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big drop in low side pressure

sdr on Sun August 16, 2009 10:02 PM User is offline

Year: 1985
Make: Mazda
Model: RX7
Engine Size: 13B
Refrigerant Type: R-12
Ambient Temp: 85
Pressure Low: 40
Pressure High: 180

The system has worked fine for years. A small leak in the system became a larger leak over last winter. An electronic leak detector identified the evaporator as the source of the leak. But the detector didn't detect the compressor leak even though there was oil on the compressor and hood. To be sure I pulled the evaporator and there wasn't a drop of oil on it. The evaporator was reinstalled. To find the leak, the system was pressurized with nitrogen and soapy water was used to look for leaks. The only leak was the compressor shaft seal. A shaft seal, compressor o-ring kit and receiver was ordered (from ackits.com). I installed the shaft seal and o-ring for the front but not the complete o-ring kit. The compressor with 4 oz of oil was reinstalled. The system was evacuated and pressurized with nitrogen to be sure there were no leaks. After two days the pressure was still the same. Next the new receiver was installed with 3 oz of oil. The system was evacuated and charged with R-12.

The pressure at idle was 40/180. Increasing the rpm decreases the low side pressure to below 20 and the low pressure switch shuts off the compressor. The high side goes to 250.

The system has a thermostatic expansion valve. Could I have damaged that when removing/reinstalling the evaporator?

mk378 on Mon August 17, 2009 8:55 AM User is offline

TXV systems typically don't have a pressure switch on the low side. It is normal for the low side to drop to 20 or below especially with the evaporator fan on low. Consider other reasons the compressor may be turning off. Is the air from the vents cold? It could be cycling on evaporator temperature like it should.

sdr on Wed August 19, 2009 9:39 AM User is offline

As you say I do not see a switch in the low pressure side. When I reinstalled the compressor I wanted to check the clutch before adding R12. It wouldn't engage and I assumed there must be a low pressure switch. Nitrogen was added and then the clutch engaged.

The only A/C info in the factory service manual was in the electrical schematic. I was told that the A/C was installed at the port of entry and not at the factory. So the manual has no mechanical info on the A/C.

I did more testing and these are the results:

Ambient Temp: 82-85 F
Evaporator fan on high, no recirc

Engine RPM LP HP Duct Temp Compressor cycling
-------------- --- ---- ------------- -------------------------
600 48 200 72 no cycling
2000 32 255 65 no cycling
2500 25 260 63 12 secs on, 2 secs off
3000 22 260 63 12-14 secs on, 4 secs off

When the compressor cycled the low side pressure would initially be lower than the steady state values shown. Again as you noted, the compressor must not be turning off due to the low pressure. Do the pressures and temperatures seem normal?

Edited: Wed August 19, 2009 at 9:45 AM by sdr

mk378 on Wed August 19, 2009 10:21 AM User is offline

Controls in these old import cars are typically real simple, though sometimes a computer is involved for no particular reason. The switch on the high side responds to both high and low pressures. The low pressure function really only protects in case of total loss of the refrigerant. Once the compressor is able to start, the high side pressure will rise and it will stay on even if badly undercharged.

Pressures look good. If the compressor is cycling it should be doing it on evaporator temperature. Is the suction line getting cold? Consider it might be a reheating issue, the refrigeration may be working fine.

knightgang on Wed August 19, 2009 10:28 AM User is offline

How much r12 did you recharge back into the system. It calls for 29oz, but with the compressor cycling as frequently as it is, it sounds like your system might be low on charge. I could be wrong, verify the amount you recharged.

sdr on Sun August 30, 2009 7:51 AM User is offline

The electrical schematic shows a frost warning temperature switch. It has a capillary tube going into the evaporator. Jumpering the switch eliminates the compressor cycling. I must have changed or damaged this switch when inspecting the evaporator. What is the downside to leaving this switch jumpered?

The temperature at the exit of the evaporator is in the low 50s F with the ambient in the low 80s.

Edited: Sun August 30, 2009 at 8:12 AM by sdr

mk378 on Sun August 30, 2009 1:27 PM User is offline

The switch keeps the evaporator from getting too cold and frosting up. Should that happen, cooling will stop because there is no airflow through the frozen evaporator.

sdr on Mon August 31, 2009 7:01 AM User is offline

Is it more probable that the evaporator will freeze in July or January (defrost mode)? Or is it equally probable or unknowable? The evaporator exit tube temperature is in the low 50s, and the inlet pressure is in the low 20's. On average the evaporator temp should be above freezing. But I don't know how the average temperature and the evaporator freezing are related, or if it is possible to know. Obviously, the switch is in there for a reason. I'm trying to determine if replacing it is worth the pain. That's assuming a new switch is available. If so, it means opening up the system which isn't my first choice.

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