Hotpoint Mistral (FF92) mis-reading temperature?

[feefifofum]

In 2010 the thermistors had the usual problem and were replaced.

Last week the same problem seemed to have recurred with the temperature in the fridge rising as the freezer reported it was -14C.

Assuming it was the same issue I ordered and received replacement thermistors and fitted them yesterday.

Unfortunately this hasn’t solved the issue.

It managed to begin cooling the fridge once last night during the initial cool-down when the freezer displayed -20C and the fridge was seen at +11C (ambient was +26C) but next time it was checked the fridge was back to +25C and the condenser was silent.

It has been left on overnight. This morning the
freezer temperature is displayed as -14C and therefore the motorised baffle isn’t operating to allow cold air into the fridge compartment.

With the freezer still empty (except for air) a digital thermometer shows the actual temperature at the bottom drawer is -27C. At the middle drawer (just below the thermistor level) it shows 11C.

This suggests to me either there is some resistance in the thermistor connections or else a different problem on the control PCB itself.

I’ve been unable to obtain the service manual for it so I am hoping some experts can help.

[kwatt]

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

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.

I am no expert on those particular cabinets though so I am unaware of any nuances that they may have.

K.

[feefifofum]

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.

[feefifofum]

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.

[feefifofum]

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.

[feefifofum]

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.

[feefifofum]

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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