Air Conditioning

mercury
villager

#1

2 mos. ago I had a.c. repaired - replaced condenser, compressor, drier, exp. valve, unit flushed. Last week while I was on the road the unit began to squeak, no cold air, and unit would not shut off. A local Mechanic replaced the fuse, compressor, condenser, clutch, drier, 0-tube, exp. valve. They could not find the reason for the high pressure reading from the condenser. At this point they are stumped - the unit will not hold more than 2 lbs. of coolant although it should accept 3 lbs. They have given up - any ideas?


#2

Without knowing what the pressures are it would be impossible to make much of a guess.
Personally, I never go by the amount allegedly required by a system. The gauge pressures are the best indicator of what’s going on inside.

There a number of reasons why a unit may hold less than shown on the tag. Capacity of the condenser and drier, whether the system has excess refrigerant oil in it, and things of that nature.

Do you by chance know what this high pressure reading was and is it known for a fact that the radiator cooling fans are operating with the A/C on?
I’m assuming here the fans are working but just wanted to ask anyway.

The part about going in 2 months later and replacing things like the compressor and condenser again sounds very odd to me.


#3

Sorry I don’t know what the high pressure reading was. I was just told that it was very high. That is why they continued to look for a cause after replacing the clutch adn then the condenser.

I appreciate your trying to help. Thanks


#4

A squeak can often point to belt slippage due to elevated pressures or a faulty belt or belt tensioner.

The high side pressure will vary based on a number of factors; state of charge, fans operative, ambient temperature and humidity, whether the vehicle is in the shade or sunlight, etc.

Inside a shop on a 90 degree day I think one could reasonably expect 225-240 PSI on the high side at elevated RPMs. (Should be checked at about 1500 RPM or so)

If you have the pressures checked again you might post them back here for discussion. Also verify what the static pressure is. The static pressure is the system pressure that exists throughout the system when the engine and A/C is off. At rest, everything equalizes and both high and low side should be the same. Normally this is about 115-120 PSI on both sides. If you were shown a static pressure of 140 PSI or something like that this would mean a system overcharge.


#5

The amount of refrigerant in the system will not affect the static pressure, only temperature will change the pressure.

The vapor pressure of the liquid in the static system is independent of the amount of the liquid.


#6

You’re incorrect. The amount of refrigerant in the system has everything to do with the static pressure. Statick pressure is the system pressure at rest.

If you don’t believe me then connect a set of gauges to a non-running vehicle. Note the static pressure. Unhook the gauges and depress one of the Schrader valves for a few minutes. (destroying the Earth according to some)
Connect the gauges back up and compare the pressure now to what it was. It WILL be lower; ergo, static pressure is lower.

If the static pressure in a tire is 35 PSI and you bleed some air pressure off the static pressure will be lower. Same thing.


#7

“The amount of refrigerant in the system will not affect the static pressure, only temperature will change the pressure.
The vapor pressure of the liquid in the static system is independent of the amount of the liquid.”

WRONG!


#8

ok4450, first off I highly respect your opinions.

I think I made an incorrect assumption and we know about assumptions. Is there liquid refrigerant present in a static A/C system? If no, then you are correct.


#9

Tardis,

If there IS liquid in the system when it is static and not running, then explain why I am wrong. The same amount of pressure will keep 1 ounce or 1 gallon liquid at a given temperature.

The A/C site below backs up what I am saying.

http://www.aircondition.com/tech/questions/82/


#10

Because 1 gallon of liquid will occupy more space (and therefore further compress the gas and increase its pressure) than 1 ounce will.
This is typical of the debates between theorists and engineers. The theorists are convinced something works one way because of the theories that they have read about it. The engineers and tried and done it, so they know how it really works. Go do some experiments for yourself.


#11

Tardis,

We are talking about vapor pressure. This is the pressure where evaporation from the liquid equals the condensation rate. Look it up on Wikipedia. That’s why propane tank levels can’t be measured by pressure, because the pressure stays constant for any given temperature until all the liquid evaporates. Do you disagree with this?


#12

You are talking about vapor pressure. The rest of us are talking about the static pressure in the AC system. I have not seen one example where the static pressure in the system did not vary with the volume of refrigerant in it. I’ll bet you’ve never even worked on an AC system.


#13

If there is liquid 134a in the system then the static pressure will equal the vapor pressure. See the site I mentioned above. If the charge is too low, then there will be no liquid in the system and the static pressure will drop when you let some gas out.

Yes, I have worked on A/C systems…several in fact from assembly to end. Anyway, let’s just agree to disagree


#14

"let’s just agree to disagree"
That works for me.


#15

Orangevega, do you have any actual A/C training and/or repair experience? I gather you do not.
There is no sense in feeding the OP some incorrect information because at this point they’ve got a serious problem that needs to be ironed out; and a problem that may involve a questionable repair. Replacing the condenser and compressor twice in a few months sounds odd to put it mildly.

Hopefully, the OP will respond back with some pressures and maybe we can determine what’s going on with that car.


#16

ok4450, I do have actual repair experience working with a professional on car A/C systems from beginning to end. Nothing near your or Tardis’ experience I’m sure.

So, agreed let’s move on and hope we get more information.


#17

I’m not sure there’s enough refrigerant in a (not grossly overcharged) car A/C system for it to exist in liquid state with the system off.


#18

Why wouldn’t an A/C system have both liquid and gas in it? A 12 ounce can of refrigerant on the shelf or a 30 pound canister has both. Run either one down to the last ounce or pound and both liquid and gas still exist.