Test thermostat

[fixbear]

with a DVOM. Measure the sensor ohms first. Not knowing all the information on your cooler, I don’t know what else you need. The controller compressor relay is small and often under designed. Larger capacity coolers like 47’s & 49’sshould have a auxiliary relay. The controllers are very compact, and it’s difficult for the manufacturers to cram in anything larger than a 10 amp relay to control the condenser, evaporator fan, lighting and defroster. Remember, that start current on a motor is 17 times full run current. The other thing I’ve noticed with Master-Built is they are a Chinese built machine that likes Secor (old Danfoss) compressors. They are built opposite of all the other compressors, as the pump is on top and not in oil. They seem to take longer to get to RPM and eat controllers faster than Copelands or Tecumchas. Especially low temp units.

Your cooler should have a wiring diagram attached to it. Also the controller has it’s own. Check the sensor to match the ohm value of it’s temp. At the controller! Look for any cuts and abrasion’s to the thermistor wire. They often get pinched in the evaporator housing.

Also check the location of the sensor. There are 3 different design ideas that manufacturer’s use. First is simple air sensing. Location is usually air intake area of the evaporator but not touching the coil or housing. This design is the quickest reacting to conditions in the box. Next is coil bonding. This design is to monitor coil ice conditions and lengthen out cycles for efficiency and provide cycle ice harvest. The third design is a bit more complicated, and only used in precise control units. Requires multiple sensors. Not something we see commonly. Your Master-Built will likely be air sensing. If someone has had the evaporator down, they may have mounted the sensor wrong. This will make it think that the box is not cold enough as the clamp or wall will delay the transfer of heat.

Emmerson (Dixal) has the market on controllers. You can find generic data on the controller on their site. But the sub letters tell who they built it for and to what spec. Things like cycle width.