Automotive Air Conditioning Information Forum

I'll say up front I'm not an experienced A/C guy, but I'm learning. I have an RV that the dash A/C was working when charged with Freon, but had a Freon leak and would leak down pretty quickly. I finally found the leak which was the pressure switch. This was complicated by the fact that the pressure switch and expansion valve were housed inside the evaporator box. After I got the evaporator box out of the vehicle I took it to an A/C shop and had it leak tested and they found the bad switch. While I had it apart I thought it was a good idea to replace the expansion valve as well since it was such a pain to get to. After putting the system back together I vacuumed it down over night and it lost no pressure so was good to recharge. It is a 2.8 lb system and I put 38 ozs of Freon in it. The compressor clutch comes on and my gauges read 60 lb low and 375 high, and at no time during the charge process has it blown cool air. After about an hour after shutdown my gauges have equalized at 100 lbs each. While the compressor is running, the line going into the evaporator is hot and the line coming out is only a little cool not cold. The dryer is also hot to the touch. I was told by the A/C shop that the new expansion valve might be bad, so I tore it all apart and replaced it,,.....exact same results. Since it was working before except for a leak, I keep coming back to that expansion valve. Is it possible that they are giving a valve that is not the proper one for this system even though it looks identical? I still have the original and my only thought at this point is to put it back in and see what happens.

Are the valves the correct ones? Are the sensing bulbs secured and insulated? You might contact Tim and let him supply the correct replacement. Two replacements with the same problem? Something seems rotten in Denmark.

That's my question. Is it possible they are giving me the wrong valve? If so would that cause the problem I'm having? Are they made for different pressures or other paramaters? All I know is they look the same. My original has numbers on it, but the new ones do not other than the part numbers on the receipt. And yes, the sensing bulbs are cliped onto the evap pipe and wrapped with insulation. Who is Tim?

Tim is the owner of this site and of ACkits.com.

What is the make, model, and year of the vehicle?

A few other thoughts and questions: I assume that this is an R-134a system and that the under-hood decal specifies 2.8 pounds (44.8 ounces) of refrigerant. You mentioned that you evacuated and charged it with only 38 ounces...6.8 ounces shy of the spec, which is significant. Was that because it wouldn't take any more, or for some other reason? Were you able to pull a good vacuum on it overnight, around 29.9 in. Hg.? And did you change the receiver/dryer?

The pressures you posted seem very high, so I'm not surprised that the system isn't cooling. What was the ambient temperature at that time, and did you measure the pressures with the A/C set on max cooling, blower on medium, windows and doors open, and engine at about 1,800 RPM? Did you notice whether the condenser fans were running at full speed?

And yes, if the TXV is an aftermarket part, I think it could be a problem, especially since the system was cooling before you replaced it. If an OEM TXV is available, I would definitely go with that even though it will cost more. If not, I would reinstall the original TXV, evacuate and recharge, and see how it performs.

I appreciate your response JohnHere. Yes this is a 134a system built on a Freightliner chassis. No underhood sticker, but Freightliner said it is a 2.8 lb. capacity system. I only put 38 ozs. in because the pressures were climbing alarmingly high quickly, so I stopped. Yes, it held a 29 Hg vacuum for 24 hours before I added Freon, and I did replace the dryer. It was 85 F while I was working on it with the A/C on max and fan speed on high. Windows and doors were closed and engine was at normal idle. But here's the kicker. The condender and electric fan are mounted underneath between the wheels. I had never had a problem with those and did not check them until this morning when I started looking for trouble again. The fan was locked up. So I think that is my problem. Just my luck I'd get multiple problems at the same time. Since it was working before I replaced the pressure switch and expansion valve I was only looking there.Tomorrow I start looking for a 16" single speed 12vdc fan. There are no markings or numbers left on since it lives in the road spray, etc, but I'll find one. Thanks again for your interest.

you can spray the condenser with a water hose and see if the high pressure drops. You need to test the system at 1500 rpm; not idle.

A nonworking fan will certainly raise the pressures, especially with the unique system design of your RV. At least you won't have to recover and recharge the refrigerant again to replace it.

At 85 degrees ambient and the fan running at full speed, the high side should be somewhere in the low 200-PSI range while the low side should be around 30 PSI.

As mentioned previously (and if you'd like to test it now), spray the condenser with water while watching the pressures, which should drop. While you're under there, check the condenser fins to ensure no bugs or other debris are partially blocking airflow and that the condenser itself isn't badly corroded.

Hope a new fan fixes this and that you don't have to get back into the TXV. Let us know.

Thanks for the help. The fan was the problem. My gauges now read 35 low and 225 high and I have cold air. It just happened to fail while I was in the process of replacing the pressure switch and THX, and I had tunnel vision and wasn't looking for additional problems. Looking back on it I do remember a burnt smell at one point while I was in the middle of it. That was when the fan let go. But it's mounted underneath, the smell went away and I just didn't put it together. Oh well!

But I do have one more question. When I was still troubleshooting, and my high side pressure went up to 375 psi, I tried to bleed off some Freon to relieve the pressure. Except for a short burst of Freon there was nothing in the system but air. I assumed I had blown something out and once it cooled down I put it back under vacuum. It pulled down to -29 hg and held, no leaks. Where did the Freon go?

jim427 wrote:Thanks for the help. The fan was the problem. My gauges now read 35 low and 225 high and I have cold air.

Pressures look good assuming an ambient of ~90 degrees. With virtually no air flowing through the condenser during your earlier testing, it's no wonder you weren't getting any cooling. Glad you found the problem and fixed it, especially now that summer is here.

jim427 wrote:But I do have one more question. When I was still troubleshooting, and my high side pressure went up to 375 psi, I tried to bleed off some Freon to relieve the pressure. Except for a short burst of Freon there was nothing in the system but air. I assumed I had blown something out and once it cooled down I put it back under vacuum. It pulled down to -29 hg and held, no leaks. Where did the Freon go?

At such a high high-side pressure, my guess is that the high-pressure relief valve (HPRV) mounted on the compressor did its job and released the refrigerant. Probably not air, though...just residual refrigerant gas. Did you happen to hear any hissing or wooshing sounds at some point and/or notice any oil residue around the compressor? Since you evacuated and recharged the system and it's now holding the charge, you were lucky in that the HPRV didn't continue to leak. Usually, once an HPRV lets go, it will no longer seal 100 percent and will need to be replaced.