Bypassing Vent Fan Thermal Fuse to Get Microwave To Work

Hello all,

I have an over-the-range Samsung microwaves (model me18h704sfs ) that stopped working due to a blown fuse. I changed the fuse and discovered an abnormally in the electrical circuit that I can’t figure out what caused it.

All three door switches, fuses and thermal fuses show continuity. There is one thermal fuse (“open” under normal condition, “close” in response to high temperature) that turns the vent fan on automatically when the temperature exceeds its ceiling. When I bypass that thermal fuse, the microwave works fine but the vent fan is always on. When I do not bypass that thermal fuse, the microwave runs with no heat the and the vent fan is off. When I disconnect the bypass when the microwave is on, the microwave continues to run but no longer produces heat (I also heard a single click sound as soon as I disconnect the bypass)

Do you guys think I need to replace that thermal fuse to solve the problem or is there a faulty electrical circuit somewhere?

Thank you for the help!


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Hi @david271

Does the vent fan ever shut off after it has been running for a while, with the microwave not in use, indicating that the temperature has been restored back to a normal level and that the thermal fuse has released?

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The vent fan continues to run for at least 30 minutes while at room temperature when I bypass the overheat-protection thermal fuse. If I leave the power on, it would continue to run indefinitely.

What baffle me is why would the microwave function normally when I bypass the thermal fuse which suppose to shut down the microwave in case of an overheat. My understanding is that when the overheat-protection thermal fuse connect, it turns on the vent fan automatically and shut the microwave down. In this case, when the overheat-protection thermal fuse connect, both the vent fan and the microwave work.

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Bit confused when you say "If I leave the power on, it would continue to run indefinitely." after saying "when I bypass the overheat-protection thermal fuse"

What I meant was does the fuse reset normally i.e. goes open, once the correct temperature has been reached without "bypassing" the fuse and the microwave is not running?

By bypassing I assume that you mean removing the fuse from the circuit, as you said that open is normal condition and closed is when high temp is experienced.

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Sorry for the confusion. By passing, I meant removing the wire from the fuse and connect both ends to each other.

The fuse does open at normal temperature, but the microwave run with no heat.

According to the service manual, the correct term I should have been using is Hood Thermal Cutoff

The part in question is on page 26.

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Hi @david271

If you remove the fuse and connect both ends of the wire together this simulates that the fuse has closed and the fan will run.

You need to remove the fuse and leave the wires disconnected simulating that the fuse is "open" indicating that the fuse has reset and is open.

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