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1972 Chevrolet Truck - R12 System Low Side Soaring

BruthaMan on Sun July 01, 2012 12:04 AM User is offline

Year: 1972
Make: Chevrolet
Model: C10
Engine Size: 350
Refrigerant Type: R12
Ambient Temp: 95+
Pressure Low: 60+
Pressure High: 160
Country of Origin: United States

Have a 1972 Chevrolet truck with R12 system. I've been reading the forum here for several months while *attempting* to get my trucks AC system working.

The problem: When I add r12, with engine at 1500-2000 RPM, pressures initially look good while adding. Low side will stay right at 30 psi. High side will start to work it's way up. 140, 180, 210...as I add r12. The second I let off idle, low side soars up to 60-70 psi (high side remains relatively unchanged), and the compressor starts trying to lockup (belt squeal).

There's a lot more to this, tough. I've been reading for several months on the forum in preparation for this. I have good vacuum pump, cheapo gauges (I'll be straight up, they are the HF specials).

First, let me run down the new parts: New replacement condensor. New AC/Delco drier. All new hoses, including aluminum hoses from drier/condensor. New AC/Delco compressor (P/N - 6598423) (replaces 1131100 1131016 1131017 1131053 1131101 1131108 5910720 and 5910495). The evaporator core is original, as far as I know. The POA valve is original. Originally used a new Murray expansion valve, but after first attempt (with same problem), I picked up a refurbished original frigidaire expansion valve.

We originally took every part out of the truck that was AC related. Even the evaporator (man, the debris from 40 years was amazing!). Thoroughly flushed the evaporator, the POA and other parts. Many parts were new, flushed those as well, just to be safe. Flushed with AC Pro, used air compressor hose with rubber tip and 60 PSI on compressor to gently air flush the ac flush out.

Compressor was new, NOS AC/Delco. Drained the oil (even though it was mineral) just to be safe. Refilled with 10 oz of ac mineral oil from Napa.

I built a POA Valve Tester to test the POA valve. It was right at 30 PSI. I adjusted it down to 24 PSI (just to confirm it adjusted), then back to 29 PSI. Checked, double checked it. Was really surprised at how simple this part was. One heck of a small socket!

With the POA valve adjusted, we started assembling. All new o'rings, lubed the o'rings. Pulled vacuum on the system for 2+ hours. Let it sit overnight, no movement. Was getting excited. This has been a very long process and I've taken more time than I probably should have, to make sure everything was done properly.

With vacuum pulled and holding at 29, we hooked up the first can to the yellow hose on the manifold gauge set. Blue to low side (on the POA valve), red to the high side on the muffler of the hoses. Both valves OFF at the knob, opened on the valve. Let the first can start. Depressed the shrader valve twice on the yellow hose at the top of the manifold gauge, to let any air trapped in the hose out. Opened the blue valve and the first can goes through with no problem.

Start the truck up, begin idling at 1500-2000 RPM. Continue to add using same process as above. Using 12oz cans. Real R12, no substitute. Should take about 3.2 lbs of R12. Have five cans, knowing I'll only need a small portion of the fifth can.

Things go good, until some time during adding the third can of r12. If idle is let off, the low side goes from 30 up to over 60 and the compressor starts locking up. This has just baffled me, because virtually everything in the system is new. On the first attempt, was using a new Murray expansion valve. I picked up a refurb frigidaire expansion valve for the second attempt this weekend. Replaced the expansion valve, pulled vacuum once, let it sit overnight, then pulled another vacuum for an hour while doing some other work.

Sight glass on the AC/Delco drier is excellent. Can see perfectly. It's definitely flowing, very fast. I see a "white" constant flow. If the compressor is disengaged, you can see some bubbles start to appear, but when it's engaged, it's flowing like a river.

I should also note, that before the low side pressure starts going haywire, the POA valve and the tube that curves around, do get cold. Not frozen, but definitely cold and sweating. Same thing from the tube coming from the bottom of the evaporator and up to the expansion valve. Inside the cab, though, it's a little cooler, but not much. Air flow is about 80. Outside temp both time has been ten degrees cooler than hell...about 95-100 degrees. I do have a heater core shut off valve that is working properly, so the heater core does not compete with the evaporator.

This last time, I went ahead and added the fourth can. I hooked up AC ground and positive to the battery, so I could control the compressor engaging. The compressor wanted no part of that fourth can being added. I added it, and would manually touch the positive to the battery to "bump" the compressor, but it would start "sqwaking" after a few second. The high side did start to climb up to about 210 (which was encouraging), but the low side was just too unstable.

There was a point where we got idle back up, and the after bumping the compressor on/off, the pressure on the low side came back down top 30 psi. But if we let off the idle, it would jump right back up again and the compressor would start sqwaking again.

This has been an ongoing process over the past few months. I took my time, read many posts here, try to get a plan in place, mostly everything on is new, but I'm stumped. Any idea what would cause the low side pressure to go so high, and make the compressor start trying to lock up? Is there anything I'm missing?

Dougflas on Sun July 01, 2012 1:10 AM User is offline

Try dropping the idle rpm to 1200, disconnect the blower motor. Low side should be 30psi. If not, POA is not regulating. Is the correct schrader in the oil bleed line port? This is a special schrader. Also, try disconnecting your gauges. HF gauges are not the best in the world. Possibly there is leakage between the high side to low side forcing high side into the suction. Do not let A6 clutches cycle by applying and removing 12 volts to the clutch.

I noticed in yur charging procedure that you did not purge the yellow hose when introducing the first can of R12. THat would allow air into the system. I did notice you bled the other cans correctly. Did you just forget to tell us about the first can?

I remember some of these systems at idle have 40psi on the low side...perfectly normal. are your belts tensioned properly?

BruthaMan on Sun July 01, 2012 5:12 PM User is offline

Hi Doug,

Thank you for offering feedback. I unaware of how to verify the correct schrader valve in the bleed line port. I do know what you are referring to. I saw no abnormalities when hooking it up to the POA valve. It's attached to the evaporator which is one of the few remaining unchanged parts.

I did forget to mention the first can, in terms of purging the line. The first can was purged, including subsequent cans. I really had not considered the POA valve might not be regulating, because I had adjusted it per instructions for another post here and built a tool to hook up to it, running 60lbs through it, then opening up the attached ball valve to let air through. If I opened it too quick, it sounded like (my best imagination here) a awkward moose call. The POA and evaporator are the only original parts to this setup.

Have also read that 40 psi is acceptable. For whatever reason, when letting off idle, the pressure soars up on the low side causing the compressor to sqwak. Advice taken on the cycling compressor, I'll refrain from that next go around. I may go ahead and purchase a NOS POA valve. I will check for the suggested shrader valve as well.

I do appreciate your advice. If you can think of anything else to consider, please don't hesitate.

Dougflas on Sun July 01, 2012 7:08 PM User is offline

After verifying POA regulates at 30psi with blower motor disabled, next thing I'd do is look at the txv. remove the sensing tube from the evap line and place it in ice water. This should shut the txv off, pressures should drop because you are stopping R12 from entering evap. Try this at idle and see if belt squeals. Then place sensing bulb in your hand. You are looking for the tXV to react. Since you made up hoses, by any chance is the drier reversed?

You can use the feel method of touching the lines. Hot from output of compressor, cooler at output of condenser, warm at input of TXV and cool at its output before it enters the evap.


When bench testing the POA, increasing the supply air from 60 to 75 should not change the 30psi reading. If it changes, the POA is mushy and defective.

I would think you can only have a bad POA, TXV or compressor. and yes, new parts can be bad. Does you "new " compressor say, Made in China?

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