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One Powerful Measurement Could Have Saved This Tech Hours of Troubleshooting...


Son of Samurai

3,987 views

Here's the situation: the tech is working on a dryer that keeps blowing its thermal fuse. The tech has already replaced the fuse once, and it's now blown again. What could be causing this, and what's the best way to tell?

We'll start by looking at the heater circuit -- an essential step in any troubleshooting plan.

Screen Shot 2020-11-04 at 12.07.37 AM.png

Pretty simple stuff. Just a cycling thermostat, a centrifugal switch, and a hi-limit thermostat. The thermal fuse that keeps blowing is the one in the motor circuit. It's a simple device: if it spends too long above a certain temperature, it will go open, preventing the dryer from running.

Now let's step through some possibilities.

1. A faulty cycling thermostat could cause the fuse to blow. If the cycling thermostat doesn't open every now and then, the heater could run for too long and blow the fuse.

2. If part of the heating element breaks off and makes a path to chassis ground, you'll have a part of the heater that is always running, which can blow the thermal fuse.

3. A plugged vent will prevent proper airflow, thus causing the thermal fuse to get too hot.

So where do you start? Is there a single, go-to measurement that you can make right off the bat that could help you immediately zero in on the problem?

Of course there is! In this situation, our tech should have made an amp measurement on the heater circuit.

Why? Because an amp measurement gives you a comprehensive overview of any AC circuit. If the circuit has the correct amperage reading, then you know that everything is in spec. You would measure amps on the heater circuit and then compare with the heater wattage, use a little Ohms Law mojo, P=I*E => I=P/E, and you got it. And if the circuit is electrically in spec, then we're left with only one possibility: an airflow problem, most likely the vent. This is something that we can very easily confirm by feeling the airflow on the output of the vent. And what do you know, in our tech's case, the vent is exactly what the problem turned out to be.

Pop quiz: Would you expect most problems in the heater circuit to cause the amps to be higher or lower than spec? Why? Great question! But rather than just tell you the answer, I'll refer you to a webinar we did on dryer case studies where one of the cases we studied was very similar to this one: repeatedly blowing thermal fuses. An amp measurement on the heater circuit would have identified the problem right off the bat. Premium tech members can watch that webinar recording here: 

 

Amps are a powerful and informative diagnostic measurement and they're so easy to do with a clamp-on ammeter. You should always try to incorporate them into your AC circuit troubleshooting, since they'll save you time and give you valuable information that might have been hard to find otherwise.

Want to step up your troubleshooting game to the next level? Check out our online Core appliance repair training course at the Master Samurai Tech Academy.

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Hmmm.  How does measuring correct current in the heater tell you that the cycling thermostat isn't stuck closed?  What am I missing here?

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Doesn't even have to be stuck closed, could also be out of spec and opening at too high of temperature. 

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  • Team Samurai
Son of Samurai

Posted

34 minutes ago, vincecapizzo said:

Hmmm.  How does measuring correct current in the heater tell you that the cycling thermostat isn't stuck closed?  What am I missing here?

In a normally operating circuit, you would every now and then see the amps drop to 0 -- that indicates that the cycling thermostat has opened, interrupting the circuit. If you don't see the amps dropping to 0 every now and then, that indicates a problem with the cycling thermostat.

The point is that you have a single test that requires minimal disassembly and gives you a lot of information about the circuit. By comparing the amp reading with the wattage spec (using Ohm's law to convert), you can see if everything is in spec, and by watching the amps fluctuate, you can monitor the cycling thermostat.

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OK, fair enough.  And while you're waiting for the thermostat to cycle, you can check the airflow.

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  • Team Samurai
Samurai Appliance Repair Man

Posted

No takers on the pop quiz?

Quote

Pop quiz: Would you expect most problems in the heater circuit to cause the amps to be higher or lower than spec? Why? 

 

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13 hours ago, Samurai Appliance Repair Man said:

No takers on the pop quiz?

 

I would say lower. If there is a probelm in the heater circuit it would only open the circuit removing power supply to the load. 

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  • Team Samurai
Samurai Appliance Repair Man

Posted

An open is easy— amps go to zero. But that’s not the question. The question is what if the amps are below or above spec (which means there are still measurable amps)— which case is most common? And what would it tell you about the circuit?

Hint: And not all problems in the heater circuit result in 0 amps.

Double extra special hint: the answer is detailed in the webinar linked in the blog post. 

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I haven't watched the add on attached webinar, but I would say it would be more common for amps to be below spec due to loose connections or resistance being higher on the heater due to stress.

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  • Team Samurai
Samurai Appliance Repair Man

Posted

2 minutes ago, MVrepairs said:

I haven't watched the add on attached webinar, but I would say it would be more common for amps to be below spec due to loose connections or resistance being higher on the heater due to stress.

You have snatched the pebble from my hand! 


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MVrepairs

Posted (edited)

Thank you.  Company makes a man better or worse, and the Samurai clan has been good company.

Edited by MVrepairs
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