Old Sears Freezer runs constantly

Have an old Sears upright freezer, probably 30 or more years old, that the compressor runs continuously. Keeps cold OK. Just won't shut off. What is the most likely problem? Thermister (if it has one)? It is a defrost model also. Won't shut off even at the lowest setting just before the off position. The circuit diagram is half missing.
The sensor appears to feed into a white box underneath the unit. The troubleshooting instructions on the door mention a cold adjustment to adjust if it stays on all the time. I can't see any adjustment anywhere.

Thanks in advance.

Bob