A/C System pressures
 

72 C20 with factory air that was converted to R134A by some shop in Ar. that says on sticker under hood IAW some standard and lists new oil type as well. Done before I got the truck. Not cooling well so hooked up a set of gauges but this being non standard setup for R134A not sure what pressures should be. Looked through the past posts but most just explain how their conversion was done. Looking to find out what kind of readings I should see to determine if I need to adjust the charge. At about 85 deg ambient and probably 65% humidity it is reading 35/310. This would normally indicate a restriction on the low side and an over charge but not sure with the current setup. There are bubbles in the sight glass but from what research I have done it is not an accurate indication of low charge. I don't want to drive the high side any higher. Normal readings on an R134A system should be 45/250 at this ambient temp. but with factory evaporator which is smaller I am not sure. Any recommendations based on what you guys are seeing?

Re: A/C System pressures
 

If you have a sight glass, I suspect that the receiver/dryer may not have been changed out for one compatible with R-134. Can you post a picture of it? Also, the AC depends to some extent on ram air flow. When I work on AC systems and the pressures go up, I put a box fan in front of the radiator to increase air flow. This will drive the high side pressure lower. In a pickup, the evaporator being small isn't necessarily a bad thing, since the cab is so small, so the efficiency issue is reduced.

Re: A/C System pressures
 

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Re: A/C System pressures
 

Low side if the poa was adjusted for 134a should be about 27.5 if it wasn’t adjusted then it should be about 30(r12 setting), at 2000rpm with a big fan blowing into the Condensor.
High side will run optimally at 2.5 to 3.5 times ambient temperature.
Your not getting enough cooling because the low side pressure is too high, poa valve is not working right or adjusted wrong.
Check that your water valve is closing off the heater core properly too.
Check your vent temps with a thermometer.

Bubbles in the sight glass just means the receiver dryer is not full of liquid refrigerant. It doesn’t mean air is in the system. Your high pressure shows the pump is working and it’s somewhat charged. If it was really low on refrigerant you wouldn’t have high pressure.

You could add a little 134@, 2-4 ounces at a time, use a postal scale to weigh the can as your adding to the low side, run it a while and check pressures and vent temps, stop adding if you see better vent temps. If the vent temps don’t get better then it’s probably overcharged.

Re: A/C System pressures
 

I get 38-42 degrees when the truck is moving. The POA was adjusted for R34, the compressor, filter dryer, hoses and compressor are new. The system was flushed and new o-rings installed..

Re: A/C System pressures
 

Following as I have pretty much the same readings on my truck. All new hoses, POA adjusted for 27.5 new dryer (that is labeled R134 and has a sight glass). It does seem to be cooling fairly well especially at hwy speeds. In my research it does seem that the values listed for R134 don't seem to match up with R12 system conversions.

Re: A/C System pressures
 

38 vent temps are not bad.

Here is a testing guide for pressures.
http://thefirstgensite.com/library/6...7S1BP91_98.pdf
It’s for a Pontiac, the only real difference is high side pressure can vary by model and year due to condensor placement and airflow.
Vent temps vary by model due to duct efficiency.

Re: A/C System pressures
 

A little further info after doing more research and checking out some of ya'll's suggestions. Just ran it again with a fan blowing on the condensing coil. Low side 45, high side 425 and climbing. Probably is over charged. Looked up the spec SAE 1661 listed as the standard the conversion was done to according to the under hood sticker and it states for 80 and up vehicles so it was not probably the right way to convert the ac system as later models operated somewhat differently(no POA, etc.) The dryer looks to be the original(still has the old bubble sight) and the POA has no evidence of being cut apart to be modified for R134A so I guess I will just order a new dryer and send the POA in to be rebuilt and recalibrated and go from there. May be a good time to replace the O-rings and front A/C compressor seal as well to ensure the correct material to be compatible with R134A. Any other suggestions are welcome. Thanks

Re: A/C System pressures
 

The overpressure valve at the back of the compressor opens around 400psi I think...
The poa valve is adjustable without cutting it apart, small nut inside the outlet, pressurize the evaporator at the line where the expansion valve connects, hook up the low pressure gauge to the low pressure port on the poa valve, adjust pressure to 27.5 or so for 134a using the nut, it’s an odd size.

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May just go ahead and replace the condenser with a parallel flow while I am at it. Haven't found one for this truck yet though. May have to make one a different application work. Good to know I can adjust the POA. Wonder if can I get a rebuild kit for it as well?

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Parallel flow condenser available from Old Air and probably other vendors.
Got mine from Old Air.
RP

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Sorry for the long post but have been doing lots of research the past few days. Talked to Classic Air and Old Air yesterday and they claim to only offer universal fit parallel flow condensers. I have been unable to find a receiver/dryer that says it is compatible with R134A(the stock one isn't). Most people have just used the stock one anyway either through lack of knowledge or lack of a resource. I doubt if the TXV orfice size is correct for R2134A but I don't think it ever gets addressed. Maybe it is not significant. All the auto supply stores want to sell you PAG 46 or 100. The Harrison A6 should have PAG 150. There have been many failures attributed to not using the correct oil. I am going to dump some of the charge today and see if that helps. Hopefully with the truck off I can dump charge slowly and not loose too much oil in the process. Pressures are 110/425 with a fan. A problem I created myself when adding freon without gauges to try and clear the sight glass. I may have a restriction in the system somewhere as well. Any advice on where to look? I am thinking the TXV orfice or the inlet screen on the compressor. Once i have rounded up all the parts ie: new dryer, hoses, o-rings and maybe a front double lip seal for the compressor I will take the system apart if dumping the charge doesn't help. Surprisingly the system doesn't appear to have any leaks. The only real reason to take everything apart is to check the POA setting as I doubt it was changed 19 yrs ago when the original conversion from R12 to R134A was done. I think they flushed the system, changed the oil and recharged with R134A. Any additional thoughts from ya'll's experience? PS: As I stated I talked to Old Air yesterday and they said they didn't have the condenser and dryer for R134A for this truck but just went to their catalog and found the kit. Maybe not a parallel flow condenser so they didn't recommend the super cool kit.

Re: A/C System pressures
 

>> I think they flushed the system, changed the oil and recharged with R134A. <<

As you said, you really don't know what they did.
You use the AC flush for the hoses, condenser and evaporator. You don't use it for the compressor. Mineral oil used with R12 is not compatible with PAG used with R134.
A conversion using the old compressor should be using Ester Oil. The Ester oil is compatible with mineral oil and works with the r134. You can't get ALL the old Mineral oil out of the compressor without rebuilding it. On the conversions I have done, I poured sacrificial Ester oil into the compressor, cycled it by hand then poured it out while continuing to cycle. Then add the correct amount of ester and close it up. This still doesn't guarantee removing all the mineral oil, so no pag.


The pictures from Old Air do not show a Parallel flow condenser.
Vintage Air appears to have Parallel flow.

A Parallel flow condenser has large tanks on both ends with flat tubes running across through the fins.

I have only done a few AC installations for myself and friends, but the accumulators and dryers I have seen in recent years have ALL been labeled as having desiccant compatible for R12 & R134 . Any new dryer shouldn't be a concern.

Re: A/C System pressures
 

Flush the system again. Get new hoses and mineral oil. Vacuum down system and recharge with r12. Enjoy ice cold air and never worry about making adjustments on this or tweaking that or swinging a dead chicken over your head...

I’ve played the r134 conversion game on these trucks and learned it’s just best to restore it as original. You’ll chase your tail and waste more time and money to save a few bucks on r134 when in reality r12 isn’t much more $$$ anymore (saw some at a swap meet this weekend for $10 a can)

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You can test your condenser capacity by spraying water on it while running, 1500-2000 rpm, pressure will drop significantly if inadequate condenser, like from 425 to 275 or so. Do that and see if the low side comes down to 30 or so with it. I doubt it will, the poa valve is probably bad which is causing the high low side pressure which in turn doesn’t provide enough cooling.
Post a picture of the dryer and expansion valve, wonder if there both actually original.

Be careful with the double lip seal, it seals forward of the original seal and the shaft could be rusty in that area which will prevent sealing, I just tried one of those seals and it wouldn’t seal up. I put the original ceramic in and it was fine.

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Picked up a superkool lit from Old Air today as well as new hoses, TXV valve with R134A in the bulb. Only thing from the old system will be the compressor(works well) and the evaporator which I will flush along with the existing hard lines. I will calibrate the POA for 134A when I get it torn apart. I have an electrical gremlin to chase as well. I replaced the fan relay behind the glove box and the control switch and now have 2 fan speeds. I think there is another one under the hood somewhere. Thanks for all the input. It may be a few days until I get around to making the repairs but I will post an update.

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Agree 100%..........