Abstract: Electronic control apparatus is disclosed for automatically controlling the operation of refrigeration units on truck-drawn trailers which includes an undercool comparator (240) connected to a resistor (239) in a voltage divider including a temperature selector (13) and having its output in parallel with an overcool comparator (44), either of which will actuate a malfunction indicator (15) in the event of undercooling or overheating conditions during a chilled food setting for the temperature selector (13). An overcool comparator (91) that is turned on when the selector (13) is moved to a frozen food mode operates a control element in the form of a transistor (99) that serves to disable the output of overcool comparator (240) and prevent the actuation of the heat load device (23) by effecting grounding connections in the circuit.

Abstract: Electronic control apparatus is disclosed for automatically controlling the operation of refrigeration units on truck-drawn trailers and providing a signal to the truck driver in the event there is a malfunction in the refrigeration unit that could result in damage to the cargo. An electronic automatic temperature control circuit includes solid state comparators which are responsive to a temperature sensor and to a temperature selector. The comparators provide output signals that selectively actuate solid state control elements which in turn control the actuation of the various load devices in the refrigeration unit to maintain the temperature in the refrigerated chamber within a selected temperature range. When the temperature in the refrigerated chamber is above a selected temperature setting to the extent that damage to the cargo might be caused, a solid state malfunction circuit controls the actuation of a solid state relay and in turn a malfunction indicator.