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Troubleshooting a Samsung Dryer: Won't Start but Always Runs


Son of Samurai

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Join me on another short troubleshooting journey -- this one taken from a real tech's experience that he posted on Appliantology.

You're troubleshooting a Samsung dryer that won't start-- the control panel will let you select a cycle, but it won't actually run if you press the start button. But the real head-scratcher is that, despite not being able to start a cycle, the machine always runs when the door is closed.

Sounds tricky, but as you'll see, the troubleshoot is pretty simple if you have a solid troubleshooting strategy. Any good troubleshooting strategy always starts with the schematic, so let's go there.

Screenshot 2021-01-20 084012.png

Pretty standard for a Samsung dryer. Our first order of business is to select an LOI -- a load of interest. That's the load that's not doing what it's supposed to do or that's doing stuff when it's not supposed to do anything. In our case, this is the motor. It's running even when you haven't started a cycle, so something in its circuit or power supply must be off.

Now that we've selected our load, let's analyze its circuit:

Screenshot_2021-01-20_084014.png

You might already be seeing that there's only one thing that could be causing the problem here. Sure, there are plenty of things in the motor's circuit that could interrupt its power supply, but there's only one that could supply power when it's not supposed to, and that's the board. More specifically, relay 5 on the board.

Screenshot_2021-01-20_084012.png

The question then becomes why that relay is closed, even when a cycle isn't started. Could it be a problem with the board's algorithm? Are the logic circuits all scrambled? Do we have to be a computer whiz to get to the bottom of this?

Not at all. The truth is that electromechanical relays like this can just get stuck closed sometimes. It's one of the ways they fail as they age, and a new control board will get this machine running normally again.

So if it was just a stuck relay, why wasn't the heater circuit running? Why wouldn't the machine start a cycle? Well, all these control boards run on algorithms. Don't get scared by that word -- it just means that the engineers have designed the board to make certain decisions if it receives certain inputs. If this, do this. That's all it boils down to. In this particular case, we can pretty safely assume that the board was programmed to refuse to run when it sensed (through a sensing line) something strange like the motor running out of cycle. That's the safest thing for it to do when it senses something is wrong.

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I've ran into this a couple times, but for me, the control board would still work as usual, so the customer could just close the door, the motor would start, choose a cycle and start, the heat would activate, then they would just wait for the cycle to finish and even though the motor was still running, the heat cycle would be done and they could open the door and take the clothes out.

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Son of Samurai

Posted

2 hours ago, justjad said:

I've ran into this a couple times, but for me, the control board would still work as usual, so the customer could just close the door, the motor would start, choose a cycle and start, the heat would activate, then they would just wait for the cycle to finish and even though the motor was still running, the heat cycle would be done and they could open the door and take the clothes out.

It all depends on how the board's algorithm responds to the unexpected. In your case, it sounds like the board wasn't bothered by the motor running out of cycle -- perhaps it had no way of even knowing this was happening.

The machine in the blog post has several sensing lines, so it's able to tell when, for example, the centrifugal switch is closed. After that, it's all up to how the engineers have programmed the board to react.

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Scottekarate

Posted

@justjad I've seen this so many times with LG and Samsung. In each case the customer reports, won't heat and motor always runs. I show them that yes, in fact the heater will run. My favorite thing to do here is give the relay a rap with the handle of a screwdriver and lo and behold, normal operation returns. Then I order the board, it takes a week or two to arrive, we call to set up the return and inevitably the customer states that the unit has "worked fine since you left, do we have to put the board in?"

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heyweb1500

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I'm an amateur, so bear with me, but when that relay is closed, is that how L1 is supplied to motor?

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6 hours ago, heyweb1500 said:

I'm an amateur, so bear with me, but when that relay is closed, is that how L1 is supplied to motor?

Yes, the L1 is hard-wired to the relay on the board at connector 5 pin 2.

When relay closes, the power (L1) goes to the connector 5 pin 1 to run the motor.

But if relay gets stuck closed, the power is always present at the pin 1, hence the motor always runs (when the door switch is closed).

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