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How do you change a Waeco thermistor in the CF range of fridges? And how do you know when the thermistor has failed?
We’ll lead you through the process.
First though, we’ll explain what a thermistor is and why your fridge has one.
What is a Thermistor?
A thermistor is a thermal resistor. In simple terms, its resistance changes as temperature changes. The electronics in the fridge read the resistance and convert that to a known temperature.
So what you see on the temperature readout, is simply what temperature the thermistor is detecting at the bottom of your fridge.
How To Decide If The Thermistor Is Faulty
The CF range of Waeco fridges have a known fault. The thermistor often fails after several years.
If you have a temperature problem with your Waeco fridge, then there’s a good chance the thermistor has died.
Waeco install the thermistor at the bottom of the fridge on the underside. It usually sits in about the middle of the base of the main compartment. Being on the underside and up against the metal compartment, the thermistor is prone to attack from moisture.
Where does the moisture come from?
It’s simply condensation between the outside of the compartment and the thick layer of foam insulation. And because the thermistor is on the bottom of the fridge, it tends to sit in damp conditions for long periods of time.
Eventually moisture makes its way into the internals of the thermistor, causing it to fail.
Now this takes a while to happen. In our case, the fridge was over 14 years old. (See a long-term review of our Waeco CF-50 fridge here).
How do you know if the thermistor has failed? When you start seeing weird temperatures displayed on the temperature readout, you can be pretty sure the thermistor is dodgy.
For example, at first our CF-50 fridge consistently displayed 00 no matter what the actual temperature was.
If we then set the fridge below 0°C, it would pull the temperature down to the setpoint and operate correctly. After a while though, the temperature display was showing random numbers… 53, 28 and eventually 80.
80 is the maximum temperature the fridge will display, so is a good indication the thermistor has shorted out internally. I’ll explain this further below.
When this happens, the fridge runs constantly. It’s trying to bring the temperature down to the setpoint. The result? Everything in the fridge freezes solid!
I was confident the thermistor had failed because of how the fridge was behaving.
However if you want to know for certain, then this how you test it…
How to Test a Waeco Thermistor
Waeco CF fridges use a 10k ohm thermistor. Resistance decreases as temperature rises. So if the internals of the thermistor have shorted out, there will be zero (or nearly zero) resistance.
And if the resistance is zero, then the fridge controller will think the fridge temperature is at or above 80°C.
That’s why when you see 80 on the fridge’s temperature readout, you can pretty well guarantee the thermistor has died. Make sense?
If you want to test your thermistor, set a multimeter to 20k (20k ohms) unplug the cable from the circuit board and touch the probes onto the metal terminal tabs.
In the photo below, it shows me testing the thermistor after I removed it. This is because I forgot to take a photo while it was still installed… whoops!
You’ll notice the reading on the multimeter is zero ohms. This means it has short circuited internally, so it has basically no resistance. If you get a reading like this, then your thermistor is definitely dead.
One thing to be aware of. I tested the thermistor again about a week after I had removed it, as shown below.
It had a resistance of about 3.3k ohms, so it was working again to some degree. I expect this is because the thermistor had a chance to dry out.
If you only use your fridge occasionally and it intermittently shows a fault, this could be why. The thermistor probably has a chance to dry out between uses, so will work for a while before playing up again.
How to Change a Waeco Thermistor
I could write a long and detailed step-by-step instruction manual on how to change the thermistor. Or I could just show you a video instead…
A few things to note from this video:
- The position of the thermistor varies between models. Some are underneath (our CF-50 CoolFreeze) while others are in various places around the side walls, including behind the compressor/evaporator unit.
- The location of the thermistor, where it disappears into the foam insulation and where it terminates on the printed circuit board varies between models. Verify the cable you’ve identified as the thermistor cable is indeed the correct one before cutting it.
- The large plastic base is difficult to remove and difficult to re-fit. Because the manufacturers fit the base and then fill the cavity with insulation foam, the foam tends to stick to the base. The foam also moulds around indented items like moulded handles. So the lid really doesn’t want to come off.
- The best way to remove the base is to keep tapping along all the edges with a round bar and hammer. Don’t get too aggressive or you’ll crack the cover.
- Some people leave the plastic base on and cut a large access hole in it. You could do this, although I’m not too sure how you’d patch it up neatly again.
- If you take this approach, you could cut the old and new thermistor cables and join the wires. I decided not to do this because each join is another potential place for condensation to enter.
- The insulation foam is easy to cut. I used an old serrated knife, which cut the foam easily.
- When you glue the new thermistor in with the heatsink plaster, place a small weight on top of it and leave to dry for 24 hours.
- While the fridge is apart, give it a thorough clean inside. Also check the printed circuit boards for any corrosion or dust build-up.
The Waeco thermistor in the CF range of fridges is a well-known weak point. Given ours is over 14 years old, I’m not too worried about it though.
Changing the thermistor is a bit of a mission and probably more difficult than it should be. However, these fridges are like most things now… they’re not designed to be repaired.
But with some patience and a bit of thought, you should be able to replace the thermistor easily enough.
This fridge has given us many years of faultless service. If I had to buy a new fridge, I wouldn’t hesitate to buy another Waeco (or Dometic as they’re now called).
And with the thermistor replaced, there’s no reason why our fridge won’t continue to run for many more years.
Go here to see the Dometic range of fridge/freezers for camping and 4WDs.
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