling, momentarily jumper the Low Pressure Switch, though it will probably ice up while your checking pressures. Compare to Temp/Pressure Chart in your Manual which will be close enough.
R134 at 30 psi is about 36 degrees; with superheat probably 42 degrees and inefficiency gives you the vent temp you're seeing. Make sure there isn't an air stream below the ECM which would indicate leaking ductwork or below the Evaporator Case, especially if someone swapped it out, indicating a bad seal.
Because R134 freezes at a lower pressure than R12 (28 psi vs 30 psi), adjust the Low Pressure Cutout to 22.5 psi. Get it on Max and pull the Blower Motor Harness to drop system pressure and note where the cutout is. The Low Pressure Switch has an adjustment screw between the terminals. Turn it counterclockwise to lower the threshold (1/2 turn at a time) or clockwise to raise it, or simply buy a new switch - and in my experience, they're all set for R134 meaning if you're still using R12, it needs the threshold raised.
Take pressures noting the temp of the air at the Condensor (usually 10 degrees above ambient), Main Fan on (disconnect the fan switch), engine steady at 1200 to 1500 rpms. If the compressor is cycling, momentarily jumper the Low Pressure Switch, though it will probably ice up while your checking pressures. Compare to Temp/Pressure Chart in your Manual which will be close enough.
ok so your saying that if im getting 58* through the vents that i have a leak somewhere? i put UV die in to see if i can spot a leak and i cant find one... could it be under the dash? are you sure i have a leak or is there something else i can do to lower my vent tempitures????? i dont think i have a leak
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
No gas leak, he's saying that you might have ducts inside the dash that are leaking the cold air out before it gets to the vents...or the exchange is blocked or leaking the cold air out. To be efficient and get you ALL the cold air inside where it counts, the dust work has to be tight, the evap box has to be sealed and everything has to be in new condition. It just takes a little detective work to find where the cold is going...
orifice....sorry, you have to open the system to do that and THAT could be a problem. The orifice is a screen and IF its dirty, it reduces the efficiency of the liquid/gas transition in the system. Thats what the orifice tube does...meters the spray into the evap core.
orifice....sorry, you have to open the system to do that and THAT could be a problem. The orifice is a screen and IF its dirty, it reduces the efficiency of the liquid/gas transition in the system. Thats what the orifice tube does...meters the spray into the evap core.
Le Mans Master
Joined: Feb 2005
Posts: 6,479
Likes: 565
From: Clearwater Florida
........... One important part of the freon change-out ... R12 to R134 ... is to change ALL the o-rings from the black ones to green ... this is for oil and freon compatibility ... R134 will eventually dissolve the black o-rings ... including the one that seals the body of the compressor ... changing the orifice tube was also important for max efficiency ... when charging , you should put a fan in front of the car and overide the engine cooling fan so it stays on ... run the engine at approx 1500 rpms when checking pressures ...
