I don't have any shop manuals which outline diagnosis of a POA type A/C system, and my friend who is an HVAC guy by trade is on vacation, so rather than wait 2 weeks for him to get back, I'll ask here.

The Vintage Air system in my Chevelle works great, but the A/C in my beater 1973 Ranchero isn't working. It's a POA system, R-12. It has a brand new compressor and expansion valve. The system will not blow cold air.

I evacuated and recharged the system tonight and took some readings and made some observations. When running, the receiver/dryer ices up, as does the POA valve, but the outlet of the evaporator (where the capillary bulb from the expansion valve clamps to) stays at ambient temp. The expansion valve itself ices up.

When running at 1500rpm, ambient temp of 75F, system pressures are:
Hi side - 75psi
Low side - 5 in/Hg vacuum
POA pressure - 10psi

With the system running at the same conditions, but with the capillary bulb from the expansion valve immersed in icewater, the system pressures are:
Hi side - 70psi
Lo side - 0psi
POA pressre - 10psi

To the best of my knowledge, the hi side seems awfully low (s/b around 150 at an ambient temp of 75F), and the POA valve pressures are way off-- they should be around 30psi (I'm at 20 feet above sea level).

The car has the original receiver/dryer, but the system has never been open to the atmosphere.

Or should I get the problematic POA valve out of the system and convert to a CCOT system?

Thanks for the help.

Sorry, I'm confused, if the system has never been open how did you change the comp. and txv? The drier should always be changed, every compressors warranty requires it. I think you have a blockage in the drier because it is icing up and the suction pressure is so low. Did you pull a deep vacuum on the system before you charged it back up?

Jimmy H.
whatsupdoc

I should have elaborated... by the system not being opened, I meant that nothing in the system was disconnected and left exposed to the atmosphere for any length of time-- it was always buttoned up until I changed the compressor and expansion valve.

When I got the car, there was very little freon in the system, so I recharged it. It didn't work, and the hi-side pressure was low, so rather than do more investigative work I thought that the low high-side pressure was a bad compressor, so I changed it... then when that didn't work, I changed the expansion valve. I'm not usually a parts-changer type of person, but for some reason I didn't spend any time trying to diagnose the real problem.

I probably should have changed the receiver/dryer, but this is my beater, so I was trying to do things as cheaply as possible and decided to leave the original in there.

As far as the vaccum goes-- yes. I service the system using a computerized evacuation/vacuum/leak check/charging station, I don't use the parts-store cans (and NEVER will). The system does hold a vacuum, and there is no trace of any leaks (I used a flourescent tracer dye).

I'll purchase a new receiver/dryer today and give that a try. Unfortunately, this darn Fo$# car has both a receiver/dryer at the evaporator and an additional large accumulator between the POA valve and the suction side of the compressor, so there is the possibility of two blockages.

Should I change/convert the POA at the same time I replace the receiver/dryer, or should I test the system with my current POA once I change the receiver/dryer?

[This message has been edited by Gokou (edited 07-29-2002).]

Well, I just changed my POA to an "update" valve and I'm less than satisfied with it's operation. It uses a clutch cycling switch with about a 15 PSI differential between cut-in and cut-out pressures making the vent temps run from 40 degrees to over 50 degrees. I think that 50 degrees is unacceptable from a system that would hit 38 degrees on a 100 degree day with a POA valve. So much for updating http://www.chevelles.com/forum/rolleyes.gif.

Jimmy H
whatsupdoc

Try this site, there are some very knowledgeable auto A/C mechanics there http://www.aircondition.com

[This message has been edited by doc j (edited 07-29-2002).]

[This message has been edited by doc j (edited 07-29-2002).]

[This message has been edited by doc j (edited 07-29-2002).]

[This message has been edited by doc j (edited 07-29-2002).]

Whew, that was a wascly wabbit!

I updated this year also with the new POA valve that cycles. Im getting the same readings betwwen cycles and its frustrating because my other chevelle with the stock POA would freeze me out.

Another update... I posted over at aircondition.com and it was suggested to me that the receiver/dryer probably lost it's dessicant, which clogged the system.

However, I just flushed the entire system. No blockages anywhere. Even the old receiver/drier wasn't blocked up.

Does this mean my POA is shot? Or is there a chance my brand new expansion valve is defective?

More than likely your POA is shot/clogged. Just order a new updated version or possibly try Ebay. I saw some on ebay a few weeks back that were the original type for like $150. Just take your new expansion valve off and make sure its clean still. It should be fine but my ac friend did say he didnt trust the ones discount auto or autozone sells.

Gokou. The only description in my manual is http://www.si.hhs.nl/~rob/images/other/POA-exp-test-72.jpg

The cycling switch on the update valve should be adjustable. There is a little screw in between the prongs.

The POA valve is used on many cars. Can't you find one on the salvage yard?

Rob

[This message has been edited by rusty66 (edited 08-01-2002).]

I'll try some scrapyard POA's first. I've already spent about $225 on the A/C system, which isn't bad, but I only paid $1000 for the vehicle, so I'm reluctant to shell out money for a new POA or a CCOT conversion.

I'm banking on the POA being bad, because now that I have established the system is free of blockages, the POA is the only original component left which could be causing the problem.

Guess I'll hit the boneyard tomorrow morning and take a couple POA's off the cleanest cars out there.

GOKOU how much R-12 did you put back into the system? Those old systems took a BUNCH of freon. I had a 73 LTD wagon a long time ago with poor cooling and I charged the system up with what I thought was a proper amount of freon and was not satisfied. As I recall it eventually took around 60 ounces to finally make it work properly and then it really made cold air.

The readings you have make me think that the system is a long way from full. Especially the lo high side reading. The compressor can't develop proper discharge pressure with an inadequate system fill.

Having said that the system I was working on used the Tecumseh style 2 cylinder compressor. If yours is the 6 cylinder Fridgidaire type then the R-12 requirements may be different.




[This message has been edited by charbilly2001 (edited 08-01-2002).]

My Ranchero takes 4.5lbs of R-12. That's a LOT of CFC's! The ford system has a rather large receiver/drier AND a large accumulator. It did have the full 4.5lb charge when I took the readings.

[This message has been edited by Gokou (edited 08-01-2002).]