Identifying NTC sensors

[brokentrak]

I have found something larger to repair this time: a fridge-freezer with electronic control. I got it for free because the electronic control was acting up. Upon removing it, I found that it had a bad case of swollen electrolytics and a burned relay. Replaced the electrolytics and the relay and the first startup was a success, but all is not good yet.

After it has cooled down to it's setpoint and the compressor switches off, it sits for about an hour and then goes into error mode. The error given is to do with temperature sensors. It has two sensors: one in the lower part of the fridge compartment and one on the evaporator plate. The controller is unable to distinguish which of the two sensors has failed so the official statement from the manufacturer is to replace both sensors at the price of 45€ a piece  .

After doing some investigating, I found that one of the temperature sensors is a standard 2K NTC thermistor. The other one is a bit more complicated: according to the service people the other sensor is the same 2K NTC as the first one but it has a pre-resistor which value is unknow.

 Since the error occurs when the fridge has cooled down I suspect that it may be something to do with cool temperatures. I thought about putting both sensors in a glass of warm water and measuring the resistance difference to determine the value of the pre-resistor. Are there any other ways to find out the resistor value of the other sensor?

I have ordered some Mitsubishi CH25-3G202FB NTC-s to replace both sensors at the same time. If all else fails, I have ordered a chinese universal temperature controller to replace the original one. I would not like to go that route as I prefer to keep my stuff as close to original as possible.

[abyrvalg]

How does it look? A “silver bullet” on a cable? I’ve cut such type to pieces to find out what kind of NTC was there (my one failed open circuit). The outer metal can is very thin, it was easy to cut the rim a bit then just peel it off. That reveals an epoxy body, quite brittle, so I’ve heated it in a flame then carefully cracked piece by piece with wire cutters to reach the parts and see the markings.

[madires]

Bad defrost sensors (the NTC at the evaporator) are a common problem. The freeze-defrost cycles will brake them after a while. To determine the series resistor in your case simply measure the temperature and the total resistance of the sensor at the same time. Since you already know that the NTC is a 2k type look up a NTC datasheet for the resistance at the measured temperature. Subtract that value from the total resistance and you'll get the series resistor. If you take a glass of water with a nice temperature like 25°C it will make things more easy. And don't forget that the NTC has a tolerance. Usually they don't use precision NTCs, maybe 5%. The controller board might have an adjustment like a hidden service menu.

[NivagSwerdna]

After doing some investigating, I found that one of the temperature sensors is a standard 2K NTC thermistor. The other one is a bit more complicated: according to the service people the other sensor is the same 2K NTC as the first one but it has a pre-resistor which value is unknow.

They are thermistors... so there are only a few variables... NTC versus PTC... you seem to have good evidence they are NTC.... So now you only have two real variables... resistance and temp coefficient...  Ignoring temp coeff for a second... so... do they measure open circuit, short circuit or a reistance... and how does that resistance change due to temperature?
The reason I ask is that given the controller has obviously been through a hard time it might be more likely that the sensing is broken rather than the thermistors themselves... but thermistors do fail as described above.