Msf-49nm will not stay below freezing!!

The compressor runs during defrost to supply hot gas to the inside coil

There are different models made for holding different products. Yours has a 1/3 hp compressor for above 20 degs. It may have 0 on the thermostat and may well pull down that far, but -20 would be out of it's reach. Where as it's sister model is made to freeze to below zero has a 3/4 hp compressor. Larger compressor for the lower temps.

Change the temp control,check to see if it is iced up on the evaporator

This is normal. All commercfial freezers must defrost 3 to 5 times a day, or they will freeze over at the coil and then stop freezing. A timer in your freezer turns off the compressor and turns a heating element on for 10 to 15 minutes. During that time the coil heats and melts frost from the coil. Usually within 30 minutes the temperature will briefly rise to 20 t0 25 degrees F and then quickly drop down to 0 degrees F. The food rarely defrosts in this short period of time..

http://www.turboairinc.com/TB-Service/Service%20Manual%20%20-%20April%202008/MSF-23NM.MSF-49NM.MSR-23NM.MSR-49NM.MSR-23G-1.MSR-49G-2.pdf
this web address should take you to the manual for your unit. check the settings on the controller first. If that does not resolve, let me know.

get a sheppards hook a parts store can help you tell them your prob

here you go: http://www.turboairinc.com/TB-Service/Service%20Manual%20%20-%20April%202008/MSF-23NM.MSF-49NM.MSR-23NM.MSR-49NM.MSR-23G-1.MSR-49G-2.pdf
takes alillte to load so be patient
hope this was helpful

I deal with MasterBilt which is a private lable made by TurboAir.
I have several units but only one does this icing up thing. Look for a bad seal on the door gasket. These really ice up if that does not seal almost perfectly. Look for premature termination of the defrost cycle. Check that the heater is pulling amps. The wiring diagram at "turboairinc.com" is very poor but it shows 2 heaters for the refrigerator. They list a diagram for the freezer but it's not there. I know for sure that there is a heater for the evap but don't know about the drain. I've seen some have it and some don't. there tech support line has much to be desired but it's worth a shot. Also, the bottom hinge on the door fails fairly regularly. This allows the door to drop just enought to disengage the switch at the top of the door and stop the evap fan from running, thus causing a freeze up.
Good Luck . You got your hands full with this one.

It sounds to me very likely that your thermostat isn't functioning properly. change the settings and wait to see if it responds. If the thermostat is working it should shut down the compressor when you increase (rise) the temperature and aversly start it when you drop the temperature. Always let the unit sit for 3 minutes for the pressure inside to equalize before restarting. Good Luck

ice cream is always the last to freeze and a temp of minus -10 degrees is usually required to get it to get hard. if you put the ice cream in the door it will be especially tough to freeze. a good thermometor is a good investment as you can observe the temps. usually ten degrees or 20 degrees is what they run. yes they do cycle on and off as long as they hold temps thats good. the outside of the freezer gets warm because the hot compressed refrigerant is spread through piping near its surface giving up its heat of compression . setting the stat to max will freeze the ice cream eventually but utility bill suffers . good luck