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HIgh low side pressure, POA or TXV?

barraza on Thu May 13, 2010 2:13 PM User is offline

Year: 1972
Make: chevy
Model: truck
Engine Size: 350
Refrigerant Type: r-12
Ambient Temp: 80
Pressure Low: 75
Pressure High: 100
Country of Origin: United States

This system was working fine last year. It quit cooling and I noticed the TXV was wet with oil. I replaced the TXV, dryer, and all o-rings. I also ended up replacing the condenser, as it was damaged loosening fittings for the o-ring change. Evacuated system normally, and started charging with r12. I know the system is not charged fully. The POA frosts on its downstream side. What's going on??

Dougflas on Thu May 13, 2010 4:45 PM User is offline

POA can show frost when charging until fully charged. Charge by weight until full charge is in system. If you did not measure the charge, charge until sight glass almost clears. Wait for a hot day and finish charging until sight glass clears; then add an oz or two. You should be ok.

barraza on Thu May 13, 2010 6:24 PM User is offline

How do I charge when the low side is equal to the pressure in the bottle? What is keeping the low side pressure from going down, but still allowing frosting of the POA?

Edited: Thu May 13, 2010 at 6:27 PM by barraza

Dougflas on Fri May 14, 2010 5:37 AM User is offline

shut off the vehicle. Put the r12 in warm water to build its pressure and charge into the high side as liquid. Did you pull a vacuum before charging? POA may be stuck closed. To test POA, Run engine with blower motor disconnected. Low side should be at 30 psi if POA is working.

barraza on Fri May 14, 2010 9:38 AM User is offline

Sorry, I am a little confused. I am hesitant to put any more r12 in when the low side is not going below 75psi. My understanding of the POA is that it regulates pressure in the evaporator. Shouldn't the evaporator be incapable of maintaining 75psi if the POA is working, assuming the compressor is pulling lower pressure than that from the POA's downstream side? When the POA is closed, is it completely sealed, or is it bypassing enough to have the pressure drop that is causing the frosting? I assume the POA has enough capacity to regulate the exap pressure no matter what the TXV is doing, corrrect?

Dougflas on Fri May 14, 2010 7:09 PM User is offline

If you're charging, high side is 100psi, low side is 75, you will be reading the high side which will be low due to low freon charge level. The low side is reading the pressure of the container of the freon supply. POA will frost up because charge is low. How much freon 12 have you gotten into the system?

barraza on Fri May 14, 2010 9:55 PM User is offline

Not sure how much total charge I have in, but the low side reads 75psi with the bottle closed. So I am pretty sure I am not reading the bottle pressure. I just went out and did a cold system off startup, no r12 bottle attached. The pressure is about 75psi low, and 100psi high, lots of bubbles in the sight glass. I took the TXV bulb loose from the Evaporator and put it into a glass filled with ice water, then into hot water - the pressures stayed the same. Why should the low side pressure ever be this high on a running system?

Dougflas on Sat May 15, 2010 5:39 AM User is offline

the txv test was the next step. It appears your txv has gone south. You should have seen a change when the bulb was placed in ice water and then held in your hand or hot water.

barraza on Sat May 15, 2010 11:52 AM User is offline

I had an old POA on the shelf, that I had removed from a car 20+ years ago because I had thought it was bad but never knew how to test it. I rigged up some test fittings and it blew straight through, ie it never was able to hold any pressure on the gauges - evidently it is stuck open. Then I removed the POA from the truck I am working on and ran the same test. The pressure on the gauges always reads the same as the compressed air source, but a little leaks through the POA outlet - this one is evidently stuck closed. This is what is causing the excessive low side pressure. The small amount leaked through the stuck closed POA is what was providing the pressure drop that was frosting the POA. I tried upping the pressure to approx 120psi and it still never opened

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