I've used a Johnson Controls thermostat on my fridge for about 2 years now, well the fridge died last week so i bought a chest freezer this time around. I've noticed if i set my controller to say 15 degrees C it will actually chill it down to 12 degrees C, I have the differential adjustment set to 1 degree C and always had it set that way, and i doubled checked and its still set to 1.
Is it just because its a freezer? i can't be the only person using this controller on a freezer, a temp swing of 3 degrees C is around 5 degrees F...seems like a bit much.
Check the location of the prob. The distance from the evaporator can make all the difference in the world. It's a bit hard to mount a probe on a frost free freezer. Spray some water on the freezer walls and you will see the frost line from the coil. This is where you need to be and you can also see where you can put a small screw for the mounting bracket.