I installed a new thermostat in my true cooler but it just runs all the time never shuts down?

the cooler should cycle several times daily. as set point is reached unit turns offuntil inside temp rises to set point an it comes on again
instruction manual covers that
https://www.manualslib.com/manual/1306105/Mastercool-Mcp44-C.html?page=5#manual

any time you omit the thermostat the heater wont work but the engine will operate at a cooler temp. I live in a hot climate year round and we southerners leave out the tstats on older jalopy.

If the new thermostat sensor is secure and where it belongs, it has the wrong charge in the system. That cooler takes a critical charge. The factory charge is in ounces if refrigerant. The system needs to be evacuated completely with a vacuum pump and recharged with an accurate scale that weighs in ounces. That will solve your problem. A refrigeration tech carries the necessary tools that an hvac tech doesn't. Hope this helps and good luck.

Are you referring to the compressor not shutting off down below or the evaporator fan inside the cooling area not shutting off? Evaporator fan always runs the compressor is controlled by the thermostat you could have a bad thermostat if the compressor never shuts off.

As a refrigerator, it should not go below about 34 degrees. If it is going down to 30, it may be freezing up on the coil. If you get a chance, do this: unplug the refrigerator and let it sit for 4 hours with the door open. If the coil is frozen it will defrost. You can feel the coils inside the box at the top back inside the unit. If you feel ice at any time it is an indication that it is getting too cold. Once the inside coils freeze over, the air can't flow through the coils and the unit will get warmer and warmer. When it is defrosted, set the thermostat just above the off setting. Let it run until the compressor shuts off. See what the temperature reads on a thermometer inside the cooler. If it is not cool enough move the thermostat one number colder and let it run an hour. Take your time, let it equalize before you change the thermostat. Also if the cooler is empty it is hard to set the thermostat because when you open the door all the cold air escapes and the temp will change rapidly. If it is full of cold food, opening the door won't change the temperature very quickly. Also check the radiator coils at the bottom of the cooler behind the grill. If it is clogged with dust or grease, the compressor may overheat and shut off prematurely.

you might have a leak going on clean the coils and check the fan.

If you have a blocked capillary tube that had nothing to do with a power outage. You could get this if the condenser got EXTREMELY dirty. The cap tube runs up through the wall this is true but I have run external line sets up the back wall to bypass this sort of problem. You have to know where to drill so as not to hit the old line set or the electrical wiring which is also in the wall.

It sounds like the thermostat is "stuck closed". Try unplugging the cooler, MAKE SURE ALL POWER HAS BEEN DISCONECTED. Remove thermostat cover with thermostat on it. Carefully pull one of the wires spade connectors off the thermostat. Insulate the end of the wire you disconected with electrical tape. Now plug the cooler back in. If the compressor stays off the thermostat is the problem. Let me know if that helps. I know of a common replacement cold controll for that unit:)

These units are bad about that. you have to mount the cap sensor in the small tube that runs thourgh the evaporator for proper operation. laying on top of the coil does not let the thermostat sense properly.

If your cooler is constantly running too cold the thermostat is probably bad. I can hook yu up with a thermostat. ;) www.northeastcooling.com