How I repaired my dishwasher using fundamental principles for troubleshooting

I have a Samsung DW-FN320T dishwasher, which is about 5+ years old. It was not used very often because of maid at home. But now due to COVID lockdown the dishwasher started getting used heavily – almost twice a day because authorities banned maids to contain the spread of the disease.

One day though, it stopped working – the symptom, it showed heater error which as per the manual means either of temperature sensor, heating coil or some low sensing device is dead. So I called up my usual home repair technician, he came and found some plastic part of a mixer-grinder blade in the drain basin of dishwasher. Meanwhile we also discovered that the drain pipe is broken so he replaced that, but that still didn’t solve heater error.

He was not very positive about this because it seems Samsung has phased out this model in India (and they don’t sell dishwashers anymore) and availability of spare parts is a problem. He left saying he’ll try to procure the heater sensor.

But I wasn’t convinced that there is a heater related problem, because the dishwasher was completing cycles partially – for a cycle of say 2h 30m it would complete about 20-30 minutes then stop with heater error and I also observed it wasn’t pumping water to the spray arms in some cycles occasionally. The problem of pump not working was completely random and there was no pattern irrespective of wash program selected.

In addition to this, before the machine stopped working completely there were days when dishes wouldn’t come clean or the soap won’t dissolve but dishwasher used to show END (wash cycle complete).

So I opened up the dishwasher (a hell of a job that is) and then checked all the wiring for any loose connection etc. The problem here was – motor not running sometimes, so I was suspecting some kind of loose connection or faulty component such as a relay (which drives the motor) on the controller board.

To check what is happening I used a simple multimeter – put it in AC mode and inserted the probes in the connector of the motor. Then I started the dishwasher in the smallest cycle (pre-wash) – when I discovered the motor was getting power but it wasn’t spinning. This was weird, because if it isn’t spinning that means motor is dead, but a dead motor does not spin occasionally!

The shaft the motor was accessible from outside, so to check if the shaft is jammed I tried to turn it manually using a screwdriver (with the dishwasher being off) – it was moving freely without any resistance to motion. Then turned on the dishwasher again and started pre-wash – the motor started.

This made me suspect the capacitor of motor, because in single phase motors a capacitor is used to phase-shift the current in second winding which gives the starting torque. Also whenever the motor was turned on by the controller and it didn’t spin there was a sound coming from it. In one such instance, when power was being applied to motor (confirmed using multimeter), turned the shaft manually and it did start spinning.

The dead capacitor

This confirmed the doubt on capacitor. When capacitor loses it’s capacitance due to age/other factors, it is not able to provide the current required to produce the starting torque to spin the motor. Once a motor starts though it continues to spin due to inertia.

New capacitor

Now the next challenge was to finding a compatible capacitor (3 uF) to replace this – since it has a bolt thread, it was fixed using a nut on the motor itself. Considering the urgency I had planned to wire up two ceiling fan capacitors in parallel (which are typically 2.25 uF) in case I don’t get a new one. Luckily a friend of mine helped in procuring a 3 uF capacitor but it didn’t have the bolt thread.

Mounting

So in order to fit the new capacitor I did a simple jugaad / hack – put a simple nut-bolt on the mounting hole and tied it on the bolt using cable ties.

After fitting the capacitor and making the machine upright, I ran some test cycles in short long ones to check if the problem is fixed – it was fixed. The motor started spinning immediately without getting stuck / jammed at any time.

The lesson from this exercise is that one should always think from basics when approaching a problem to be solved, this applies to any field – whether it is electronics, electrical or computer science.

Also often people say – what is the use of subjects taught in school in real life. AC motor working was a part of 11th grade when I was in school. Yes, the subjects taught in school do have some use in real life. That’s how I was able to fix my dishwasher and saved a lot of money in that process in spite of not being an electrical or electronics engineer.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: