Hotpoint FFA97 Thermistor

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  • May 11, 2014 at 8:10 am #80621
    rufusw5
    Participant

    Hi All,

    I have a Hotpoint FFA97 which wasn’t cooling the fridge (It’s Frost Free), on further inspection the fan wasn’t cutting in. This fridge has 3 thermistors going to the controller, with coloured labels, green, red and black. The red one is in the bottom of the freezer so I guess this is freezer temp, green one in the freezer fan housing and I have no idea where the black one is… (Must be the fridge temp, assuming it’s not serviceable)!

    I disconnected the thermistors and the fan kicked in..

    So I isolated them one at a time by taping over the terminals one at a time and found that with the green one disconnected the fan worked.

    I put the red and green thermistors in ice water and compared the resistance, they look to be the same thermistor (I know that doesn’t mean they are….) and the green one does seem slow to respond, and gets hung up at about 26k, where the red one comfortably goes to 29.5K.. so I think I’ve found a faulty thermistor.

    I left it on overnight as it is, the fridge seems cold and the freezers freezing. It actually seems ok.

    So what I’m asking is… what does the green thermistor do? I’m assuming it’s for controlling the defrost heater.. so it should be ok to use like this while I get one, it just won’t defrost?

    And.. is the thermistor listed on this site (Model: C00216810, Hotpoint green thermistor) the part I need?

    I can’t find the correct part listed anywhere…….

    May 12, 2014 at 6:46 pm #413496
    rufusw5
    Participant

    Re: Hotpoint FFA97 Thermistor

    Just a thought… I’ve used thermistors before as temperature sensors and they’re not expensive components. They usually cost about 20p. Looking at Hotpoint prices would certainly lead me to think otherwise, I know genuine thermistors are potted, which has the obvious benefit of preventing water from seeping in.

    Now most thermistors won’t be suitable for potting, but should be heatshrink sealable (Heatshrink over the sensor and shrink it to seal the sensor in). You’d need to use glued HS to make sure it seals properly or they’ll fail frequently (Looking at the ones made for Mistral FF’s I guess this is what happens to them).

    Just wondering if anyone’s tried using regular thermistors?

    The one’s in the FFA97 freezer section seem to be about 10k.. Might be worth a try…

    It actually seems to work fine as is, since these are NTC thermistors it’ll be stuck at max cold for the green one due to infinite resistance when disconnected, so probably thinks it’s frozen up.

    If i was writing the software, I would measure the temp difference between the two freezer thermistors over a time frame, using an algorythm i’d be able to tell whether it was iced by the difference in rate of change.. I expect when iced up the green one would warm up far slower than the red one due to it’s ice blanket.

    But to protect against being stuck in defrost mode I would have a defrost timer, maybe 24 hours minimum between defrosts

    So my assumption is that without that thermistor it’s defrosting on the timer, probably too frequently, which might not be a bad thing given that these apparently ice up pretty bad.

    It does make you wonder though.. What if the icing up problems these are known to suffer from are a result of a bad thermistor, slightly out of range, so it doesn’t defrost often enough?

    Looks like I’m talking to myself here anyways.. no harm in thinking out loud I guess

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