Hotpoint CTD85A thermistor/heater/timer problem?

[HOLLYW00D]

Hi,

First time posting here so apologies if I've got anything wrong in terms of forum etiquette. I also apologise for the long post - I just want to be thorough.

In November last year, my condenser dryer stopped working. It threw up error codes f-08 and occasionally f-10 on the LCD display. The dryer basically stopped heating up and would keep stop/starting on every cycle.

Ive always ensured I emptied the water and cleaned the filter prior to every use. I must admit I only cleaned the condenser maybe twice a year but it was always pretty clear anyway.

However, took the top and sides off to Hoover out the fluff inside and also checked out the heater element - I noticed the element itself was damp and was corroded. One of the elements wasn't even connected due to corrosion!

I ordered a replacement from hotpoint which came complete with the two thermostats and thermistor - I fitted it and it ran fine for a few days then it threw up the f-08 code frequently.

It's been like this ever since. I select a cycle, it starts fine, runs for about 15 mins, stops, adjusts the remaining time to dry, then it continues to run.

However, it always stops/starts every few minutes, it runs for 30 seconds or so (sometimes longer) then it stops for a few minutes (sometimes shorter/longer) yet the countdown timer on the display reduces accordingly.

It seems to get stuck on the last 10 mins everyt single time - I deliberately leave it to keep going until the machine beeps and showed f-08 and I have to turn it off at the wall.

I've tried resetting the thermostat so many time but makes no difference (it has never clicked when I push a pin inside the hole) - the element looks fine this time too although there is evidence of moisture at the bottom and around the bulged metal backplate/cover.

I bought a basic multimeter to test the two thermostats, they both show continuity so I assume they're fine? - never removed the thermostats to test them - I just unplugged the connectors - is that OK?

I've tried to get my head around thermistors (front and rear) too but suspect I don't fully understand it...

I once again never removed them to test. I set my multimeter to ohms (20k) I touched the probes to confirm the meter works and attached the probes at room temperature on the rear one - the meter just showed "1" which suggests that there is no reading registered. I blew a hairdryer onto the thermistor for about 30 seconds and re-checked but the meter read the same.

I repeated this method on the front thermistor which gave exactly the same results.

I assume both the thermistors are faulty?

By chance, I still had the old heater element I replaced so I tested the thermostats and the thermistor on that one which also did the same...

Any help would be appreciated because I'm tired of trying to polish a turd hahaha

Cheers!

Hotpoint CTD85A
Serial number - 502010224
Part replaced was "heater (one shot+cyc) C002258828

 

[yardie]

Ok sounds good testing but with the thermisters change the meter to a higher ohm range setting(go up one setting at a time), these are not simple open or shut switches like the thermostats but resistors that change resistance according to temp. Might be 100kohm thermisters! Not exactly sure what rating they should be.

Also with the thermostats test again but with meter on highest ohm range, are the results the same?

Now apply heat to thermostats with a hairdryer, when it operates you should get zero ohms on lowest range setting.

All tests with stats etc disconnected.

 

[HOLLYW00D]

Cheers, mate!

When I tested the thermostats, I used the continuity setting on the multimeter - should I have been using ohms then?

Also, can you clarify...Should I also physically remove them to re-test or is ok just to disconnect the wires to them?

Sorry if I'm being thick...

 

[yardie]

Just disconnect wires.

Continuity setting should be fine with thermostats to test they are closed circuit but I would use ohm range for testing open circuit, your meter probably detects anything below 20ohms or there abouts as having continuity.

 

[HOLLYW00D]

Hi Yardie,

I wasn't brave enough to try it out on the appliance so I practised on the old "faulty" heater element which has the original thermostats plus what I've been told, a thermistor. This was just out of curiosity - I'll give it a go tomorrow on the one currently on the dryer.

The heater element has three circular thermostats/thermistors or whatever they're called (sorry) on the rear. See attached pic.

As you look at it, the left one is black with a white spot on it and has a hole which supposedly you can insert a pin into for resetting?

The middle one is white - this is the one I believe is the thermistor.

The one on the right is a plain black one.

I did as you suggested and tested these on the Ohm settings from lowest to highest (200 - 20m). The two black ones had similar readings at 200 Ohms and 2000 Ohms. The following were done at room temperature.

The one with the White dot read 00.7 to 00.8 at 200 Ohms / 00.1 at 2000 Ohms.

The plain black one read 00.8 to 01.3 at 200 Ohms / 00.1 at 2000 Ohms.

Both the above thermostats had a zero reading at 20k, 200k and 20m Ohms.

The White one had zero readings at 200, 2000, 20k, 200k but read 0.43 at 2m Ohms???

Then, I put all three onto a griddle to heat up for a about a minute or two(not sure what temp though) and I re-tested them - the two black ones gave a zero reading across every Ohm setting and the White one gradually reduced to zero as it got hotter until it got to zero too. It started to rise up on the meter as it cooled and the other two black ones "popped" a few minutes later and gave similar readings to when tested at room temperature.

I'm not really sure what any of that means apart from perhaps my original ones were OK?

Anything else you think I should do tomorrow or is repeating this process suitable in your opinion?

Thanks again, man!

 

[yardie]

Sounds like the thermostats are shot on your old element.

A thermostat is basically a switch that opens and closes at a certain temp. Different stats have different operating temps indicated by the coloured spot.

On a dryer the stats are usually cut out rather than cut in, so at room temp the switch is closed (0 ohms), when it gets to a certain temp the switch opens i.e cut out (infinite ohms in any ohm range setting).
Any reading less than inifinite when stat is open is indicative of a failing stat just is any reading above 2 ohms (taking into account the test probes) when the stat is closed.

You are safe to test the stats on your machine as long as it is unplugged from the mains. Use a hairdryer to heat the stats if still in situ, just keep the dryer moving around and not in one place all the time. Usually you hear it click when opening/closing.

Just watch out, some stats are a one time operation and are sealed so have no reset button/pin hole.

A thermistor will have a resistance at room temp and will vary as it is cooled or heated up but will still have a resistance as opposed to a thermostat that is either no resistance(infinite) or zero ohms (short cct).

I don't think its a thermister you have there.

 

[HOLLYW00D]

Hello again,

I noticed that on my old heater, the thermostat colours were as previously mentioned in my earlier posts, however, on the new one, the thermostat on the right (previously just plain black) now has a yellow dot on it. Does this mean it's the wrong one? I gave all my details to Hotpoint when I ordered it so I assumed it's correct.

Anyway, here's what I got on the test just now :

Black thermostat with white dot - read 00.4 (20 Ohms) at room temp - heated up on griddle before it clicked after 1 minute 22 seconds - tested it and it read as "1" on the display (I assume that's infinity/open circuit) - it cooled down, clicked again so I tested it again and it read 00.3.

Is that OK?

White thermostat - read 0.57 (20m Ohms) at room temp - heated up - reading slowly reduced to 0.01 after being on heat for over 2 mins - got worried so I took it off. As it cooled, it gradually rose to a reading of 0.57 again.

Should this thermostat also have gone to "1"?

Finally, black thermostat with yellow dot - read 00.3 at room temp (20 Ohms) - put on heat, clicked after 32 seconds so removed from heat and tested - it read "1". After a while, it clicked again so tested and it read 00.3.

Is that OK?

My head hurts!!!

 

[HOLLYW00D]

Sorry, just realised a mistake. The two black thermostats were tested at 200 Ohms (not 20)

 

[yardie]

Seems as though both black stats tested ok.

As for the white one, yes its a thermistor. Higher resistance at room temp, heat up and resistance decreases.
Connect to white one again and set the meter to its lowest ohm range, do you get '1'?
Now change meter setting to next setting up, if its still reading '1' step up the meter setting again until you get an actual reading.
Reply with the meter setting and meter reading.

To be honest as you have already replaced the heater incl stats the problem is more likely to be with the front thermistor or possibly the control board. Especially as you still had problems after replacing heater/stats.

Where is the front thermistor that you have located? Looks like there are 2 at the front.

 

[HOLLYW00D]

Hi,

I re-tested as you said. The White thermistor only gives a reading when the multimeter is set to 20m Ohms. Every other range reads "1". At room temperature it read 0.58 - I re-heated it and once again, it went over two minutes and still never cut-out. It still read 0.01 but began to rise as it cooled.

There is only one front thermistor on my model, I took this off to test it at room temp - it registered 0.38 once again only at the 20m Ohm setting though.

I heated it up the reading gradually reduced down to 0.00 but it didn't click like the others. I took it off the heat and the reading gradually rose as it cooled.

I now have a separate problem, I took this front thermistor out and didn't make a note of which terminal went where - there aren't any markings on it so I can't tell which way to put it back?!?!?

Will it damage anything further if I connect it the wrong way?

Help!

 

[yardie]

Only has 2 terminals, doesn't matter which way!

Reduce the meter range as you heat it when it gets down to zero in the mega range.

Thermistors do not click open/shut.

Thermistor unlikely to go all the way down to zero ohms.


Does look as though the thermistor is working but the question is: is it working correctly - in the specified ranges?

Have you checked that the fan is attached properly and definately turning, circulating air flow.

 

[HOLLYW00D]

Ha ha I assumed it would be OK but got paranoid.

So, do you reckon it's a dodgy thermistor causing this?

Do you still need me to re-test both thermistors on heat but lowering the range on the multimeter every time the reading gets to 0.00?

what should I look out for to tell if they're knackered?

Really appreciate all your help so far, mate - sorry for being a bit thick.

 

[yardie]

Yes test anyway changing meter range as it nears zero in the mega range to see what it drops to.

Results from both front and rear can be compared.

Also test the old thermistor from the old heating element.

Are there any markings on the thermistors? It is possible they will have different ratings. I.e at 25degs read 560kohms, at 100degs read 20kohms. These are not specific figures nor what you should expect just as an example.

 

[yardie]

Just for your info.
A cut-out Thermostat (normally closed) whilst closed will allow current to pass through it, as it heats (different stats operate at different temps) the thermostat will open and not allow any current to pass. So effectively an on/off switch.

A cut-in Thermostat (normally open) whilst open will not allow current to pass through it, as it heats (different stats operate at different temps) the thermostat will close and allow current to pass. So effectively an off/on switch.

A thermistor is really just a resistor but its qualities change according to heat. At ambient room temp say 25degs its resistance will be at a specified resistance, as it heats up the resistance drops, as it gets colder its resistance increases. Thus allowing more or less current to pass according to temp.
For example, @ -40degs 330kohm, @ 25degs 100kohm, @ 100degs 20kohm
Of course specifications and ratings differ considerably.

 

[HOLLYW00D]

Hi, mate!

Thank your for that info - sorry it's taken so long to reply.

I haven't got round to re-testing but out of curiosity, I wanted to look at the pump. When I opened the grey plastic cover on the bottom right of the dryer, the little chamber that the pump/float sit inside was about a quarter full of water?

Is that right or should it be near enough empty?

I'm wondering whether there is a actually a problem in this area because after I drained this water out and used the dryer, it worked perfectly 3-4 times before it displayed the F-08 error again.

I repeated what I did and it worked fine again two out of three times yesterday.

Coincidence or possibly a genuine issue?