The thermostat on my Adler Barbour small vertical evaporator stopped doing
its job last week. It did not turn off the compressor when the box got cold
like it used to.

Today it magically started working again. Why did it do so?

It must have had something to do with the fact that I added some more R134a
refrigerant to the system. The only thing I can think of is that enough
refrigerant had slowly leaked out over a period of four years to the point
where there wasn't enough left in the system to cool the evaporator
sufficiently at the area where the sensor tube from the thermostat attaches
to the evaporator which is at the tail end of the system as seen from the
point where the copper tubing attaches to the evaporator.

Adding refrigerant apparently allowed the box to get sufficiently cold at
the sensor attachment point whereas it was not getting cold enough there
with insufficient refrigerant in the system. BTW, I added perhaps two
ounces. There is now 15 PSI on the suction side fitting. 13Volts, 4.9 amps.
(Solar panels operating at optimal on sunny day) Ambient temperature 70
degrees F.

So, the lesson is before you buy a new thermostat check your system pressure
to make sure there is optimal refrigerant in it.

Wilbur Hubbard