Part · Viking built-in refrigeration
What the defrost thermostat does in a Viking refrigerator — and how it fails
In one sentence
The defrost thermostat is a thumb-sized bimetal switch riding on the freezer coil, there to let the heater work only while that metal is genuinely iced and to cut it off once the frost has gone. — Viking Repair Denver Pros, (720) 903-2603
What it does
Melting frost costs energy, and no appliance wants to spend a second more of it than it must. This switch is how the machine knows when to stop. Two metals are bonded together inside it; they expand at different rates, so the strip bends as it warms and snaps its contacts apart. Held against the coil it is watching, it keeps the heater circuit made while that metal is still iced, then breaks the circuit once the coil has come up past freezing and the frost has turned to water. Defrost ends, cooling resumes, and the switch cools and resets itself ready for the next one.
How it fails
There are two endings here, and they look nothing alike.
The common one is a switch that stops closing. Its contacts pit a little each time they carry the heater, and eventually they no longer make a circuit at all. From that day on nothing warms the coil, ice grows across it, and the cabinet slowly suffocates behind the build-up. It is the same picture a dead element gives you, which is exactly why both get measured rather than guessed at.
The rarer one is a switch that stops opening. Welded shut, it lets the heater run far longer than the coil needs on every cycle. That shows up as water where there should be none, ice cream that has gone soft, and a machine working much harder than the kitchen is asking it to.
Separating those from the element and from whatever schedules the cycle is a matter of order. The element is metered first because it fails most often, this switch second, the scheduling last of all.
Can you check it yourself?
No — and this is why.
No. It rides on the coil on the far side of the freezer back panel, wired into the same loom as the heater, and there is nothing about it to see or adjust. The useful thing you can bring to the call is the pattern: how fast the ice returns after it is cleared, whether one shelf glazes before the rest, and whether the unit has been left standing open at any point.
Repair band
Refrigeration fan or defrost system $280 to $690
The $89 visit buys the diagnosis and the figure for your own unit. Nothing is opened further and no part is ordered until you have seen that figure and said yes. Viking Repair Denver Pros, (720) 903-2603.
Follow-up questions
Why not change the heater and this switch together?
Often that is what happens, because the two sit side by side and nearly all the labour is in reaching them. A part that has measured sound is not fitted anyway, though, just to round the job off. Either way the work falls in the defrost band below, and the figure is agreed with you first.
Can this switch make the fresh-food shelves warm?
Yes, by a roundabout route, and that is usually what prompts the call. Ice builds on the coil until air can no longer be pushed through it, and the shelves outside the freezer live entirely on that air. The frozen side holds its own for longer, so people ring about limp salad rather than about frost.
Symptoms it causes
If your unit is doing one of these, this part is on the list of things that get tested.
- Written by
- Clifford Nakamura
- Role
- Service technician — controls and intermittent faults
- Last checked