What's a good R134a pressure at rest?

jjdrewski

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#1
Hi all: I,ve had a shop do a lot of chassis and A/C work on our 95 Mercury Mystique 2.5 l. During the recent 90 plus degree weather we've had, the A/C barely got down to 55 degrees at the dash outlets. Suction pressure was 50 plus psi running. It even cycled the compressor and high speed fan by means of the high pressure safety switch(new, also hoses, dryer, reduction valve new) After the hot spell passed, I got 83 psi at 73 degrees f ambient.before starting the system. Does it seem to y'all that there is too much R134a in the system? Book calls for 26 oz. After running the system on the road., it produces 37 degrees at the dash outlets, day temp in the mid 70s.. Do I need bnother trip to the shop, or is this OK? Thanks.
 
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#2
Correct low side pressures are measured at 1,500-2,000 engine rpm, AC on, doors open, blower at maximum speed. 50 psi with a CCOT system is about right above 90*F ambients. If you had an old POA system, would see a low side of 28 psi for some really great cooling, but they started making AC systems cheaper than better. Low side pressures would even be greater at hot ambients in high humidity conditions. Contrary to popular believe, high humidity air is less dense and has a much lower cooling capacity. Here the culprit is the condenser, has to be clean with maximum air flow, lower high side pressures translate to lower low side pressures for better cooling.

If the compressor ain't spinning, static pressure, doesn't tell you a thing about charge, can be either just 2 ounces or 50 ounces in the system, pressure would read the same. Strictly a function of relative ambient temperature, the higher the temperature, the higher the pressure. Find a P-T chart. If zero, that is when you have major problems with a PAG system, system has to be flushed, new accumulator, new PAG oil, draw a deep vacuum, then recharge to specifications. After the leak has been found and repaired.
 

jjdrewski

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#3
Thanks Nick. Its that old 28 psi that I was comparing to. We did have high humidity as well as 90s temperature, so what I saw fits with what you said. I'll let it go for now as it is cooling well with oat in the mid to hi 70s. John Drewski
 

Gus

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#4
The old rule of thumb was, with ambient air coming out of the registers, with the blend door set at full cold, and A/C off, that temp reading should drop at least 30F when the A/C is commanded on......i.e., if you have 85F air coming in, it should drop to at least 55 or less with the A/C on......to get even more cooling, put it in MAX mode......that way, you are not introducing outside warm air, and you are also reducing humidity in the car.......
 
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#5
Another old rule of thumb would be that system pressure "at rest" should be equal to ambient temperature. Just remember that "ambient" temp rises if the engine is hot or the sun is beating down on a black car sitting on hot pavement.