I've inherited my parents' bar fridge that they inherited from the previous occupants of their house. It's old - an Ariston - and the ID plate on the back is in Italian, so it's travelled a long way to Australia and I'm reluctant to "put it down". They told me it has a tendency to ice up fairly quickly, so I've fitted a new gasket - the most obvious deficiency (the old one was seriously leaky). This is another $50 reason for not getting rid if it and trying to fix it (the problem is coming...)

I also noticed that the tube running from the expansion valve through the back of the fridge into the evaporator actually runs up between the back of the fridge panel and the heat exchanger, and at one point was actually pressing against the heat exchanger. So I've put a block of polystyrene between them to at least stop them from touching.

THE PROBLEM:
I've had it running for a couple of days now and it's certainly cold but already it's icing up all along the aforementioned tube and on the evaporator plate as well. The compressor runs continuously, even when I turn the "temp" knob right down to "stop". When I switch the power off and on, there's a lag of a few minutes before the compressor kicks back in.

Inside the fridge there's a long wire about the thickness of a coathanger sheathed in a clear plastic sleeve that's coming from the plastic "control box" up the top. This was floating around loose so I've fixed it to the back inside face of the fridge. I assume this to be a temperature probe, but I thought if it was part of the thermostat there would be a "reservoir" at the end of it? Should this wire be closer to the evaporator/freezer box, which is suspended from the inside roof of the fridge? ie would having this in the wrong spot be making the fridge "think" that it wasn't cold enough?

I've had a look at a few websites and some engineering textbooks and I understand how the cycle works but not how it's controlled. Is the thermostat a "hydraulic relay" where the "sensing fluid" acts to open and close a valve in the cooling line? or does it connect to an electrical relay that switches the compressor on and off? or does the compressor have a pressure sensor on the expansion valve side that detects when the working fluid is moving/stopped? what does turning the control knob on the fridge actually do? is it changing the threshold/switching temp of the thermostat? if so, then why doesn't the compressor cut out when I turn it down? Is it OK to insulate the tube running from expansion valve to evaporator to improve efficiency? can I buy some sort of tape to do this?

I'd appreciate any enlightenment from readers, and as a last resort I'll pay someone to come and look at it, but I'd really prefer to try and sort it out myself, and hopefully learn something in the process.

Thanks in advance,
Cheers, Ben
Univ. of Queensland, St Lucia, Australia