No dryer heat on Hotpoint WD640

[GJPalmer]

Hi I am new to this forum,

The dryer section of our Hotpoint WD640 has stopped working, I powered down the machine and tested the one shot thermostat which I found to be an open circuit, ordered a new one shot thermostat from UKWhiteGood which I have fitted…

Heater box is now working but there seems to be no control on the heat element, is there any way of testing the control thermistor for correct operation?

I know it is not good practice to do live testing but I have checked the voltage on the dryer element this seems to be fixed a steady voltage of 230v AC.

I am used to testing live circuits since I am an industrial maintenance electrician.

This voltage does not alter as a temperature of the heater box goes up.

Thank you for any help or advice given on this matter.

– GJPalmer

[helo_75]

if you are an industrial maintenance techinician, and you think that the voltage to the element changes as the temperature goes up, what exactly do you repair?

the element is switched on and off by the pcb as the thermistor dictates the temperature

just take the heater box off and look for blockages of fluff, and i think youll find the problem….. and please do it with the plug out… its always a safer option, and any fault can be found with a basic multimeter, with the power off

[GJPalmer]

Re: No dryer heat on Hotpoint WD640

I have already removed the heater boxed and cleaned it of all the fluff after first removing the 13 amp plug top.

What is your preferred method of testing the heater box assembly?

Also how do you check the PCB with a multimeter then please?

Thanks for any reply

[timdowning]

Re: No dryer heat on Hotpoint WD640

How long did you test for voltage for on the heater?
It can take quite a while for the the dryer to reach temperature before the power is switched off to the element!!!

The resistance of the thermistor will alter with temperature. The resistance will be in Kohms and will go down with an increase in temperature.

[timdowning]

Re: No dryer heat on Hotpoint WD640

P.S

The thermistor can be checked and tested with the machine unplugged!

[GJPalmer]

Re: No dryer heat on Hotpoint WD640

Thank you Tim for your posts.

I have only run the dryer up for about 15-20 minutes did not see any reduction in the applied voltage to the heater element.

I then unplugged the machine and did a resistance test on the thermistor this was at about 950 Ohm’s, was not able to check the temperature on the surface of the heater box.

Is there a set resistance on the thermistor when the heater voltage drops off?

If so can I test it with a resistor of that value?

Thank you

– GJPalmer

[iadom]

Re: No dryer heat on Hotpoint WD640

A simple question, have you actually tried the machine with a load of clothes since you changed the TOC?

It is very unlikely the dryer would get sufficiently hot in 15 to 20 minutes to cause the thermistor to signal to the relay on the PCB to switch the current to the heater off. The voltage is not ‘reduced’ it is switched off.

To test a thermistor safely, remove it from the machine and then boil a small cup of water, check the resistance of the thermistor then immerse the metal face of the thermistor in the hot water for 15 or 20 seconds and test the resistance again, if it has altered then in most cases the thermistor will be fine.

The thermistor should measure 26kΩ @ 20°C.

I would also add that to keep running the machine then turning it off whilst it is still hot to test the thermistor is a sure fire way to kill the TOC again. 😉

Jim.

[GJPalmer]

Re: No dryer heat on Hotpoint WD640

😀 Thank you all for your help problem sorted I did not try the unit out long enough and was expecting the voltage on the heater to be a pulsed output i.e the applied voltage on full at first then cycling on /off at a lower pulse rate when the temp is at its desired level. This method of temp control is some times used on injection moulding machine heating. A thermocouple is used to input an analogue input P.L.C card and this information is then used by the program to send a signal to a SSR(Solid State Relay) when the desired temp is near it setpoint the mark space ratio signal to the SSR altered to give a shorter on time this is to cut down temperature over shot
Once again thanks for everyones help

[Hamgray]

Re: No dryer heat on Hotpoint WD640

Hope it’s ok to add to this older post.
I too have the F13 error on my WD640.
The thermostat trips out when the dryer metal box overheats.
The cut-out is of the style that can be reset with a paperclip.
Would I be right to think that my Thermistor (temperature sensor) is not working? I guess it’s the item with two thin black wires connected to it? Seem to recall it was open circuit when i checked but see here It should be 26K. If o/c would that mean the control panel powers the dryer until the thermal cut-out trips?
Guess I could test it with a variable resistor wired in to see if the heater stops with a low resistance feed?
If so, any ideas where I can obtain a replacement in the UK.
Have already removed the metal lid, cleaned all inside (no fluff as hardly been used). Motor spins ok with no rubbing marks from (plastic) fan.
Many thanks for your help.

[Hamgray]

Re: No dryer heat on Hotpoint WD640

Can anyone please tell me what the resistance of the sensor should be when it signals to the PCB to turn off the heater?

[iadom]

Re: No dryer heat on Hotpoint WD640

Very unlikely, the manuals do not give this information.

As long as the thermistor is around 26k? @20C and its resistance rises and falls when heated or cooled then it will be OK.

In fact the earlier thermistor used on these was fairly reliable. your problem is far more likely to be a blocked rear filter, also constant tripping and resetting of TOC will cause it to fail more often.

At the very back of the unit with the whole of the metal dryer box removed you will see a very small plastic gauze filter, usually with a small hose attached.

Remove this filter and using a small craft knife or something similar, cut out the gauze ‘windows’ and discard them. This was common practise for Indesit company engineers.

I am also a little suspicious of the cut out on your machines, these should not have a resettable TOC fitted. Has it been repaired in the past?

[Hamgray]

Re: No dryer heat on Hotpoint WD640

Thanks for the update.
Had another go at the machine today by wiring in a variable resistor in place of the thermistor and changed the resistance hoping to get the power to the element to switch off. It didn’t, but I did get the F13 error again. Seems that a short or open from the thermistor gives that error. Tested the thermistor in hot(ish) water and the value went down to 4k, so appears to change resistance.
Is testing the dryer empty going to cause any strange symptom?
Wondering if, without wet clothes is not a good idea?
I’ll have a look at the box you mention, guess it’s the condenser.
Although can’t quite see which part I need to look at.
Thanks a lot for you advice.

[iadom]

Re: No dryer heat on Hotpoint WD640

No it is not the condenser, It sits on top of the condenser which is actually moulded into the outer drum. If you remove the dryer box as a complete unit then you will see the tiny filter at the back, it usually has a small, grey corrugated hose attached but the hose was not used on some models. It just lifts out. Cut out the gauze and replace, see attached picture.

If you do clear any obstructions I would certainly replace the TOC with the correct one, I repeat it should NOT have a resettable TOC. The TOC should be either a small, round black plastic one that can only be replaced be opening up the heater box and unscrewing from inside. Or the later TOC which was located inside a thin yellow sheath attached to the top of the dryer box.

You didn’t answer my previous question, has the dryer side had any repairs done to it in the past?

[Hamgray]

Re: No dryer heat on Hotpoint WD640

So far I have only removed the top cover of the dryer metal chamber. I guess to see the filter, the whole box has to be removed – ie. including the metal base of the dryer? Have to pluck up courage to do that – looked a bigger job!

I bought the machine new in November 2004, but it’s not been over used. Maximum 3 washes a week (sometimes only 1or 2) and usually using the quick ‘E’ wash (1 hour). The dryer part has probably only been used less than 10 times in it’s 10 year life.

It has never been repaired and (so far) given good service.
The overheat cut-out definitely is the resettable type by pushing an open paper clip between the connector tags from the top of the switch. I read on another forum that some do have this, so tried it!

I’m really grateful for your advice, I’m itching to get it working again. I may have to buy a new thermistor anyway, as I have now broken the plastic clip which holds the small connector to it. So far the solvent glue is holding for testing purposes!

[iadom]

Re: No dryer heat on Hotpoint WD640

I would advise getting a new TOC as well if it has tripped so many times it will be weakened.

It is possible that resettable cut outs have been fitted on production, not come across one myself TBH.

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