The compressor is switched on and off by the amplifier based on the temperature of the evaporator. If you leave the compressor on full time the evaporator will freeze up and the pressure in the system exceeds the max pressure of 15 bar. Normaly there are 3 switches in the system that need to have their vallues within range, upper and lower pressure and evap temp.
I don't agree with what you've said in the little bit I've highlighted in orange Michael.
If the compressor is allowed to keep running when the evaporator has already accumulated a lot of frost, the pressures (both
high side and
low side) would actually keep falling. (Neither of them would rise.)
But this fall in evaporator temperature/pressure would create it's own problems:
- The refrigerant flow would drop to the point where oil (that normally circulates with the refrigerant) would get trapped in the evaporator (because it would get so viscous/cold there and the flow rate need to carry it back out would be too low). This would starve the compressor of lubrication, and
- The frost layer would increases to the point where it would block the gaps between the evaporator fins, thus decreasing the flow of cold air into the cabin (ie. stopping the evaporator from working) and making the evaporator temperature, refrigerant flow rate and refrigerant pressures further plummet.
So that thermostat (that has its sensor attached to the cold evaporator fins/tubes) wouldn't actually cause high refrigerant pressures if it were to fail in the
closed-contacts position but it would indeed still cause problems (as outlined above).
Wow, thanks guys. This may be way over my head. I see the switch that is on the refrigerant line but I'll have to look to see whats connected to the evaporator. I wouldn't be surprised if something is wrong with the switch coming off the fluid line since the copper line has been sitting there for 3 or 4 years disconnected from everything. I may have to get an A/C specialist in on this one. But, I will trace the wiring to these switches. I can say that the wiring going to the switch on the copper line has no power at all. So, I would guess that if that switch was the problem, I'd at least have power going to it, but not out of it. No?
Post a pic of the switch that's disconnected (and on one of your refrigerant lines) Scott so hopefully we can then determine if it is a
high pressure cut-out or a
low-pressure cut-out.
(Note: My wiring diagram doesn't show a high-pressure cut-out switch but all large refrigeration/AC systems have them and many small ones do as well. So you never know.... Perhaps you have one too).
If that's you're low pressure cut-out switch that's disconnected then I certainly wouldn't recommend running it like that..
The
evaporator-temperature-sensing thermostat (aka
low temperature cut-out switch) is normally adjustable and is normally what you actually twiddle with to select colder or warmer settings. So if your system has one of these, just check behind it to see if you can spot a length of capillary-tubing coming out the back. If you can, follow it with your eyes and you should find the far end of it embedded somewhere in your evaporator fins where it should be sensing your evaporator temperature.
Failure of this thermostat is not uncommon. And since it is operates by means of a pressurised fluid that controls a bellows that operates a set of electrical contacts .... any leak in the sensor capillary line means that the thermostat will think the evaporator is completely frosted over (and so it will INSIST the compressor isn't allowed to start up). ..... because low temperature equates to low pressure when your dealing with saturated vapours .... which is what you have, or SHOULD have I should say, inside that thermostat capillary tubing that entends in to touch your evaporator fins/tubes.... (What I'm saying here in this dribble...is that these thermostats normally fail in the "open-contacts position" and that such a failure could well explain why your magnetic clutch won't engage without your extra live-wire feed.)
