Pressures for 134a in our vw.

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JHZR2

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My wife had been complaining that now that it's getting warmer, her AC hasn't been working well. It's not hot out, but we get in the 70s, so a car parked in the sun can get warm inside.

Put the gauges on tonight with the engine somewhat warm and ambient of 72. Static check was fine, and we knew the car was cooling somewhat when on the highway, just not doing well around town, so there was no concern of a major loss of charge.

At first it was sitting with low at around 35, high at 140. My gut was that the high side was too low, so I added. It got cooler from the vents, and kept getting cooler until the pressure was too high for my liking. I stopped at the point where low was 37, high was 220psi at idle. Temperatures had kept decreasing, which was an important factor to me (fill the accumulator).

At 1500-2000, low side drops to 27, high side goes up to around 240.

To me, that's too high for this ambient condition, but wanted thoughts. The car's fans were running, but my supplemental fans may not have blown enough to help the condensor. And again, the temperature had kept dropping. Vent temps were around 40 when I stopped.

Thoughts?
 
Is the condensor free of debris and bugs? That can cause thermal issues like that.

A/C are coolest at speeds over 30mph

Just be careful when adding more foo foo juice it's easy to put in too much.

If the condensor is ok try evacuating and recharge of the system. May have a leak somewhere.

Jeff
 
Originally Posted By: Chris142
a good rule of thumb is 2.2x ambient temp. 154 on the high side should have gotten you very close. your overcharged or not getting enough air through the condenser


Even if I'm overcharged, the air kept getting colder.

Oddly enough, I knew my 98 Chevy was undercharged, because it had a leaky accumulator switch which I replaced. Put the gauges on it, and the low was over 40 while the high was around 110. I filled it such that the high was around 150 and left it. Low side never changed.
 
Nope. Certainly not excessively hot.

Forget the static number but it was correct indicate fully saturated vapor in the system from a static pressure versus temperature chart.

Apparently this car uses a Sandrn compressor that continuously runs it does not have a clutch. It uses a swashplate design so the compression ratio changes dynamically.

I think our issue maybe this:
http://volkswagenownersclub.com/vw/showt...7114#post497114
 
the static pressure number will only tell you that you have some refrigerant that is condensed to a liquid in the system. it does not tell you how much refrigerant is in the system, the only purpose of looking at the gauges before starting the engine is to know the compressor will kick on because there is a enough refrig. in the system to satisfy the low pressure switch. and if your below the pressure value for a given ambient temperature then you definitely know you are low on refrigerant... just not how much.

the 2.2 x ambient temp is a poor rule of thumb, and i'm not sure if it applies to a system with a thermostatic expansion valve, i think it applies more to a fixed orifice ac system. the temperature at the condensor will vary widely as the fans kick on and off so it is very normal for pressures to rise up to 250 psi on the high side with the car not moving, it's based on the heat transfer happening or not happening at the condensor. what ends up happening is you go by the 2.2 rule and are underfilled, the evaporator coil has all the refigerant boiled out and turned to gas before it gets half way through, the other half or more of the evaporator is gas which does not accept any heat and the vent temps are warm. the more correct way to do it when you're not adding refrig. by weight is to do like you did keep adding until you get your vent temps down around 50° F when it's less than 85° F outside this is telling you the evaporator is taking out as much heat as possible because it's nearly full of boiling refrigerant. and you stop adding refrig. when you see vent temps get warmer or see high and low pressures rise well outside normal range, remembering it's hard to know what normal range is because you don't exactly know refrigerant temperature in the sytem. if you are able to turn your ac fans on manually and put a large electric fan in front of the car, you'll observe your high side temps are much lower.
 
I really wouldn't concern your self so much with the high pressure.

Now I am assuming Dodge cared enough about having a pressure sensor in the A/C lines on my dad's Caravan to make it semi-accurate.

The Snap-On scan tool reads the high side pressure always hovering around 240psi in warm weather that I could tell at least.

I just think it is the nature of the beast old R12 systems operated at lower pressure due to a better working refrigerant. To get the same effect these days high pressure and tighter fin spacing are required.
 
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