A/C repair & restoration options

[Pete_89T2]

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*

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)

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.

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.

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.

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

[jackhild59]

Originally Posted by Pete_89T2

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)

Glad I could help, Pete. I carried my EE major girlfriend through thermo as a freshman. Except I wasn't an Engineering student-she nearly quit school in frustration that I 'got it' and she didn't. She quit me instead, lucky break!


If that temp switch is gimp, you could have attained vacuum on the low side, but not that likely.

I would advise to charge a couple of oz of liquid refrigerant into the system then run the car until everything gets good and hot. Go back to the spray bottle and look for your leak.

Pro's use nitrogen and pressure the system up to 200psi to look for leaks.

Originally Posted by Pete_89T2

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.



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

I recommend you replace the receiver/drier. The descant-bead retaining bag is soluble in synthetic oils. You do not want those bead spread throughout your system. Any modern (post 1996) manufactured desiccant is designed for R134a and synthetic oils.

Also,be aware that there are two different systems on the FC. One is installed in Japan and the other was installed at the port of entry. Type is not related to Sanden/Nippondenso system. If you lookup the part number at nearly ANY parts store, they always get the wrong one. Take your old part in with you to save the trip. An easy way to distinguish is the hold down bracket for the drier. If your bracket is welded on, get a replacement that is welded on. If it is clamp on, get the drier that is clamp on.

Whether you recharge w/R12 or Freeze12, use Ester oil for lube. Add a couple of oz of oil to make up for losses around the compressor seal. DO NOT use PAG in any system that has pretty much ever had mineral oil or R12 in it. Bad reaction. I don't know if you can easily even find mineral oil any more.

[Pete_89T2]

Originally Posted by jackhild59

If that temp switch is gimp, you could have attained vacuum on the low side, but not that likely.

I would advise to charge a couple of oz of liquid refrigerant into the system then run the car until everything gets good and hot. Go back to the spray bottle and look for your leak.

Pro's use nitrogen and pressure the system up to 200psi to look for leaks.

Looks like pulling the temp switch out for testing will be a major PITA, since it's attached to the evap core. I suspect it's fine, given how it's wired in the electrical circuit and the symptoms - I have minimal cooling which means the core is well above the freezing point; compressor runs so I know the switch is normally closed as it should be. What I won't know w/o testing is if the switch will open when temps drop below freezing. For now I guess I'll take that risk, evacuate the system and see if it holds a deep vacuum. If the vacuum holds, any leak that exists is of the slow/dynamic variety (i.e., happens only when system is running, normal operating shock/vibe/temp environments). If the vacuum doesn't hold, then I'll need to find & fix the leak before doing anything else.

Originally Posted by jackhild59

I recommend you replace the receiver/drier. The descant-bead retaining bag is soluble in synthetic oils. You do not want those bead spread throughout your system. Any modern (post 1996) manufactured desiccant is designed for R134a and synthetic oils.

Agreed, this is inexpensive insurance. I guess the only question is are these post-'96 receiver/dryers compatible with R12 and will any residual trace amounts of mineral oil that may be left behind in the system after evacuating cause problems assuming I switch to an Ester oil lube on recharge? Unless I crack open lines & remove the compressor to physically drain it, there will be some mineral oil left behind. I was hoping that replacing the receiver/dryer with a new one and the evacuation procedure would suck out enough of the old mineral oil so that a physical tear down & chemical flush of the rest of the system's components would not be necessary.

Originally Posted by jackhild59

Also,be aware that there are two different systems on the FC. One is installed in Japan and the other was installed at the port of entry. Type is not related to Sanden/Nippondenso system. If you lookup the part number at nearly ANY parts store, they always get the wrong one. Take your old part in with you to save the trip. An easy way to distinguish is the hold down bracket for the drier. If your bracket is welded on, get a replacement that is welded on. If it is clamp on, get the drier that is clamp on.

Whether you recharge w/R12 or Freeze12, use Ester oil for lube. Add a couple of oz of oil to make up for losses around the compressor seal. DO NOT use PAG in any system that has pretty much ever had mineral oil or R12 in it. Bad reaction. I don't know if you can easily even find mineral oil any more.

Interesting - I was aware that some FC A/C systems were port installed (US) and some were factory (Japan), but I thought all the ND systems were factory installed, and all the Sanden systems were of the port/dealer installed variety. Reason being is I believe all the USDM turbo models shipped with factory A/C, and I've never seen a Sanden system installed in a turbo. I have seen Sanden systems in the base model RX-7's, which were likely port/dealer installed options. I didn't know Mazda mixed & matched those pedigree variations. Good tip on the bracket - no doubt will save me some grief when ordering parts!

Since I don't have a vacuum pump to evacuate the system, I'll have to farm part of this job out. Fortunately I found a local shop who will work with R12 and seems competent and charges pretty reasonable rates. Game plan is to bring it to him to evacuate the system & recycle whatever R12 I have left. If the evacuation shows I have no leaks (holds deep vacuum for a long time), I will take the car home & install a new receiver/dryer. Then bring it back, have the system evacuated again and charged to spec with R12, Ester oil lube and some leak detect dye. If it doesn't hold vacuum on the first evacuation, I'll have him test for leaks with the pressurized nitrogen, and depending on what he finds, I'll either have him fix it there or fix it myself and bring it back for evac/charging as previously described.

Thanks for the help!

[jackhild59]

Yes the new drier will be compatible with mineral oil.

Buy your drier local so you can verify before you pay your money.