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4 Fridge only works with thermostats/thermistors disconnected
My 2007 (?) Bosch Logixx KTR18420GB/01 small simple fridge with 'Maxx Freedom Performance' has been erratic in its temperature control for many months. Using an external thermometer and ice blocks from the freezer, it was possible to keep it in use.
I recently checked all the refrigerator components (PTC relay, start capacitors, pump windings, thermistor response (2 off), and some control board component (capacitors, triac, diode), but couldn't find any signifiant fault (I did change the 4uF start capacitor as it had dropped to 2.9uF (no difference) and re-soldered one iffy joint (no difference)
By chance I found the pump would run and cool if I disconnected the two NTC thermistors (thermostats) with a starting wattage of about 60W quickly dropping to about 40W, as the start relay cut out.
But after exactly (to 10 secs) and reproducibly 15 mins, the pump stopped again. Then it started again exactly 45min later and so continued in this safe mode at about +3C, I guess to protect the fridge contents. Do other fridges have this feature? why are there two thermistors? could I use a universal fridge thermistor?
This is not a long term solution as the fridge will need a longer cooling time in summer or in heavy use and the only manual over-ride is to turn the fridge off and on again when it stops cooling to restart the cycle. (you can't change the on/off ratio by adjusting the temperature on the front panel)
Any ideas what else I can do, given that most of the parts are obsolete and no service manual is available online.
crwdns2934109:0crwdne2934109:0
Hi @dce
18W isn't that much current to be using. Depending on whether the mains are 230VAC or 240VAC that's only 75-78mA. Might depend on how many LEDs (and what type) are on I suppose as they might require between 10-30mA each to light up.
Does the problem occur if only 1 thermistor is disconnected?
Have they got any marking on them indicating model or type etc?
Knowing this may be able to find the specs for them to know if they're showing the correct resistance for the given temp or if they're faulty.
Wondering if one thermistor was for temperature control and the other for a defrost monitoring function as either one if faulty could turn off the compressor.
Where were they located?
The parts diagrams I found online for the model doesn't even show them.
crwdns2934271:0crwdnd2934271:0 jayeff crwdne2934271:0
Thanks jayeff,
I have just found out that my energy meter is faulty at low wattage. With a different energy meter the standby wattage is 2W not 18W (I've edited my original question). Sorry.
I'm guessing that one thermistor is in the cabinet, behind a cover that I don't seem to be able to get off and the other hidden where the cold pipe comes in, so I don't know the type. One thermistor measures around 10-13kohm and the other 14-34kohm depending on the temperature. Either way they both increase in resistance as the temperature goes down as expected for a ntc thermistor). I don't know why it needs two temperature sensors for such a basic fridge (no defrost or fans etc). I haven't tried with just one thermistor (I'd have to rewire the plug)
crwdns2934271:0crwdnd2934271:0 Dave Empson crwdne2934271:0
@dce
On p.22 of the user manual it states that there are two temperature zones in the refrigerator so perhaps this is why there are two thermisitors.
Just verifying that when both thermistors have been disconnected the compressor starts but then cuts out after 15 minutes, is this correct?
Usually compressors are turned off when the set temp is reached as indicated by the resistance value of the temp sensor(s), and then restarted again when the temp has risen above a preset upper limit to drive the temp back down again to keep it within the designed lower and upper temp range.
With the thermistors disconnected the resistance value seen by the control board is infinite resistance i.e. open circuit which should indicate that it is very cold so I would have thought that it shouldn't have started at all. Unless because there is no resistance at all it assumes that it needs to be cooled
What is the Bosch ean number shown on the product label?
crwdns2934271:0crwdnd2934271:0 jayeff crwdne2934271:0
@dce
What LEDs are on (if any) when the compressor stops after 15 minutes? Are any flashing?
crwdns2934271:0crwdnd2934271:0 jayeff crwdne2934271:0
Thanks jayeff,
Yes, without the thermistors, the pump starts as soon as the fridge is turned on and stops after exactly 15mins. One LED is on eg below the +5C or whatever it was previously set to. I can change which LED to turn on. It then flashes for a few seconds before becoming steady. The LED below 'super' also can flash for a few seconds before becoming steady (if the super button is pressed). But none of the buttons have any effect on the pump which is not surprising as it does not know about the temperatures.
Yes, all fridges have warmer and colder zones, but there only appears to be one (unopenable) position for the thermistor in the cabinet (fig 11 on the right). I guess the 15min turn on is useful for service engineers so they can check the system whatever the temperature.
The EN number is 153 and class is SN. Other idents are FD8310 001947.
253100230459019479 and 5700006768. R600A 0024 g Type KG KTRR18A 30/06bar. Not sure what most of theses numbers are except SN which means ambient +10 to +32C.
crwdns2934271:0crwdnd2934271:0 Dave Empson crwdne2934271:0
crwdns2934275:02crwdne2934275:0