Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

[Mark P.]

Lets assume I do not have a leak.

I just recently installed a new condenser, shaft seal and expansion valve in my 67 small block car with factory AC. I triple evacuated the system and to verify no leaks under vacuum, I did a drop test over 24 hours and the system pressure did not rise above 500 microns.

Prior to adding refrigerant via a Dial-a -Charge I pre-measured the volume of R-12 that would be lost in the hoses and manifold gauges (2.5 ounces). I added 3 lbs 3 ounces to dial-a-charge cylinder and transferred the charge to the car. I have bubbles in the sight glass. Before I add additional refrigerant I was wondering if others have to say about how many additional ounces I should add and what is the maximum charge (liquid volume) that can safety be added to a AC system?

The system is cooling in the cabin, but I did not take any measurements as the ambient temperature is 98 degrees and my IR gun showed 220 degrees on the upper radiator hose.

Thanks in advance for your responses---Mark

[Russ S.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

I am not familiar with the dial a charge so I would wonder, Is it accurate?

[Dick W.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

The manual specs are close, but not always exact. The same way with the dial a charge. I use a set of digital cylinder scales and find that some systems will take a little more or a little less. I have found, in addition to guage readings and sight glass that if you will feel the top tubing on the evaporator core, when the cold feels "sharp" you have adequate refrigerant.

[Mark P.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

I am not familiar with the dial a charge so I would wonder, Is it accurate?

Russ

Hopefully my picture of the dial-a-charge will come through for you. I pressure tested the cylinder before use and used a TIF leak detector to verify the cylinder was leak free prior to charging the R-12.

The cylinder is graduated in ounces and will hold up to five pounds of liquid. The scale is graduated for pressure temperature compensation. You can see liquid volume via a sightglass in the cylinder.

Mark

[Dick W.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

It has been a long time since I have seen a Dial-A-Charge. Years ago I used to work on Thermo King mobile refrigeration units. I am thinking the units used 15 or so pounds of R-22 for normal cooling. Long time in the winter to recharge one, the DAC sho' would save a lotta time when you had a heater on it.

[Michael H.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

I assumed you added some oil to the sysytem...........

Mike

[Arland D.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Mark,

The old rule of thumb was to add refrigerant until there were no bubbles in the sight glass. It has worked like a charm for generating very cold air.............

[Mark P.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Thank You for your responses.

Mike---I added 12 oz of mineral oil prior to charging the R-12.

Dick---I also used an electronic scale to verify the accuracy of the dial-a-charge.

I understand that the glass needs to be free of bubbles. Prior to the repairs, I did have a fully charged system free of bubbles and the system provided cool air to the cabin. I just found it interesting that both my compressor and the service manual both indicate a 3.0 lb requirement and I am 99% sure that I have added 3 lbs to the system and yet I am not fully charged if one defines fully charged as a sight glass free of bubbles.

I was just curious if anyone else had experienced anything similar to my charging experience or if anyone had any data that reflected how the refrigerant was added during the production of these cars.

Mark

[William F.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Compressor says 3 lbs. but I believe Chevy Chassis Service Manual says 3 lbs. 4 oz. Just get sight glass bubble free.

[Dick W.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Compressor says 3 lbs. but I believe Chevy Chassis Service Manual says 3 lbs. 4 oz. Just get sight glass bubble free.

That is what I remembered but did not post it as I do not trust my memory anymore.

[Ray G.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Hello Mark;
Did you drain & add oil to the compressor as per the service manual ? To much oil is as bad as not enough.
Are following the SM exactly while testing the system ? Including proper RPM and an electric fan blowing through the condenser ?
Experience shows the correct basic procedures will get the system operating properly.
While in A/C school, as a young mechanic, we were told if your cabin temp., as measured at the passenger side A/C outlet with the windows closed on recirculate, is 25 degrees colder than outside ambient temp. the system was working properly.
Hope this helps your project.
Ray

[Mark P.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Bill was correct. I did a little more research and verified the charge requirement is 3 lbs. 4oz. I added 5 ounces and the glass cleared.

Now I have another problem. I have extremely high suction and head pressure. After 5 minutes of operation, the suction pressure is reading 60 lbs and the head pressure is 270 lbs. I let it run for 10 more minutes and the pressures continued to rise to 70 lbs of suction pressure and 330 lbs of head pressure before I shut the system off.

Is it possible that a POA is stuck open and is not throttling the evaporator and thus causing the abnormally high pressures?

I would appreciated any feedback-----Mark

[Adam S.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Hi Mark, I worked for GMC Trucks late70's & 80's & was the master A/C line tech guy for that truck dealership. I did all GMC gas pickup's, heavy duty GMC Brigadier ,Generals, Top-kicks , Ken-worth trucks & so on. I helped teach A/C part time at the trade school too. My experience with the slight difference with measured charges coming out short was most of the time there was not enough oil in the systems. Took me some time to figure this out after I set the charge on the fancy Snap on refrigerant pump/scale machine. I had to top off some of the systems when I was young & in a rush to get out of there on a Friday. I also noticed that different subcontractors that made the rubber hoses were a slight bit different on fitting ends. Hope that helps a little............

[Tom L.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Your suction pressure and discharge pressure are obviuosly high. Have you looked at a pressure/temperature chart? If you do you'll see that at 60 psi. the evaporator temperature is 62*, you'll get no or little cooling. At 70 psi. the evaporator temperature is even higher, about 71*, even less cooling. High discharge and high suction pressure usually indicate an overcharge. Here's a link to a P-T chart if you don't have one.

http://www.parker.com/literature/Spo...llanous/1F.pdf

Good luck!!

[Mark P.]

Re: Why was 3 lbs of R-12 Not Sufficient to Charge my AC?

Still No Cooling.

Test 1 Conditions: Hot water valve disconnected and plugged. To eliminate the potential of the high head pressure being caused by a overcharge, last evening I bled off just enough gas to show bubbling in the sight glass. Blower fan on high and discharge temp of vent in cabin is 93 degrees. Ambient temperature is 90 degrees. Portable fan blowing on condenser.



Time 1 minute into operation 64 lbs of suction pressure,220 lbs of head press, 90 degrees discharge at vent
2 minutes 62 lbs of suction, 260 lbs of head press and 85 degree duct temp.
3 minutes 61 lbs of suction, 280 lbs of head press and 78 degree duct temp
4 minutes 63 lbs of suction, 300 lbs of head press and 75 degree duct temp
5 minutes 65 lbs of suction, 305 lbs of head press and 77 degree duct temp
6 minutes 68 lbs of suction, 315 lbs of head press and 80 degree duct temp
7 minutes 69 lbs of suction, 315 lbs of head press and 82 degree duct temp

At this point I beginning to think the new expansion valve is stuck open. I remove the J bulb thermal element and place in salt water ice bath that has a temp of 28 degrees.

I did not get but one minute into the test as the head pressure was 350 and rising. The suction pressure was 29 psig and I take this to be the pressure at which the POA is set.

Thinking perhaps that I may have freed up the expansion valve I decided to re-install the J bulb on the suction line. Blower fan still on high with the temperature of the discharge vent now at 97 degrees. Ambient temperature is now 95 degrees. Some bubbles in the sight glass. Portable fan still blowing on the condenser

Suction press Head Press Discharge Temp of vent
Time 1 minute into operation 85 suction pressure, 310 head press, 92 degrees at duct
2 minutes 82 suction, 330 head, 89 duct
3 minutes 81 suction, 340 head, 87 duct
4 minutes 81 suction, 340 head, 87 duct
5 minutes 82 suction, 345 head, 89 duct

Suggestions Please? I do have on the shelf a older expansion valve that was functional but the bulb is the corkscrew style. It was functioning 3 years ago before I decided to rebuild the engine bay.

Mark