Charging AC System - Run Both Front & Rear Systems?
08-13-2012, 04:19 PM
#32
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Andrew-
FWIW, I plug both hoses into their respective ports, attach the yellow hose to the bottle, and plug the vacuum pump into the double-top-secret port where the pressure transducer for the fan normally sits. That way I can completely evacuate the whole charging manifold all the way to the bottle, no risk of air entrainment in incoming refrigerant stream. I know it's not the standard recommendation, but call it "cheating with a purpose". For those with pre-S4 cars, the pressure safety switch sits on a similar R12 port with Schraeder valve.
Related thoughts:
-- Cars converted to R-134a need both of these Schraeder valves replaced with updated versions, with HNBR seals intended for the synthetic compressor oils.
-- DO NOT remove the vacuum pump hose without verifying that the new valves will actually hold under vacuum. My vacuum pump has its own check-valve built into the last pump stage so no worries. Other pumps may not have that feature. If the Schraeder valve you are using does not seal under vacuum, it's probably better to use the center manifold hose and put the sensors back on.
And to the original question, it's a good idea to vacuum at both ports. You never know when one of the Schraeder valves in the charge ports will not open correctly. More than a few systems have been very quick to evacuate and seem to hold vacuum very well, only to find later that the depressor in the end of the hose didn't actually open the Schraeder valve.
Note also that the R12 charging hoses have a depressor in one end only; Putting that end of the hose on the manifold instead of the car will leave the valve closed. There's a small tubular seal in that end too, looks like a really short section of rubber vacuum hose. It's there to maintain the seal integrity while the hose is put on and taken off, while the flare seal isn't tight but the valve may be depressed. Both the valve depressor inserts and the rubber seal inserts are available as common replacement parts, and must be used together.
FWIW, I plug both hoses into their respective ports, attach the yellow hose to the bottle, and plug the vacuum pump into the double-top-secret port where the pressure transducer for the fan normally sits. That way I can completely evacuate the whole charging manifold all the way to the bottle, no risk of air entrainment in incoming refrigerant stream. I know it's not the standard recommendation, but call it "cheating with a purpose". For those with pre-S4 cars, the pressure safety switch sits on a similar R12 port with Schraeder valve.
Related thoughts:
-- Cars converted to R-134a need both of these Schraeder valves replaced with updated versions, with HNBR seals intended for the synthetic compressor oils.
-- DO NOT remove the vacuum pump hose without verifying that the new valves will actually hold under vacuum. My vacuum pump has its own check-valve built into the last pump stage so no worries. Other pumps may not have that feature. If the Schraeder valve you are using does not seal under vacuum, it's probably better to use the center manifold hose and put the sensors back on.
And to the original question, it's a good idea to vacuum at both ports. You never know when one of the Schraeder valves in the charge ports will not open correctly. More than a few systems have been very quick to evacuate and seem to hold vacuum very well, only to find later that the depressor in the end of the hose didn't actually open the Schraeder valve.
Note also that the R12 charging hoses have a depressor in one end only; Putting that end of the hose on the manifold instead of the car will leave the valve closed. There's a small tubular seal in that end too, looks like a really short section of rubber vacuum hose. It's there to maintain the seal integrity while the hose is put on and taken off, while the flare seal isn't tight but the valve may be depressed. Both the valve depressor inserts and the rubber seal inserts are available as common replacement parts, and must be used together.
08-13-2012, 05:59 PM
#33
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You can test your pump in service for a check-valve. With everything connected and vacuum applied to the system, just turn the pump off. -IF- there are no major leaks -AND- the pump has a check-valve, the system will hold vacuum with the pump stopped. Then pull the vacuum-pump hose connection at the car end, and see if it still hold vacuum with just the Schraeder valve for sealing. Some have such weak springs that vacuum will draw them open.
The reason for using the alternate port is to allow the vacuum pump to evacuate the charge hose between the bottle and the manifold. If that's not an option, you can sweep/flush that line with refrigerant, leaving only the manifold cavity between the middle port and the two valves for air to hide out. It's not a whole lot of space, but every cc of air that gets in the system is a contaminant that reduces system capability. Total system performance is totally dependent on attention to details like this.
08-13-2012, 06:10 PM
#34
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08-14-2012, 08:24 AM
#35
NEVER charge liquid through the low side, you will wash the oil out of the compressor and possibly damage the reed valves.
Rule of Thumb:
Liquid through the high side, engine off.
Gas through low, engine running.
If you want to speed up the charging process the refrigerant container should have a higher pressure than the system so there methods to do this, such as using a refrigerant heating blanket on a 30 pounder, a heated charging cylinder or warming up a small refrigerant can (ie. 12 or 16 ouncers) in a container of hot water.
Rule of Thumb:
Liquid through the high side, engine off.
Gas through low, engine running.
If you want to speed up the charging process the refrigerant container should have a higher pressure than the system so there methods to do this, such as using a refrigerant heating blanket on a 30 pounder, a heated charging cylinder or warming up a small refrigerant can (ie. 12 or 16 ouncers) in a container of hot water.
