hoping someone here can help diagnose an AC problem. with gages hooked up and about 75 outdoor temp i have 30psi suction and about 150 head pressures, but very little or no cooling. i have bubbles in sight glass, but when i try to add freon(r-12) the suction pressure won't rise at all and i don't get rid of the bubbles in the sight glass. i did notice frost build up on the small line from POA valve to evap box after 15-20 min of running. compressor sounds normal and clutch is pulled in. was thinking of replacing POA and Expansion valve, but not sure thats my problem. any ideas?
I ma no expert but I read an article in a Muscle Car magazine several years ago that said when your lines frost up it is a bad expansion valve. I had the same problem frosting up and changed mine and it fixed the problem. I was lucky to find an NOS part. The aftermarket jobs look a bit different and have more coil to them if you care. The POA valves are much more expensive. Also never hurts to chance receiver/drier can. If you do not know how old it is change it as they go bad often.
If you have suction on the low pressure side (negative pressure), then there is a blockage in the system. Usually can find the blockage by noting the point where the freezing up stops. If it is freezing up on the line between the expansion valve and the evaporator, then the expansion valve has lost the CO2 charge in the capillary tube coming out of the front of it and going back to the blub clamped to the large evap. line. I have seen it happen many times. You will have to replace the expansion valve. I am not sure about the Riveria, but on the A-body cars, you have to take out the inner fender to get to the bulb and un-clamp it from the evap. tube.
i have 30 # pressure on suction side-it will not go up if i add refrigerant. the only line that frosted up was the bleed line from the POA to expansion valve ( 1/4" or less in size). this only frosted after adding more r-12 to system. the large aluminum line going into the evap is not cold, so i'm thinking it is the expansion valve not allowing flow or a problem with POA, just wondering how i tell for sure which is problem without just changing parts. thanks for the help
my system is all original, but very low miles. where do you get parts for these old systems? only the small bleed line from POA to expansion valve froze up after adding some r-12. the large suction line is not cool at all going into evap.
30 psi suction is normal. You don't want more than that. If you have 30 psi I'm pretty sure your POA and TXV(thermal expansion valve) are OK. It sounds like you need more refrigerant. How much did you add? What was the high side pressure? I am away from home and don't have my pressure data in front of me but at 75 ambient I'm pretty sure you're going to see 225-300psi high side when it's full, even with a fan blowing air into the grille. I wouldn't try to clear the sightglass. It may clear but it's more important to put enough refrigerant in the system to get the suction line cold without overcharging. Overcharging is seen when the high side pressure becomes excessive. The proper way to charge a system is to recover whatever is in the system, replace the reciever drier, evacuate and weigh in the factory charge. I realize this may not be practical so you do what you can. You'll want to do your charging with the controls set to max cool, fan on high, doors open and engine around 2000RPM.
This may not sound very professional but tapping on the POA Valve with a hard plastic hammer might loosen up a sticky valve if that's the problem. I have done this before and results have been positive most of the time to determine if it is the POA Valve gone bad or sticking. Even a couple of times the valve has stayed working with no problems even till now. Just my 2 cents.
If the small line from the evap to the POA is freezing up, then you have a problem with the POA valve. That small line is the oil return line from the bottom of the evap. to prevent the system oil from accumulating in the bottom of the evap., so there should be little flow of refrigerant through it. If it is freezing up, that is a sign that the POA is not opening and allowing refrigerant to flow through the large line into the back of it, which forces the refrigerant to flow through the oil return line instead. Classic Auto Air in Tampa, FL can set you up with a rebuilt POA valve or they can rebuild yours for you. They can also rebuild your factory dryer for you (the aftermarket ones look nothing like the original) and supply you with any parts you need for your system.
the 30 psi is ok, but there is no cooling at evap and high side is only 125-150 with consistant bubbles in sight glass. i have a 30lb cylinder of r-12 and when i opened it for about 3 minutes the bleedline frosted up, head pressure increase maybe 10lbs or so, but still no cooling at evap line and still had bubbles in glass. i pulled a vac on system for 2 hrs and recharged, but didn't have a charging cylinder so no real way to know how much( in old days we used to just charge by watching sight glass). still had no cooling, so my guess is something in the metering side is not functioning correctly. i'm guessing no restriction because i don't have a high head pressure or low suction pressure. thanks for the input. anybody had any luck with the r-12 replacement freons-freeze 12 comes to mind? with the small cans i could measure in the charge, or i could maybe use a bathroom scale to measure my 30lb cylinder, just not sure how accurate that would be. system capacity is 4.25lbs i think.
I've been using the same brass cylindrical fish scale to weigh refrigerant for the last 18 years professionally. I feel that I can weigh in a charge within 4 OZ that way. They are fairly cheap. You need to be careful that your hoses don't interfere with your scale. I install the hoses then weigh the tank and make sure I weigh it exactly the same way. Some guys swear by electronic scales. I'd put my method against any other out there. I should mention that I don't work on cars professionally any more. I do residential and commercial air conditioning and refrigeration. If I only worked on cars I'd be using the same equipment that other shops use, and that is a recovery/recycler unit with a built in electronic scale. There are lots of other refrigerants that will work in an R-12 system but if I had a cylinder of R-12 I wouldn't dream of trying anything else. I seem to remember my oil hose freezing up before the system was fully charged as well. While it is possible that your POA is bad, I'd first weigh in the correct charge before I'd go there. They can be checked fairly easily off the car if you find you don't get the results you want with a full charge. Again, it sounds like you don't have enough refrigerant in the system. Clearing the sight glass is good on some applications but IMHO weighing the charge is more accurate on cars. You may or may not find that your sight glass will clear when you have a full charge. After evacuation, you also want to charge the system with liquid(tank upside down) through the high side with the engine OFF. If you can't get the full charge in that way, you charge it the rest of the way through the low side with the system running. You can charge liquid through the low side but you need to be careful to meter it in slowly or you can damage or blow the compressor. The rule of thumb I was taught was charge liquid 10 psi over the running pressure. If you don't feel comfortable doing that you can charge with a gas but as you know, it takes longer. good luck.
Parts are still available everywhere... and send a link of this thread to George... tell him I sent ya... gnenad@ix.netcom.com he knows all this stuff too.
i was checking the service manual and noticed that the factory muffler in my car was eliminated at some time. will that have a large effect on the necessary total charge? the manual seems to indicate a problem with the expansion valve on the troubleshooting chart, as the POA is still controlling the low side pressure, but the expansion valve is not allowing flow thru evap coil. so i will change that and evac system and measure in a charge with scale and see what happens at that point. i don't think it matters whether you charge with vapor thru suction side or liquid thru high side as long as you get the correct weight in system. i have usually just charged with vapor so as not to risk compressor damage. thanks guys.
Success!! ok, here's the outcome to my AC issues. i changed the expansion valve and drier with NAPA current replacements. i evac the system over night and measured in 4 lbs of r-12 using my digital bathroom scale. ambient air temp of approx 80 degrees, i had the following: vent discharge air temp of 41 degrees, suction pressure of 35 psi and discharge pressure of 195 psi. at this point the suction line had frost from the POA to compressor on suction line with sight glass clear, so i added another 1/2 lb of r-12 and the line thawed and is now just sweating. i think that is pretty good according to service manual, so looks like i'm good to go. there is a little "bounce" in the high side gage-kinda vibrates 2-3 psi-hope thats ok? thanks again for all your input guys!! Now if i want the OEM look i'll have to send out my old expansion valve and drier for restoration, but for now i am COOL!!
Re: Success!! The "bounce" in the high side gauge is normal. I bet your expansion valve is just fine too but I wouldn't open a good running system just to change it. Glad you got it cool again. :TU:
