Quote:
Originally Posted by jackhild59
I wasn't going to get all scientific on you, but since you brought up Gas Laws, understand that you are making my point. The Temperature Pressure chart says that at 85*, the pressure of R12 in a constant volume MUST be 91.7psi. This is BTW the saturated vapor pressure. We must presume there is enough liquid R12 in the system to give a saturated vapor pressure.
Practically this plays out as follows:If we add more R12, both the temperature and the pressure will rise. Then, if we let the system regain it's original 85* temp, it must settle back at exactly 91.7*. Take a little out, the temp and pressure drops. Let the temp regain 85*, you guessed it, back to 91.7. If the gas in the system is pure R12, there is only 2 ways the system can vary from the stated temp/pressure chart. One way is if there is so little in the system that there is no longer liquid available to maintain the saturated vapor pressure. In this case, the pressure will drop below the 87.5*. Or conversely, we can fill the system so completely that there is no room for vapor. The system is then filled with a non-compressible fluid. Then the pressure can exceed the 85.7*
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LOL, I'm a EE, and it's been almost 30 years since I had to put up with undergrad thermodynamics, I completely forgot about the saturated vapor pressure angle... If I understand correctly, 87.5 psi static pressure would be the knee in the curve for R12 at 85*F. My static pressure was 86 psi, so it's definitely reading below that threshold, but intuitively if all the R12 leaked out, wouldn't the static pressure read zero or pretty close to it? (equalized to atmoshperic)
Quote:
Originally Posted by jackhild59
Given the low static pressure and the very low lowside/highside pressures when running, your system is likely very close to empty. Look for leaks.
It could also be contaminated with another gas that would cause the pressure to vary from the chart. Some would cause the pressure to be lower, most would cause the pressure to be higher.
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Agree with hunting for leaks, but don't think contamination is too likely since the system was never opened up, at least since I've owned the car. I suppose if the receiver/dryer was failing due to age it could cause contamination; ditto for the compressor.
Quote:
Originally Posted by jackhild59
Ok, I oversimplified. The evaporator has a temperature control switch that keeps the coil from freezing. It switches the compressor off at about 33* and is located at a mid-point on the coil that keeps the coil from freezing up and also gives the best performance.
In a properly (or moderately low) charged system, the compressor switches off at about 18psi. and back on at about 35-40psi. If the switch does not switch the compressor off, either the switch is bad or there is not enough freon to cool the coil down to 33* to cause the compressor to switch off. I personally have never seen this occur. Usually low systems cause the compressor to rapidly cycle-on and off. If it is cycling on off in under say 30 seconds it is called short-cycling.
You are correct: The compressor should not be able to run constantly at 6psi low side.
Diagnostic of this switch is on page 44/45 on the S5 FS. S4 is identical.
I hope all this helps you solve the problem. Good Luck.
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Checked with the FSM, and the pressure switch is plumbed into the high side, located in the line between the condensor & receiver/dryer. It's wired in series with the thermal switch you mentioned. Both are normally closed switches, so when you switch the A/C on, the compressor will stay running (clutch engaged) as long as the pressure switch is seeing at least 2.3 kg/cm^2 (FSM #, converts to 32.7psi), AND the the temp of the evaporator thermal switch is above 38.3*F. This would explain why my compressor was running constantly - evaporator isn't getting cold enough, and the pressure on the high side was well above 32.7 psi