[jeff5may]

Ok so it has a mechanical defrost timer. When you initiate defrost, a few things happen. The compressor and circulation fan should stop. The defrost heater should energize. With that rig, the evaporator has a defrost termination thermostat wired in series with the heating element(s). Bimetallic klixon switch like in a clothes dryer. Make / break window between 40 and 80 degF.

The defrost box is super quiet like an old school electric alarm clock. The only time you will hear it is when manually advancing it or when the contacts open and close. If it has stripped gears or a tired motor, it will not change modes like it should. So if you advanced it to defrost, it may not ever go back to cooling mode. Then again, it may defrost once and get stuck again in cooling mode. Defrost mode usually lasts half an hour, cooling mode a day or two. Most are wired into the box thermostat, so the timer only runs when the thermostat is calling for cooling.

What I do with these to check the defrost timer is easy. Once you find the "secret" knob, put a timing mark on it. If it makes a few revolutions in a week, call it good. Same thing with the ice maker motor. Just because it moves a little bit in a couple hours doesn't mean it is working completely.

Checking the heater can be done with the kill a watt meter. Advance timer to defrost mode and watch the watts. If there is frost, the element will run for more than a minute or three as frost melts and finds the drain. Once the solid stuff is gone, the heater will cycle quickly until time runs out. If the freezer section is still cold, you may see some steam like a defrosting heat pump. What happens when will tell you as much as poking wires with a voltmeter without taking anything apart.

EDIT: DOH! Should have refreshed my browser before posting! Had errands to run this morning, you already diddit.