A/C diagnostic help

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So here I am again having A/C issues. Over the winter I installed a Classic Auto Air system using the Sanden compressor I had. After the install I took it to a shop where they pulled vacuum for 30 minutes. It held vacuum, they charged it and I had very cold A/C for about 3 weeks. I last tested it last weekend and it was blowing cold. This weekend I took the car for a ride and had only warm air. When I checked under the hood I could see the compressor clutch was activated, the pressure line out of the compressor was hot, the line out of the condenser was at about ambient temperature, and the expansion valve at about ambient as well. This was weird and made me think that the expansion valve was clogged or something. Then today when I tried to diagnose with the manifold dials, the compressor was not activating. Now the pressure switch was cutting the compressor off because the pressure was low. I jumped the switch for a couple seconds and the compressor clutch activated. I connected the manifold gauges and confirmed that the pressure was low. These are the dial readings with the compressor activated and not:

Compressor off: low side @ 20+ psi; hi side @ 25ish psi

Compressor on: low side @ -10 psi; hi side @ 50-60 psi

So it appears that I lost refrigerant pressure, which may indicate a leak. What it is strange, why was the compressor activating the day before but blowing warm air and then the next day I had a low pressure issue? Also, if I have a refrigerant leak, should I be able to see an oily area to indicate where the leak is? All looked clean along the A/C system.

Thank you.

 

Mike King

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Is the entire system new?

Were all the o rings new and lubed prior to assembly?

What refrigerant was used? Some hydrocarbon refrigerants leak through modern single barrier wall hose walls. R134a does not do this.

You will not always see an oily stain where there is a leak. did you a/c technician put UV dye in the system. For example a compressor front seal may only leak in one spot per revolution making leak detection very difficult.

Ask your local a/c technicians if they have nitrogen/hydrogen (N2H2) leak detection abilities. This is the best way to find a leak.

If the gas used is R134a the above pressures are very low. Static pressure (off) for R134a at 20C (68F) is about 70PSI and the high side would be approximately 150-175PSI at 1500RPM.

The negative low side pressure could also indicate a partial blockage in the system usually between the condenser and the evaporator (IE: Tx valve).

 
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Is the entire system new?

Were all the o rings new and lubed prior to assembly?

What refrigerant was used? Some hydrocarbon refrigerants leak through modern single barrier wall hose walls. R134a does not do this.

You will not always see an oily stain where there is a leak. did you a/c technician put UV dye in the system. For example a compressor front seal may only leak in one spot per revolution making leak detection very difficult.

Ask your local a/c technicians if they have nitrogen/hydrogen (N2H2) leak detection abilities. This is the best way to find a leak.

If the gas used is R134a the above pressures are very low. Static pressure (off) for R134a at 20C (68F) is about 70PSI and the high side would be approximately 150-175PSI at 1500RPM.

The negative low side pressure could also indicate a partial blockage in the system usually between the condenser and the evaporator (IE: Tx valve).
All was new, except the compressor, which was about a year old. I flushed the compressor with oil, empty it all out and the refilled following the instructions in Sanden's manual.  Every o-ring was new and lubed with Nylog. The refrigerant is R134. I don't know if the put UV dye, but I will assume not.

Do you know how good are those A/C leak detectors, like Elitech?

 
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In hindsight, I found an area that had what appeared like too much "dust" near where the refrigerant hoses connect to the evaporator box by the firewall. It didn't seem oily so I disregarded it, but maybe that's where my leak is.

 

Mike King

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I'm not familiar with that brand of leak detector but refrigerant leak detectors are pretty much on a par with each other. Definitely worth a try if you have enough pressure left.  Check all fitting connections for tightness etc. and also check the service valve cores for tightness. To check the compressor front seal turn the compressor center hub slowly by hand with the detector underneath. Gas is heavier than air. dust can collect on the suction hose (Large hose) due to it being cold and wet when the a/c is operating. you could also try pressurizing the system with nitrogen and using soapy bubbles (Big Blu) to find the leak.

A hydrogen leak detector(N2H2) is more precise if you can find someone with one. It will only detect hydrogen.

 
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I'm not familiar with that brand of leak detector but refrigerant leak detectors are pretty much on a par with each other. Definitely worth a try if you have enough pressure left.  Check all fitting connections for tightness etc. and also check the service valve cores for tightness. To check the compressor front seal turn the compressor center hub slowly by hand with the detector underneath. Gas is heavier than air. dust can collect on the suction hose (Large hose) due to it being cold and wet when the a/c is operating. you could also try pressurizing the system with nitrogen and using soapy bubbles (Big Blu) to find the leak.

A hydrogen leak detector(N2H2) is more precise if you can find someone with one. It will only detect hydrogen.
Mike. Thank you for your help. Since I have a vacuum on the low side with the compressor running, can this happen only when the expansion valve is clogged or can there be other circumstances like low refrigerant level? I am trying to figure out if this condition is a certain indication of one or several issues.

 

Mike King

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Your static (a/c off) pressure appears to be low which could make the low side run in a small vacuum when operating.

The people who commissioned the a/c should be able to leak test it for you. They can put some more gas in it, test run a/c to check operation, leak test the system, reclaim the refrigerant, repair the leak and/or fault and then recommission the system.

Blockages can occur in the condenser, receiver drier, tx valve and the evaporator coil causing the low side to run in a vacuum. Test running the a/c with the correct amount of refrigerant and gauges connected will confirm whether there is a blockage or a leak. 

 
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I ended up finding a leak on one of the high pressure hose fittings. I ended up purchasing an Elitech detector. I put a little refrigerant to increase the pressure to 50 psi and found the leaky hose. Then used soapy water and it was obvious by the bubbling. A quick test of the A/C at 50 psi and worked.

In hindsight (and I knew it, but!) I made a mistake. What happened is that I didn't find anyone in town that could crimp the hoses. I found a hydraulic shop that did it but their crimping is different than the A/C line crimping. For hydraulic hoses the crimping leaves axial markings compared to the circumferential indentations for A/C lines. I should have biten the bullet and purchased the Mastercool crimping tool from Amazon. Now I will have to remove the hose and recrimp it. I may have to do this for all of them. Oh well! First I will try reducing the leak with one of those bolt clamps. Maybe I get lucky until I can properly evacuate the system and redo all the hoses.

 

Mike King

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Glad you found the leak. It definitely would be worth while re-crimping all of the fittings. R134a consists of smaller molecules than R12 so it will leak easier. If you only repair one crimp the system will probably then start to leak at the next weakest crimp.

 
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