Repairing a Microwave Oven--Shorting a fuse for testing purposes

I have a built into the cabinet microwave oven that will be a pain to replace. Last week it just died. No power at all. The outlet it is plugged into is fine. I took it apart and found that two of the thermal fuses are bad. I want to make sure that this will completely fix it and do not want to waste money if it will not.

My question. Will I die or burn my house down if I short the two wires that connected to each thermal fuse, just for the very, very short and limited time to see if it powers on? Run it for 10 seconds? Or should I up my homeowner’s coverage and life insurance before doing this?


Current leakage in the transformer could be explosive if the fuses are not in circuit.

Id recommend against shorting across the fuses. I’m not familiar with microwave internals but if the fuse blew there’s something wrong. Jumping them will probably mean arks and sparks, you might get smoke and fire as a special bonus.

Until a microwavologist stops in I will recommend to you the first steps I usually take when troubleshooting electrical problems. Visually inspect the components. If they look good then smell them. After that it’s meter time.

Fair enough. I will not do that and just order the fuses. This model has three thermal fuses and one, well, I’ll call it a “regular” fuse for lack of a better term that I know. I put the meter on them, and one of the thermal fuses and the regular fuse has continuity. Two of the thermal fuses do not, so I assume they are bad?

Next question, just more of a confirmation—there is no polarity on the fuses, right? In order words, it doesn’t matter which end the wires go on?

Fuses don’t have polarity. No continuity does mean they are bad.

Installing new ones without finding out why the first ones blew will probably result in more blown fuses. It will also give the failed component the opportunity to fail more violently.

Do you have the make and model handy? Maybe there’s a wiring diagram on the net that can help determine possible causes.

I do have a manual online and troubleshooting step #1 was check the fuses and replace if bad.

A common rule for replacing fuses is to replace them once only. If they blow the second time, it isn’t the fuse.

In a recent Adam Savage’s Tested video he fielded a question from a viewer asking if there was anything he wouldn’t fix. Top of the list was microwave ovens. I concur. They are cheap to replace and will turn you into a raisin if you mess with them. They won’t just kill you stopped heart dead. They will kill you un-restartable stopped heart dead.

Now this is bugging me even more. I tested one of the fuses for continuity and didn’t find it. I ordered a new one on Amazon and didn’t find continuity, so I send it back. Now I get a SECOND one from Amazon and no continuity. It’s this one:

I know my meter works. But I put a lead on each prong and get no continuity. Did they send me two bad ones (in addition to the bad one inside my microwave) or am I doing a stupid?

That’s a thermostat, not a thermal fuse.

I looked up the part number, and that particular one is normally open at room temperature. Its purpose in most built-in microwaves like yours is to automatically turn on a blower if someone is using the stove underneath the microwave and too much heat is generated from the stovetop cooking.

It should measure open at room temperature.

Well, shit. What about this one:

Should it measure open as well?

And I ask because it is the identical part number to another (what I thought was a thermal fuse) that tests closed.

Hmm… good question.

I’m finding some sites that list it as normally open and others that list it as normally closed.

Did you order a new one and are you testing that, or are you just testing the existing one?

On that diagram you see parts 110 and 115. There are two part 110s. (and there is a “regular fuse” somewhere). I tested the “regular fuse”—continuity. Part 115 tested open, one of the 110s tested open, one of the 110s tested continuity, and the 115 tested open. So I (mistakenly) ordered 1-110 and 1-115 (the ones that tested open).

The 110 I ordered has not arrived, but the 115 did. I pulled it out of the bag and the new 115 tested open. I sent it back. I got another 115 today and it tested open so I came on here to post.

Perhaps you are correct and the 110 that is testing continuity is the one that is bad. Again, either that or you are wrong, and two new parts are bad—unlikely.

It’s hard to tell exactly what goes where as far as the wiring is concerned from those diagrams, but it looks like one of the 110s protects the magnetron and the other protects the high voltage transformer. I would expect those to be normally closed, so chances are the one that reads open is bad.

Which one reads open, the one next to the magnetron or the one next to the transformer?

The one that tests open is by the magnetron. The one by the transformer has continuity.

Well, we know that the two 110s should be the same when outside the microwave, correct? Either both open or both closed.

So when I get the new one, test it, and then put the corresponding other one I have in.

Clean off the Magnetron, transformers and circuit boards thoroughly before putting it back together. They build up crud, and are likely the reason your fuses blew. Pay particular attention to the areas around the fuses. I’ve had one thermal fuse blow on an oven. Before replacing, I tested the system with an alligator clip. It neither killed me nor blew a mains fuse. If you try that, keep the run time short. Your situation is a bit odd. Usually only one thermal fuse blows at a time; so you might get ready for fireworks before you plug it in.

Oh, you’ve only got one dead fuse. That’s far less scary. Be sure to clean the magnetron, and also the cover over the inlet through which microwaves enter the oven.

Thanks, so which one is the bad one? The one that tests open or the one that tests closed?

Open, high resistance, is blown. A good thermal fuse will conduct.

If you look above, Engineering_Comp_Geek is telling me that these are thermostats and not thermal fuses.