70 GMC, 134 retrofit problem

[Likeg]

Long story. but here we go. AC system on my 70 GMC has not been used in 10yrs (estimated). System was closed during this time, but no charge. Replaced A6 with sanden compressor, new hoses for 134, new parallel flow condensor (25x14), new expansion valve, new receiver dryer, and POA eliminator kit. Bought the POA kit long before I read about re-calibrating the old POA. Flushed EVAP, and blew out with compressed air, before installing all new components. Pulled a vacuum on system for 5-6 hours at 29 and system held vacuum for 24 hours. Put in 3 cans 134 ( no oil additives) and pressures were low. Came back next day to add more 134 and system was empty. Obtained a leak tester, shot a little bit more in the system and found the leaks (o rings) and repaired problem. Pulled vacuum again for 1.5 hours and waited another hour to to see if it would hold. It did so I added about 45 oz or 2.8 lbs 134. Original system called for 3.2lbs of r12, but I dont know what the modified system capacity is now. Pressures at idle were 30 L and 270H. Ambient temp when work was done was about 85 F. According to the charts I have found it should be about 40-45 L and around 275 H. Problem is that the lowest I can get out of the vent is 60 F. I drove it to get air over the condensor and still no lower than 60f. EVAP outlet was cool to the touch and sweating. I did not add oil to the system, compressor came with oil in it and directions stating not to add oil. I need to get the vent temp down around 40-45, because once it hits 105 here this summer I won't be happy. Any suggestions will be appreciated.

[Cusser]

Pressures at idle were 30 L and 270H. Ambient temp when work was done was about 85 F. ...Any suggestions will be appreciated.

Wait for a pro to respond, because I'm NOT one, and while I work with both R-12 and R134a, I've never done a conversion.

That said: pressures are typically measured at 1500 to 200 rpm, not at idle; and your high side pressure at idle is quite high. How good is your fan clutch, and are all the radiator shrouds present?

[Likeg]

The fan shroud is new as well as the clutch, I will check the gauge readings again at 1500-2000 rpm. If both the low and the high increase I will assume the system is overcharged. If something different such as high side is elevated and low side too low, I will assume I have another problem. Only thing I haven't done is put the tacky tape over the sensor for the expansion valve. Wanted to make sure it was working correctly before I did that. I will post the gauge readings at 2000 rpm.

Thanks

[Likeg]

Update:
80 deg F ambient temp
Hooked gauges up, increased RPM to 2000.
clutch off = 150 H and 42 L (highest readings)
Clutch on = 225 H L 36 highest reading, then low drops to 20 within about 15 seconds and high rises to 275.
Once it hits 20 on the low side the clutch kicks back on and the cycle repeats.


Tested again at idle (550 RPM)
Clutch on 225 Steady H, Low raises to 40 steadily drops down to 20 and then clutch cycles off.
Clutch off: 130 H , Low rises to 40 and clutch kicks on
and we rinse and repeat again.

Lowest vent temp = 58.

Trying to provide as much info as I can,

Thanks

[bohica2xo]

Make sure the expansion valve sensor bulb is making good contact with the suction line at the evaporator - and cover it with something. Presstite tape is required for final install, but you can wrap a shop rag around it several times & secure it in place for testing.

An exposed sensing bulb will make testing difficult.

When testing, make sure the doors are open, and the cabin fan is on the highest speed. You want full heat load on the evaporator.

You may need to increase the charge level. Do the 2,000 rpm teat again with the bulb covered & the evaporator at max load. Post the pressures & ambient temperature.

[Likeg]

Finally got time to get back to this project
80 deg ambient temp.
Sensor bulb has been wrapped.

Results at 2000 RPM
clutch off: high side drops to 120, low side steadily rises to 42 and them clutch kicks on.
clutch on: high side 225 max, low side steadily drops to 20 and then clutch kicks off.

vent temp lowest was 54 deg.
when clutch kicks off vent temp steadily rises 10 degrees, drops back down when clutch is on.
It seems to be short cycling the clutch at high RPM. clutch stays on for about every 30 seconds, then cycles off. The clutch doesn't cycle as often at low RPM.
According to the 134 chart the high should be around 175 to 210 at 80 deg,
The POA replacement cycles the clutch on around 20 and off around 42, so the low side never gets to high.
Just cant figure out if the high side elevated pressure are due to overcharging or undercharging or the POA replacement is working correctly or not.
Does anybody have any experience working with the POA replacement?
Anyboby got any ideas?

[Dougflas]

Refrigeration side of system seems ok. You may have what we call a "reheat " problem where the heater core is affecting the vent temps. try clamping or by-passing the heater core. Also, make sure your TXV bulb is mounted on top of the evap tube; definitely not the bottom. You do not want the bulb to sense the oil which is flowing in the bottom of the tubing.

[Likeg]

Tested system one
More time. Lowest vent temp 50-52 at 2000 rpm. Pressures not bad high 255, low fluctuates between 20 and 42, thus kicking compressor and off. 85 deg ambient temp. I really am beginning to believe that this POA replacement is the POS many people have claimed that it is. I dug out the original POA (thank god I am a pack rat) and ran the POA test. Adjusted it to 26.5 and it holds pressure there. I am most likely going to put the POA back in and run the sanden compressor constant just like the old A6 would have. I don't think the poa replacement allows the compressor to run at long enough intervals to get down in the 40's vent temps. Couple of questions: when I take the poa replacement out, there will be no switch at all to cycle off the compressor if the pressure get to high for some reason. Do I need to install a pressure switch on high side? Original system did not have this. Is it safe to flush poa with acetone to remove any debri in it? I don't want to screw up this working valve. Per forum advice I have not removed Schrader valves in original POA, especially oil line side. Hate to tear this down again, but 50 deg vent temps ain't gonna cut it when it gets 110 here, I also claimed heater hoses as suggested earlier, no difference

[Dougflas]

You should have a high pressure switch on a conversion. The POA kept the evap from freezing. The A6 did not cycle. I would not use anything other than a flush solvent in a POA and not a powered flush. You have a 50 year old part so be careful.

[bohica2xo]

The down side to the cheap "bang-bang" control. Instead of a constant evaporator pressure of 26 psi, you get the average of the 20 to 42 swing. Sort of.

Yes, you should install an HPCO. Hopefully your POA is ok. Make sure the oil bleed line is clear & the low pressure check for the oil is installed.

I really wish they would quit calling that eliminator an "improvement".

It was 90f here at midnight last night. Your GMC OEM system was made to deal with that.