FredR

As I remember the LP switch triggers at 2.2 barg so you are very close to that point if my number is correct. The compressor has a fixed compression ratio so if the system settle out pressure is low it will have a low suction pressure and a low discharge pressure whe it achieves that ratio and that will control how much duty you can get which will not be much if any at all and what you do get will also be controlled by ambient conditions.

Sounds as though you have lost refridgerant- time to start looking for signs of oil leakage I rather suspect. Your suction pressure is 14.7 psia and discharge is 64.7 psia so the compression ratio is just over 4:1 which is the maximum you can achieve with a single stage compressor and avoid excessive heat generation- a sure sign your gas has gone AWOL as those numbers are way too low but the compressor is Ok.

Rgds

Fred

StratfordShark

Thanks Fred - reassuring that compressor ok anyway.

I will start looking into leak testing while there is still enough pressure for compressor to run!

The suction side of compressor is pulling pressure down to almost nothing when it runs, so I'm still puzzled why the LP switch isn't cutting the compressor power, and then allowing the low pressure to rise again till it cuts in again. Maybe I'm misunderstanding how the LP switch works?

dr bob

The "low pressure" safety switch is in the high-pressure plumbing. So the critical pressure you are watching is high-side pressure. You are right at the edge of the switch holding and allowing the compressor to even start.

StratfordShark

Of course! Thanks Dr Bob, I completely forgot it is the high side service port at receiver-dryer, though in mitigation the gauge set I picked up had the high and low coupler colours the wrong way round (lost in translation in China I think).

So today I shot some R134a into the system via the low side port, and the A/C was blowing ice cubes in the cabin again. I must have a leak somewhere, but relieved all major components are essentially fine, and that the leak is not catostrophic as with hole in the condenser or something. Can see flow through the sight port on the dryer, with occasional bubble.

Will track down the leak, so will replace the receiver-dryer if/when I have to open things up to fix the leak.

FredR

Now that you know you have a fugitive emission the first thing to remember is the size of the leak is not too important compared to the location of the leak.

The usual suspects in likely [?] order of occurrence /are:
1. The compressor dynamic seal and/or body joints [look for signs of compressor oil]
2. Expansion valve[s] [do you have a rear a/c?].
3. The various joints with older type seals in them.
4. Hoses - are they original or modified for R134 service?
5. Pinhole leak in the condenser or [heaven forbid] the evaporator.

Best way of detection I have used is with dye in the compressor oil that shows up in UV light. Sniffers seem to be a bit hit and miss but that might have more to do with those trying to operate them. I think they tend to go if you break wind!

Good luck with the search.

StratfordShark

Thanks Fred that's a very helpful list of likely failure points - anywhere but evaporator please!

I do have used sniffer on way - just sounded technically elegant compared with the UV method, but I'll try that if sniffer fails to identify the problem.

No rear A/C so at least only single expansion valve to check. They are the original hoses - the conversion was made with new service valves and complete evac by a real 928 A/C guru - has bee running fine since for several years, but I'm aware R12 hoses are less of a barrier for R134a.

Thanks again