thermistor in dometic rerigerators

[nickcrowhurst]

Our Dometic RM2611 refrigerator recently froze everything hard. This happens when the machine gooes into "limp mode", and runs continuously when a fault is detected in either the thermistor or the associated circuitry. (The thermistor is the sensing device clipped to the fins inside the top of the refrigerator, and is used instead of a thermostat. Thermistors are semiconductor devices whose electrical resistance varies with temperature). I needed to isolate the problem to either the thermistor or one of the circuit boards. To test the thermistor, gain access to the circuit board at the rear of the refrigerator (all power off, 12v and 120v, and touch your fingers on the metal body to earth static before touching the circuit board!!), and remove the clip connecting the two wires from the thermistor to the circuit board (left edge of board, second plug down). The two probes from a digital multimeter in the resistance mode can be inserted in the ends of the connector. The thermistor is tested by unclipping the sensor and placing it in a glass of cold water and melting ice, which, after a few minutes, should be at about 32 degrees Fahrenheit. The resistance should be between 7k and 10k Ohms. On mine it was 8k, and when my wife held the thermistor sensor end between her fingers, this reduced to 2.7k Ohms. I deduced that my thermistor was good, and I replaced the circuit board with one from Dinosaur. This cured the problem. I post this as I could find no reference to this with a search, and it may help others faced with isolating a similar fault. Don't touch this job if you're not conversant with electrical safety procedures. Nick