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Save button pressed shows Evaporator thermistor temperature

I’ve done some more investigation and concluded that the temperature reported when the Save button is pressed (when the zone being displayed is freezer) is from the evaporator thermistor. I checked this after raising the thermistor’s temperature to 25C and then checking what is displayed.

I emptied the appliance, removed the evaporator shields, and defrosted rapidly using a fan then removed the thermistors and testing their resistance at the standard specification temperature of 25.0C using a mug of water and the digital thermometer.

Evaporator Thermistor (Yellow): 2.66k? @ T25
Air Thermistor (Green): 2.69k? @ T25

I would surmise from looking at the PTC on the control PCB that they are both +/-20{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} tolerance. I doubt they’d need to be (expensive) +/-2{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} tolerance.

Now I know I can see the temperature of the evaporator when the appliance is refusing to keep the fridge or freezer below -20C it should help in isolating the true cause of this intermittent loss of refridgeration.

I’m wondering about the temperature range the evaporator fan operates within since I discovered previously (using a USB video camera) that the fan switches off when the freezer compartment gets too cold, as well as when it gets too warm.

Any theories are welcome at this point!

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Compressor and Motor test; an imaginary Chiller compartment

Four days later and the issue returns again!

This morning the fridge was reporting 20C and the freezer -18C; compressor sounded to be running and there was heat in the condenser pipework. The compressor itself was quite hot to the touch (but not burning).

I tried switching off power for 20 minutes and then restarting via the self-test mode (holding down Save button as power is switched on). That didn’t seem to solve the issue – the compressor was running, heat was in the pipes to the condenser, but the freezer temperature was remaining constant around -17.8C.

I decided to strip down the rear of the unit. I removed the PTC Relay, Motor Protector, Running Capacitor and associated electrical wiring from the compressor to reveal the three electrical pins.

I then did a resistance test. Top pin is Common (C), lower-left is Start (S), lower-right is Motor (M). I measured the resistance between C and the others, and between S and M.

The resistance of S and M (stop tittering at the back, there!) should equal the sum of the other two resistances (give or take 0.5? or so).

C > M = 12.4?
C > S = 16.6?
S > M = 27.6?
Difference = 1.4?

This might indicate a developing issue in the compressor windings but is unlikely to be the cause of the current issue – unless the compressor’s thermal cut-out is being triggered (but then it would go silent and that doesn’t happen).

I have the technical data documentation for the Matsushita compressor model DB110C22RAW5 fitted to this FF92P. I checked it was operating within the ranges. Ambient temperature is specified as -5C to 43C.

I also decided to remove and strip the condenser fan motor. After removing it I took the fan blade off and cleaned it then removed the rear cover of the motor to reveal the PCB and spindle.

There was indication on the PCB of some kind of caked-on spillage which I decided to remove in an abundance of caution using Isopropyl alcohol and a sharpie blade.

I de-soldered the PCB from the motor windings and inspected the component side. The large wattage resistor has obviously been rather hot, having scorched the PCB and burnt off its colour bands, but it measured at 48.7k?.

I took copious and detailed photographs of everything and will organise them on my web site shortly.

I reassembled refitted and powered it back up via the self-test mode. So far it appears to be cooling the freezer compartment. The digital thermometer indicates a drop of -0.1C approximately every 15 to 20 seconds.

I also made a strange discovery that I’ve never noticed before despite having pressed the control buttons in every combination.

Now, when the zone indicator shows the freezer compartment, if I press the Save button the display changes to show a non-existant central chiller compartment and initially reported -41C but later shows -42C.

I’m not sure how that happened or whether that would have displayed previously but it did make me wonder – is it possible the control unit thinks it has a chiller compartment, sees the temperature has hit -41C and thinks “I’d better turn the compressor off because its too cold” ?

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Re: Hotpoint Mistral (FF92) mis-reading temperature?

Solved! Investigation revealed that the Insulation Displacement Connector (IDC) for the evaporator thermistor (yellow) had a poor contact such that once frozen (and shrunken) it introduced more resistance into the circuit which fooled the controller into thinking the evaporator was colder than in reality.

This was a symptom of having to re-use the connector block from the already installed device because the connector block is included with the replacement device in most replacements now.

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Re: Hotpoint Mistral FF91, freezer only reaches -9°C any ide

I found the returning issue on our FF92P was a bad connection for the evaporator thermistor which led to increased resistance.

Many replacement thermistor units no longer have the Yellow Insulation Displacement Connector (IDC) so you have to use the one connected to the installed faulty device.

Despite taking some care it seems that the IDC tool didn’t give a perfect connection so that once frozen (everything shrinking of course) the connection proved to be poor and increased resistance.
Being a Negative Coefficient Thermistor (NTC) meant the controller believed the higher resistance meant the evaporator was colder than in reality and therefore stopped the compressor prematurely.

I’m also working on identifying a replacement thermistor that anyone can buy and make for a few pennies instead of spending at least £13 per device.
After all, the thermistor itself is only ~4mm square – the rest being a 2-core 5A 240V flat cable. The cost of the thermistor itself should be much less than £2 even in single quantities (according to RS Electronics and Farnell).

I’m developing a theory for why the thermistors fail but need to do some tests on fault vs good ones. I’m beginning to think the resin the component is dipped in might be the problem. The resin
encases the thermistor and is then shielded by a rubber sleeve.

Anyone who would be willing to send, through the royal mail, their faulty evaporator (yellow) and freezer air temperature (green) thermistors to me could help in my endeavour. If sending them please affix sticky labels to each identifying them (especially if you’ve removed the IDC coloured connectors). Send me a private message to get my street address.

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Re: Hotpoint Mistral FF91, freezer only reaches -9°C any ide

I’m currently dealing with the same issue with a FF92P. See my thread “Hotpoint Mistral (FF92) mis-reading temperature?” for background. I replaced the air and evaporator thermistors last week and thought it fixed.

The compressor continues to run along with the condenser fan (at back-right bottom corner) and the condenser itself is clean and free from dust/dirt).

Earlier this evening, after a complete natural defrost, I had left the evaporator insulator and back cover off to check that the evaporator fan was operating by using a USB video camera fitted with Infra-red LEDs.
Unfortunately this meant that the cold air wouldn’t get pushed up into the fridge.

What I found was the evaporator fan only operates when the the temperature is above -27C as measured at the ‘green’ air thermistor.

I had already seen that the evaporator fan begins operating when the temperature is below -15C (according to the freezer temperature display) which usually seems to be about -22C measured by a thermometer at the air temperature thermistor.

With the evaporator insulator cover left off the air temperature had reached -32C but the fan had switched off presumably because the fridge wasn’t showing any signs of cooling or that the freezer compartment was ‘too’ cold.

I replaced the evaporator insulator and back cover and left it to cool properly.

At this moment it has been cooling for 5 hours. I checked it after 4.5 hours and it had reached -17C according to the unit and the digital thermometer (placed next to the air temperature thermistor middle-front of freezer compartment).

I had expected it to have reached its operating temperature in both freezer and fridge by this point.

I removed the freezer rear cover (but not the evaporator insulator) and saw there is a good build-up of frost across the evaporator.

I closed it up with the digital thermometer probe lodged next to the air temperature thermistor and am letting it cool down again. It currently seems to be changing by -0.1C every 10 or so seconds.

The obvious issue to check is whether the compressor is running when the freezer is failing to reach temperature. If it is, then the next thing to check is that the evaporator seems to be frosting up and the condenser (underneath, front-right, behind the air grille) is warming up as it sheds the heat.

All that indicates that there isn’t a gas leak to weaken the cooling effect. Then you’re left with the electrical control circuits; mainly the thermistors and the fans.

The condensor fan (bottom rear-right corner) should be blowing air over the condenser fins and helping the compressor (bottom rear-left corner) to shed excess heat.

The evaporator fan should circulate cold air within the freezer compartment and, with the evaporator cover in place, that should force air into the fridge compartment via the tube top-right of the evaporator (behind the evaporator capilliary tube).
There is a motor-operatored baffle in the fridge top-right behind the light cover which should open the tube when the freezer is below -15C.

Warm air from the fridge returns to the freezer via the tube top-right of the fridge which enters the freezer on the lower-right side behind the evaporator.

Hope this helps you at least understand the mechanics!

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Re: Hotpoint Mistral (FF92) mis-reading temperature?

One week later and the problems return!

Wednesday morning (yesterday) the fridge was at +18C and freezer at -18C (according to digital thermometer).

Freezer couldn’t/wouldn’t lower the temperature any further.

Moved all the food out last night into a second fridge-freezer and left it outside to warm up.

I have just begun testing it this afternoon. I removed the evaporator covers so I can check what is going on. Ambient temperature was 23.5C. Freezer reached -15.6C inside 30 minutes with an even spread of frost across the evaporator. It reached -22C within the hour. The evaporator has not been running each time I’ve quickly opened/closed the freezer door to check.

No error codes regarding evaporator thermistor failure.

I’m wondering if the clean-up of the terminal block might be related although I think that is for the defrost circuit.

Could do with the circuit diagram; I have the service manual on order but it won’t arrive until next week.

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Re: Hotpoint Mistral FF92S Fridge temp high, freezer works O

Frank, did you fix this?

I’d suspect the motor-operated baffle between the freezer and fridge compartment, or something I experienced a few years ago: ice build-up at the entrance to the tube in the top-left of the freezer compartment.

Make sure you can blow air without restriction through that vent pipe using a hair dryer or similar. You’d need to remove the condenser cover in the freezer compartment to check it.

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Re: Hotpoint Mistral (FF92) mis-reading temperature?

The fridge/freezer stabilised at +4C/-22C yesterday afternoon and maintained correct operations overnight so I’m considering it fixed.

It does seem that frequent power-cycling whilst testing can upset it.

The mis-reading of temperature seems to be the expected consequence of the displated temperature being an average of past values.

When the unit is cooling down the historic temperatures will have been much hotter which would explain the difference between the thermometer reading and that displayed. Now the unit is stable the values are about the same.

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Re: Hotpoint Mistral (FF92) mis-reading temperature?

The freezer compartment reached -22C according to the display but the digital thermometer showed -32C. The fridge hadn’t started cooling.

I was confused as to why the fridge hadn’t started cooling and then realised it might be because I had left the freezer’s evaporator cover off, which might prevent the fan-driven air from being pushed up the pipe to the fridge when the motor-driven baffle opened.

I re-fitted the evaporator cover. The freezer was showing +5C when I turned it back on but the compressor didn’t start.

I then noticed that when being powered on the display briefly flashes “P1” for about 1 second before the temperature display is shown.

I noticed several other similar reports here in the forum but the experts seemed unaware of this “P1” code or its meaning.

I also saw some suggestions that power-cycling the freezer frequently is a ‘bad thing’ and to leave it to stabilise.

After leaving the freezer for about 30 minutes I returned to find it had started cooling again. I’m surmissing the “P1” code might relate to initial compressor _P_ressure issues.

Currently the freezer reports -9C and the thermometer shows -11.2C, so the differential is now down to +2.2C.

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Re: Hotpoint Mistral (FF92) mis-reading temperature?

kwatt wrote:It sounds like it’s struggling to reach temperature which, from what you describe, would usually point toward a refrigerant system problem rather than a control issue.

Thanks for the swift reply. I stripped it down earlier and did a thorough inspection. I noticed that the terminal block in the freezer compartment that joins 4 wires on spade terminals was blackened and showed small signs of corrosion developing.

I removed it, cleaned the terminals, applied Vaseline and re-assembled. The freezer is currently cooling again.

I have the digital thermometer touching the air-temperature thermistor (green terminal) so I can accurately compare the temperature being displayed.

So far, at -12C it appears that the freezer is displaying a temperature difference of +3.2C.

Whilst I was checking it over I compared resistances of the removed thermistors and the replacements. With an ambient temperature of 22C all four seemed to be around 2.7k?.

As I post this the digital thermometer reads -25C and the display shows -22C. So far the fridge baffle doesn’t seem to have opened though.

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Re: Advice on changing brushes

I’ve got the very same issue with the same (cheap) motor. Can you tell me the exact make/model or other references for the replacement Zanussi Vacuum cleaner motor you used?

I stripped the Ronning Motor TD-8419M23(900) [1] down, tested the commutators for shorts (between each the resistance was ~ 90 Ohms), cleaned and polished them, and replaced the brushes with a pair from another identical motor. It began to arc during testing and can, I think, be thrown away.

In stripping it down I had to drill out 8 studs (using a 4mm drill bit) that retain the blower blades and air flow chamber and replaced with M4 bolts (13mm and 20mm for the chamber).

The carbon brushes are 7mm x 6mm x ~ 30mm with ~40mm tails and 72mm springs. They are retained by a folding brass flap where the tail is spot-welded. When being bent this easily snaps so I used solder when fitting replacements from another identical motor.

I have ordered some 7mm x 6mm x 20mm replacements just in case they help but I think the issue is with the communtators.

[1] http://www.ronningmotor.com/index.php/products/universal-series-73/vacuum-cleaner.html

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