Charging custom A/C system

[chevynut]

I've leak-checked my A/C system and I think it's all ready to go. I've never run the compressor and it's many years old now, bought new from Street & Performance. It was supposed to have oil in it when I got it and as far as I know none has leaked out. I want to charge it myself if I can, so I can service it in the future if I need to.

I've been looking at YouTube videos explaining how to charge a 134a system and they all say to check for a tag on the car that says how much refrigerant to use. Well, my car has no such tag.

I don't have Vintage Air's "standard" kit, I just bought their 56 Chevy Gen IV evaporator kit. I made my own hoses and hardlines. Condenser is from PRC and the dryer is from S&P as is the Sanden SD7H15 Model compressor. I also have a trinary switch installed.

What determines the amount of refrigerant, the volume of the entire system, the volume of the condenser, the volume of the evaporator, etc.??? I thought you could do this with gauges but I've read that you can't with a R134a system. How do I determine how much refrigerant to add?

I did find this, which contradicts what others say:

"A manifold gauge set will help to measure the psi?s in the system. The gauge will help identify when the system is adequately pressurized. For a good charge, the low-pressure gauge should read between 25 and 40 psi (pounds per square inch), and the high pressure gauge should read between 225 and 250 psi. Another useful tool is a refrigerant scale. A scale will allows you to know how much refrigerant (by weight) was actually added. You can also use a thermometer on your dashboard vents to test the temperature of the air coming out during a recharge. When everything is working properly your A/C system on max cool should be blowing out air that is around 38-48 degrees F. An A/C system is comprised of a variety of components, hoses and fittings that circulate and pressurize refrigerant. "

[Rick_L]

Refrigerant volume required is a function of total system volume. Your condenser probably has a larger volume than the one VA supplies for a stock radiator.

The last paragraph is pretty much correct except that they didn't say what the engine speed should be for those pressures. 1500-1800 rpm is a good number for that. High side pressure will be dependent on ambient temperature. Low side pressure is an indication of evaporator temperature. Too high and it's overcharged. Too low and the evaporator will ice up and block airflow. You'll probably have difficulty trying to charge the system when the ambient temperature is 70? or below. If you had a freon recycling machine and knew exactly how much you need you could use that (those have a heater to push the freon into the system) but you can't really see how the system is working.

In my limited experience, VA's spec of 1.8 pounds of refrigerant is slightly low for a system with a bigger than stock condenser.

[chevynut]

Refrigerant volume required is a function of total system volume. Your condenser probably has a larger volume than the one VA supplies for a stock radiator.

Thanks for the reply and the info. Yes I think my condenser is a bit larger than one for a stock radiator but I don't know the exact volume comparison. I think it's a "standard" size PRC even though my radiator is custom. The hoses and everything else are probably comparable. It sounds like if I use the VA charging volume spec I should be close, if not a little under-charged.

You'll probably have difficulty trying to charge the system when the ambient temperature is 70? or below.

Wow, I had no idea that would be a problem. I can get my shop to 70 or higher if I need to. What happens if I under-charge it now, and add refrigerant next summer when it's hot if needed? Would that cause any problems? I'd just like to get it working to make sure everything works, even if not at full efficiency. My trucks' and cars' A/C work fine even in colder weather. In fact, I thought the A/C ran with the defroster on in winter. Is the problem just getting the refrigerant out of the can? Why is it so hard to charge?

In my limited experience, VA's spec of 1.8 pounds of refrigerant is slightly low for a system with a bigger than stock condenser.

I was going to call them to get a recommendation but I'm not sure they would give me one with a custom setup. I didn't get any charging specs with my evaporator kit. Here's what I found from Vintage Air. How do you get the complete system to 90 degrees?????:

R-134a refrigerant charge is 1.8 lbs OR 28.8 oz.
New Sanden compressors come charged with oil. No additional oil is needed anywhere in the Vintage Air system.
Vintage Air systems use an expansion valve and will operate at a lower suction pressure than some OEM systems. Please note formula below.
Evacuate the system for 45 minutes (minimum) with the complete system @ 90˚F before charging.

A. Place temperature probe (thermometer) into center outlet.
B. Connect gauges or service equipment to high/low charging ports.
C. Place blower fan switch on medium.
D. Close all doors and windows on vehicle.
E. Place heavy-duty squirrel-cage blower directly in front of condenser.
F. Run engine idle up to 1500 RPM.

These test conditions will simulate the effect of driving the vehicle and will give the technician the three critical readings needed to diagnose any potential problems.
Acceptable Operating Pressure Ranges for R-134a Vintage Air Systems:

High-side Pressures: 160 to 270 PSI. NOTE: General rule of thumb is two times the ambient (daytime) temperature, plus 15 to 20%.

Low-side Pressures: 10 to 20 PSI in a steady state.

Center Duct Temperature: 36˚F to 46˚F.