Old analogue thermostat not cycling

[paulfedex]

I have lived in my 1986 XL full time for three years now. I have three cruisair units that have always cooled effectively with no problem. Even now they are cooling effectively but this year the thermostats do not seem to cycle the compressors on and off as they used to. One notch on the knob is off and a millimeter further is on, but on all day and into the evening. So, I am reluctant to leave them on and unattended. What could the problem be? Two out of three units are acting in this manner. The thermostat knob amounts to no more than an on/off switch. I cannot find these for sale anywhere and not really sure if the switch itself is the problem.

Paul