Hotpoint Mistral FF91, freezer only reaches -9°C any ideas?

[gibbins73]

Hi all,
I have a Hotpoint Mistral Frost Free Fridge Freezer Model FF91S.

The freezer will not drop to a lower temperature than -9°C, I have removed the panel at the back of the freezer storage, and the evaporator was encased in hard ice. I have defrosted this ice with a hairdryer, freezer still not dropping to expected temperature of around -22°C.

Does anyone have any advice on what the possible fault could be?
Also if possible what parts I need and where I can purchase them?

Many thanks in advance of a positive reply! 🙂

[feefifofum]

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!

[gibbins73]

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

Hi feefifofum,

Many thanks for such an informative answer.
I shall investigate further over the weekend!

🙂

[feefifofum]

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.

[feefifofum]

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” ?

[feefifofum]

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!

[kwatt]

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

TBH I think you may be flogging a dead horse that’s barking up the wrong tree.

It sounds really simple, the compressor may well be overheating and cutting out, restarting after it cools down. Happens a lot in warm weather as they get stressed out.

+/-20{e5d1b7155a01ef1f3b9c9968eaba33524ee81600d00d4be2b4d93ac2e58cec2d} is fine in a cabinet like that and, due to the low cost of them in relative terms, it’s about as accurate as you can hope for.

K.

[feefifofum]

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

kwatt wrote:
It sounds really simple, the compressor may well be overheating and cutting out, restarting after it cools down. Happens a lot in warm weather as they get stressed out.

I’d agree if weren’t for the fact that the compressor doesn’t stop working so the thermal cut-out nor the PTC are activating.

[kwatt]

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

Well then, that does leave you with not many places to go.

The compressor can pump weak but it only has two states, on and off. It either runs and pumps at pressure or, it doesn’t.

It does what the board tells it to. Likewise the fan motor.

The thermistors tell the board what to do in respect to switching those both on or off as required.

You know the thermistors are operating correctly so it pretty much leaves but two options.

However, if the compressor is running and it isn’t cooling, it logically leaves you one option given that it is being told to run and you know the fan motor is running.

K.

[feefifofum]

Hotpoint Mistral temperatures and resistances

Finally solved the mystery! I received the service manual (SM) today and checked it against all my observations and experience.

First thing I noticed is, the SM is not up-to-date (document # 5404310 Issue 3 March 2002 – part # C00220681). It covers the FF71 N/P/S, FF72 N/P/S, FF73 N/P/S, FF91 N/P/S, FF92 N/P/S, FF93 N/P/S, FF91 TC/LV/BL, FF92 TC/LV/BL, FF93 TC/LV/BL, and field trial models 896P, 8996P and FF73P

It describes an earlier version of the FF92 which had separate heaters for evaporator and gutter. As far as I can see on this appliance the gutter heater is an extension of the evaporator heater. It’ s a small difference but it affects the self-test mode since there is no gutter-heater test.
The SM also does not mention the self-test mode displaying the evaporator thermistor temperature when Zone is “Freezer” and Save button is pressed.
It also states that SM mode switches back to normal operation once done but my experience is that it keeps the compressor running constantly (freezer displayed -26C whilst fridge was at 2C when set to 3C; evaporator was -34C).

However, it does provide a lot of detail including the expected resistances of the thermistors. It also confirms that all three thermistors (fridge, freezer, and evaporator) have the same specification and they match the readings I’ve been getting:

+25C = 2.7k? (component specification; all)
+21C = 3k? (defrost cycle max evaporator temperature; evaporator)
-15C = 19.5k? (evaporator fan trigger; freezer)
-19C = 25k? (compressor trigger; freezer)
-22C = 29k? (compressor trigger; freezer)

Evaporator fan doesn’t operate above -15C (19.5k?) or immediately after a defrost.

I checked the components once again by freezing them in another appliance and measuring their resistance.

The solution, though, was discovered when dismantling the freezer down to the evaporator. Page 20 of the SM has a side-view line-drawing of the evaporator where the thermistor and thermal fuses clip on.

I noticed that it appeared different to the actual locations I’d refitted it after the thermistors were replaced the last 2 times.

The diagram shows the thermal fuses clipped on the lower returns of the evaporator pipes with the thermistor on the upper pipe. This is important since those pipes are set at a 60 degree angle with the upper pipe forward of the lower pipe. The thermistor *should* be clipped such that it lies in the U-shaped groove in the front of the evaporator fins so that its tip reaches the 5th fin from the right.

Let’s see if I can manage an ASCII diagram of that right-side view:

------------

|          |

| O    O   |

)      |

|   O    O |

|   H      |

| O    O   |

Q^     |

|   O    O |

|          |

| O    O   |

)      |

|   O    O |

|   H      |

~          ~

In this diagram “O” is the pipes as-if seen end-on, the ” ” are the pipes in profile, “H” are the two heater thermal fuses, and “Q” is the evaporator thermistor with its clip “^”.

I had, at some point, refitted the evaporator thermistor such that it was on a lower return and it was pressed into the center of the evaporator (where things are much colder!) rather than laying across the front of the fins.

That would lead to the unit switching off prematurely since – as the self-test evaporator temperature display revealed – the temperature was -40C or below.

After switching on in self-test mode once more, with an ambient temperature of 25C, the freezer reached -22C in about 1.5 hours and the fridge +2C within 2 hours. The evaporator temperature displayed as -28C.

It continued cooling with the fridge at 2C and freezer at -26C with evaporator at -34C. At that point I switched off for 10 minutes then powered on in normal mode. It displayed 2C/-22C (digital thermometer showed -26C) and the compressor wasn’t started.

[cmthomp]

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

Interesting post as I’ve had problems with my FF92S since I switched it on when I returned from holiday. On mine I am getting error Eb which points at the Fridge Baffle motor running continuously. Sure enough when I’ve had a look it is continually cycling. The PCB appears to show a burnt Triac so I’ve ordered another to hopefully fix the problem.

The symptoms are similar to those above though – the freezer will happily freeze down to an indicated -15C, but as soon as that is reached it must trigger the motor, realise the problem and show the error code. The freezer stuck at -15 overnight with the error code showing.

[feefifofum]

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

cmthomp wrote:On mine I am getting error Eb which points at the Fridge Baffle motor running continuously. Sure enough when I’ve had a look it is continually cycling. The PCB appears to show a burnt Triac so I’ve ordered another to hopefully fix the problem.

The Triac for the baffle motor is TY3 marked “Z7S Y227” and is a Z0107MN 4-quadrant Triac.

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