Air condition pressure

[1984C4]

Could someone tell me what the low side pressure should be with the compressor running and using R12? Is the correct way to service it with a 30lb cylinder to turn the cylinder upside down with the low pressure hose connected and open the valve and stop when it reaches this pressure? My system leaks out slowly in the garage during the cold winter. Can't find a leak in the summer with a leak detector. I have a service manual but it does not tell you how to top off a low system. It calls for evacuating the the whole system and adding 2.75 lbs. I just want to add some freon but not put a vaccum on it. The last charge was done by a shop who added a can and it lasted a year and a half. I don't have a can but I may be able to use some from a cylinder and I don't want to waste this gaseous gold. Thanks

 

[1984C4]

What, no one has ever serviced their A/C here or am I not explaining it correctly? I think it is 40 psi but I'd like to confirm that with some one who is sure.

 

[milehigreg]

My 2 Cents

I don't know off the top of my head what the low side pressure should be. I would be surprised if your manual doesn't list it. The low pressure side of an air conditioning system is downstream of the thermal expansion valve and upstream of the compressor. Refrigerant flows in an A/C system as follows: evaporator, to compressor, to condenser to thermal expansion valve , to evaporator. The refrigerant is in liquid form between the condenser and evaporator. It is a low pressure liquid only between the txv and the evaporator. It is a low pressure gas between the evaporator and compressor. The low pressure service valve is located in the low pressure gas section between the evaporator and the suction side of the compressor, aka the suction line. Even though it is a suction line it is still at a positive pressure, not in a vacuum. The pressure in your 30 pounder must be higher than the pressure in the suction line. If it's hot enough outside this may occur naturally. I place the 30 pounder in a contaner of hot water to make sure. Connect to the low pressure service valve. Start the car, turn the A/C controls to max to fully open the txv. If the charge is low there should be bubbles visible in the sight glass. Slowly open the valve either on the tank or your gage set and charge until the bubbles disappear. Cold air temperature should be in the 53 to 55 degree range. Please be careful when doing this. If you connect to the wrong service valve you could theoretically blow up the the canister or a line. Wear gloves when connecting and disconnecting hoses. You can actually get frostbite on your fingers. Good luck.

 

[1984C4]

Thanks for the reply. I just mistakenly purchased an R-134 service manifold that was supposed to be able to service both systems but alas it just included a nipple adapter set to pump R134 into an R12 system to convert it. This simple recharge is getting more and more costly. I could still use the manifold if some one makes an adapter to connect the yellow service hose to the 30 lb cyclinder. Back to the drawing board. This link is a good reference for anyone else needing some knowledge. http://members.tripod.com/~jbabs714/autoac.htm
Thanks again for replying. I was beginning to lose faith in the forum.

 

[milehigreg]

I checked my service manual last night and there is a table at the end of the A/C chapter which lists the expected low and high side pressures at different ambient temperatures and relative humidities. It ranged from about 30 PSI to 43 PSI. It also lists the expected supply air temperatures. This ranges from about 43 to 52 degrees. Check your manual and don't forget to be safe.

 

[Robertwav1]

I reference to R-12....General speaking without getting technical. On a hot day, engine at 1500 RPM, set controls to max AC. Adjust to 30 to 33 PSI suction. A variable displacement compressor should be adjusted this way to keep head down when the car is at idle and keep suction from freezing when turning higher RPM's. Try not to pay too much attention to temperature inside the car....it will vary greatly. the air comming from the ducts will be colder at higher RPM's. 134a pressure should be adjusted a few pounds higher. If your system has a sight glass by all means use it for reference. I like to go by pressures for accuracy.

 

[1984C4]

Thanks for the advice. My car does not have a sight gauge but the servicing manifold I plan on using does. Will this give me any usefull information in the way of bubbles and flow like it would if it were on the receiver/drier?

 

[Robertwav1]

Absolutely not! Sometimes it's necessary to charge the low side with the tank upside down. The glass on the manifold set lets you know the amount of liquid being sent to the compressor. Whenever you charge in this manner the low side valve on the manifold set has to be only slightly cracked to trickle liquid (letting it expand to a gas before it gets anywhere near the compressor) Always remember the compressor is a gas pump, NOT a liquid pump. Any solid getting to the compressor can ruin it. Most compressors today on a vehicle are centrifical vane type. Liquid does not play gracefully with the vanes. Charge the system with vapor, charge it slowly, this is the proper way. As you gain experience with AC you will learn to successfully charge a system without exposing the compressor to damage. Hope this helps you out....there are quite a few people here with knowledge and technics they use for charging, perhaps they can add to this.