A/C repair & restoration options

[jackhild59]

Originally Posted by Pete_89T2

You can estimate % of charge remaining from a static pressure test, based on the ideal gas law, but it's a rough estimate. Quick ideal gas law reference:

http://en.wikipedia.org/wiki/Ideal_gas_law

The gist of it is the A/C is a sealed system, so volume is a constant, and the mass/# of gas molecules is supposed to be a constant, assuming no leaks. That leaves pressure & temp as your only variables, and these two are directly proportional. Since I was reading a high static pressure, it means I still have the majority of R12 in my system. Would need to consult with a temp vs pressure characteristic curve for R12 and know the exact mass of the original R12 charge to estimate the percentage remaining though.

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*

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.

Originally Posted by Pete_89T2

This is interesting, and I probably should take a second look at the electicals... Didn't think I had an issue there since the clutch was constantly engaged while the A/C was on, turning the compressor. Does the pressure switch cut out the compressor ABOVE 18psi or does it cut out below that threshold? If it's the latter, at 6psi, my compressor shouldn't be running at all.

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.