marstair A/C cutting out on LP

[mattym]

i am looking for help with a marstair A/C which has been cutting out on LP the system has pressure in the high side, the pressure on the low side rises and falls then cuts out on LP, the system has a internal expansion valve which has been changed twice any help would be much appreciated thanks

[Brian_UK]

...system has pressure in the high side...

Yes, but does it have enough?

[monkey spanners]

Could it have moisture in the system? Does it have a filter drier and sight glass? Also what Brian said

[mattym]

yea ther is plenty of refrigerant in the system as it was reclaimed to leave pressure in th system it has a filter drier and sight glass which has been changed when the system has been worked the only thing that has not been done is triple vac

[Brian_UK]

No evacuation ???

[mattym]

yea system was vacuumed to 1torr and works for so long then works itself again.

[W.Miller]

I was gonna say a blocked capilary but I thought I would leave it to the more advanced guys to answer

[monkey spanners]

What does the oil look like? Had an old hitachi or tosh unit (forget which) that the oil had gone brown and nasty, system would work ok then kind of pump down, was told it way due to the oil going lumpy in strainers (or similar but more technical sounding reason), once it had warmed up, normal service resumed for a while.
Chucked system in skip and put new one in, fixed!

[stufus]

Ok so the TEV has been changed twice !Why??
At what back pressure is it tripping the lp switch ,and at what pressure does the manufacturer say it should trip the lp switch,(is it adjustable) is there a preset lp delay on start up via control circuit or pcb?.
If the lp switch is bridged out how does it run ? or does it pumpdown ?
Is the bulb/phial mounted correctly and insulated correctly.
What are the operating conditions indoor / outdoor.
The more info you can give the better.
Cheers
Stu

[old gas bottle]

firstly is the a moisture indicator on the sightglass (yellow/green)this tells you if the system has moisture in,if it has not got one in fit one with a biger liquid line drier ,085 or larger,vac out and recharge with new refrigerant,the old may be contaminated.

[mattym]

will try that thanks for the help i will keep you informed and let you know how it goes

[monkey spanners]

Check the evap fan/air flow too, if it stops then the tev will shut down to maintain superheat, it could shut enough to trip the lp switch.