Interesting problem. I’ve never seen that model with a electronic controller. Are you sure that it isn’t just a temp gauge? All the ones I have seen used a conventional timer, defrost thermostat system. Does your temp control say Dixel on it?Â
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Now if you have a digital control, The coil sensor is not working to tell it to stop heating. It then goes to a max defrost time.before returning to cooling mode. There will be indicators for both the fan and defrost. Fan should not be on during defrost. You can not turn off the defrost. It will ice up and possibly do damage to the compressor. The cycle can probable be changed, but I’m sure it was designed for the box. Much better to test the thermocouple to verify that is the problem. Compressor not starting or working in over load can also cause a similar problem. Check your condenser cleanliness.
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Now if you find you do have a mechanical control, There will be a timer with 4 terminals. That will have a fixed cycle and a defrost termination thermostat that will shut the heaters off when the coil gets warm enough to complete defrost. It also locks out the evaporator fan till the coil is again freezing. To prevent box warming during defrost.