Lt the unit reach temp. When the unit cycles off, turn the thermostat all the way up until the unit cycles on again.

Find the defrost timer (hopefully its a standard mechanical style) and rotate it clockwise until it clicks in to defrost. It's a bit tricky, because the defrost cycle is only about 3-5 clicks on the timer. Rotate the timer slowly.

If the heater turns on, then the timer is faulty. If the heater does not, then the defrost limiter is bad. You already eliminated the heater because it has continuity.

The defrost limiter looks like a silver "button", typically with a blue back and has two wires coming out. It should cut out at around 50 degrees to prevent the freezer from turning in to an oven.

Let me know if you need any other help, or if you need parts, as I get them very cheap.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2