Have you ever wondered why the dishwasher has stopped heating the water.
Well, things are not always what they seem. I once got asked ‘How to Test a Dishwasher Thermal Cut Out‘.
What was the thinking behind this question. Was this a misdiagnosis and that the customer was thinking they had a fault with one thing but in actual effect it was something else. This is what I call a leading question and I will try to answer it for you here.
Depending on the make and model of your dishwasher, you may have one or more devices that protect the appliance from over heating. That is a Thermal Cut Out or fuse and an NTC Device.
Although at first they would appear to be the same thing, they certainly don’t act the same. Lets have a picture.
Testing Dishwasher NTC components
Some dishwashers are fitted with a NTC (Negative Temperature Coefficient) device to sense the temperature of the water.
Variously known as Thermister, Sensor, Thermostat, Temperature Sensor, Temperature Probe, Probe Feeler – this device works by exhibiting an electrical resistance value which reduces with an increase in temperaure.
Simply put; if the water is hot the resitance value is low and if the water is cold the resistance value is high. All the machine has to do is figure out what the water temperature is by looking at the resistive value of the NTC.
The appliance control module is configured to compare the external resistive value of the NTC with internally pre-configured set-points. Depending on the resistive value of the NTC the module will provide the appropriate control function for the water heater.
How to test the NTC
For this you will need access to a multimeter.
1. Remove the NTC from the appliance.
2. Connect the NTC to the multimeter set to Ohms x1000 and note the reading. This should be in the order of 70 to 80kOhms(at 60 degs F).
3. Boil the kettle and fill a cup.
4. Place the body of the NTC into the cup of hot water – do not submerge it.
5. Note that the resistance value starts to change. Leave the NTC in the water for about 5 minutes because these devices have a long dwell time.
6. After about 5 minutes the NTC reading should have changed to approximately 90Ohms
7. Once you have obtained the readings place the NTC in some cold water; preferably with a couple of ice cubes; just to make sure the water temperature is is below ambient.
Why do I need to do this?
This allows the NTC to slowly restore to ambient temperature from a cooler place. By design the NTC is allowed to retain heat so that device does not cause the control module to keep switching the water heater on and off – called hunting. If you re-fit a warm NTC to the appliance and do a test run the appliance will think the water is still hot enough and the program will probably loose itself and not cycle correctly if at all. You will be led to believe that there is something else wrong with the appliance.
If your results compare favourably with the above test then the NTC is working correctly. If not it is likely the NTC is defective. This infomation is relative to Part Nos: 481228268051 – Temperature Sensor - and are available from our on line catalogue www.spare-parts-pro.com.
Note: Appliances that use an NTC can sometimes loose their way and the appliance will become stuck on a cycle. After testing the NTC run the machine in diagnostics mode to reset the program.
