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GM POA System Diagnosis

72PMD on Sat August 02, 2008 6:22 PM User is offline

Year: 1972
Make: Pontiac
Model: Firebird
Engine Size: 400
Refrigerant Type: R-12
Ambient Temp: 93
Pressure Low: 41
Pressure High: 280
Country of Origin: United States

I have run out of things to test/check on my a/c. Pressures are as above. Sight glass is clear. Duct temps are 55-60 degrees. These pressures & temps were during a 45-65 mph run (1800-2600 rpm). At idle, low side climbs to 50-53 psi. 72 shop manual shows 29 low/260 high at these conditions. Have added a hot water valve and insulated the evap box. Air temp directly in front of evap is usually 5-10 deg hotter than ambient. Air temp a few inches downstream of evap tends to be about 5 deg higher than low side pressure would indicate. Shutting off blower brings low side to 28 psi. Hosing down condenser brings both pressures way down. This is a stock setup, new everything, Delco reman. compressor. I have owned this car since 86, a/c has never worked very well. Is this the best this system will ever do? 96 Honda with 134 beats it by 5-10 deg at the ducts under same conditions. Any ideas/suggestions would be greatly appreciated.

Dougflas on Sat August 02, 2008 8:17 PM User is offline

First thing I'd check is the friendly POA valve. Pretty simple. Connect the guages to the low side port on the side of the POA. Engine at 1200 to 1500 rpm. Note the low side reading with the blower on high speed (max air). Now disconnect the blower motor lead. Low side needs to be at 30 lbs and stay there (at sea level). get back to us with the results.

fonebone on Sat August 02, 2008 10:19 PM User is offline

You said you replaced the hot water valve--but did you check for vacuum at the hose that connects to it? When you select temps to "Cold", and slider to AC, vacuum through the dash control must pull a min of[if I recall] 11hgs to fully shut off water flow at the water valve. With the AC on, is the hose HOT coming out from the water valve? Try pinching off the hose coming out of the water valve to heater core and send back your results. I replaced mine with a cable operated positive shutoff valve in the heater hose. Never liked the OEM design--too many problems associated with it.

JJM on Sun August 03, 2008 2:59 PM User is offline

Your low side pressure is too high. Overcharge, condenser, fan clutch, or TXV.

Correct system charge is 3¾ lbs of R-12 - or 5 12 oz cans of R-12 if you were charging with cans. Oil capacity for the A6 is 10½ oz. Was the system flushed free of oil?

Is the condenser and fan clutch new? Not a good idea to ask me what I do... and folks DO NOT TRY THIS AT HOME. I learned this trick from some old-timers. Assuming the clutch is not locked or binding, in which case this test MUST not be performed, after shutting down a hot engine, with the engine off, I place my four fingers across one fan blade while an assistant starts the engine. By feeling the resistance of fan, you can tell if the clutch is slipping. Then I let go of the fan, and if it lazily spins up to speed this indicates it's shot. I know I'm nuts, but so too were the old-timers who taught me this. But again, DO NOT TRY THIS UNDER ANY CIRCUMSTANCES!

Is the expansion valve sensing bulb firmly attached to a clean surface on evaporator outlet pipe, and properly insulated?

Perform an expansion valve quick test: With the engine running at 2,000 RPM record your pressures. Now partially cover the condenser with a piece of cardboard to obtain a high side pressure of 350 to 375 PSI (be careful here). If the low side pressure rises, suspect the expansion valve.

If you want to be more detailed, carefully remove the sensing bulb from the evaporator outlet pipe, making sure not to kink or otherwise damage the capillary tube. Prepare a container of crushed ice and water (32ºF), and another container of hot water (approximately 125º or more). Start the engine and run it at 2,000 RPM, and record your pressures with the air-conditioning on. At this point your low side will be high. Immerse the sensing bulb in the container of ice water, and the low side pressure should drop. Immerse the sensing bulb in the container of hot water, and the low side pressure should rise. If the pressures do not change, or change very little, the expansion valve is defective and must be replaced.

POA appears fine, holding at 28 PSI with the blower off.

Where did you buy the compressor? Chain store remans are typically junk.


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72PMD on Sun August 03, 2008 9:58 PM User is offline

I checked it today under the same conditions(92 deg ambient, low humidity, 45-65mph 1800-2600 rpm). Clamping both heater hoses brought the pressures down to 36/260 from yesterdays 41/280. Unclamping the hoses raised the low side to around 40psi on a second drive. At idle, low side pressure is around 50-55. Disconnecting the blower brings the low side to around 28psi. After a hot soak (sitting for a few hours) I ran it up to 1600 rpm stationary, and got 53 low, about 300 high. Blocking the condensor brought the high side to 350, low went up to around 57.

The sensing bulb on the txv did respond to temp changes last year when I installed it, can try it again next weekend. Compressor is a remanufactured by Delco, bought from what I guess could be called an AC Delco jobber. Had the system flushed, vacuumed, & charged by a competant shop, who say it's working fine. It is, under the typical Bay Area weather conditions when they check it (70-75 degrees & fog till afternoon). New Hayden H/D clutch fan, which I can hear when it engages & factory 7 blade fan. A direct drive adaptor I tried didn't make any difference.

Basically, the only items that have never been replaced numerous times through the years & are original are the heater box and the muffler. The muffler is fender mounted between the compressor & condensor, and contains the high side Schrader valve assembly. The hot water valve was not used by the factory in 72 except on B bodies, I added one a few years ago to try to isolate any possible missing seals around the heater box/doors.

Thanks for the replies and ideas. Kevin

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