At what temp should a defrost heater thermostat typically close?

[grizzly]

The latest victim in my appliance repair home self study program is a Frigidaire top freezer refrigerator model number FRT21NNGD1.

When putting this thing through its paces, I couldn't get the defrost heater to kick on after advancing the timer to defrost. After I jumpered the thermostat out, it came on and got up to 482 defrees F (according to the temp probe on my cheap digital multi meter).

The fridge hadn't been plugged in for months, so the freezer compartment was warm. I ran the compressor for about 20 minutes to try to get the t-stat cold enough to close, but either that wasn't enough time to get it cold enough to close or the bi-metal is stuck open.

Is there any rule of thumb about how cold it needs be to get it to close?

Is 482 degrees about right for these heaters?

I had no idea they got that hot. For some reason I was thinking the only got up to about 200 F. Or is this one of those things that varies by manufacturer?

[AccApp]

Def limit should close at about 20° F. Some are marked with the approx closing temp. Don't worry about the heater's temp, just know that it'll burn you if you touch it. Could also cause a nice fire if left unattended.

[That Guy]

The temp range is listed on the defrost thermostat/switch. It depends on the design of the refrig. and where its mounted, on or near the coils.

I've seen ranges all the way from 90 degrees F down to 35. Usually the range is more like 50 -20. Meaning the T-stat will kick out at 50 degrees and kick back in at 30 degrees. Hence the -20. Most T-stats have numbers on them telling you their range. Dryer T-stats have something like 155 -40. Of course many new ones have their ranges in metric form now.

I usually check the Defrost T-stat when the unit's evap. coils are still frozen over. Otherwise it may take five or more hours for the T-stat to kick in. Don't run the defrost heating element for very long, the plastic in most refriges melts fairly fast and the heater its self hardly ever runs more than 30 minutes at a time and never really glows more than red.

[grizzly]

Thanks guys, I'll look at the thermostat and see if mine has a temp range printed on it, I didn't think of that, and never had it removed from the coil for closer inspection.

You are right about not wanting to run it very long, I only had it running (in a warm freezer) for 5 minutes or so, and the plastic under the element was already starting to get a hint of that burning plastic smell going on.

[BrntToast]

first thing i do with the frigidaires that dont defrost is visually inspect the DTS cause most of the defective ones i see the plastic top is actually popping off them see that, change it

also any frigidaire fridge with the timer with the see through case, change the timer cause its going to fail, they are garbage, even though we are a factory servicer we refuse to reuse that same timer and after some argueing with our rep because of repeat failures frigidaire now pays us to use the camco d55-719 timer as warranty replacement in their products