Indesit Innex xwde1071681x F15 approx 15-mins into drying cycle

[carl0s]

Hi. We have an Indesit Innex xwde1071681x.
It gives an F15 fault about 10 minutes into the ‘air refresh’ cycle, which seems to just be a gentle drying cycle without the water intake spurts.
On a normal drying cycle, it gives the same F15 after about 15 – 20 minutes.

I have checked and cleaned everywhere – I think – pump and hoses, condenser plastic thing at the back, the green “filter” between that and the drum, which does looks slightly warped and misshapen now. The heater/fan unit is clear too. Wiring all looks OK and undamaged.
The heater element works, things get very toasty on top, and the blower fan runs like it should.
The heater unit’s NTC thermistor (a 20K one), seems to give reasonable readings – approx. 20K at ambient indoors, down to ~2k with a kettle of hot water running over it if I recall correctly.

During the drying cycle, I can see that the relays briefly cut power to the dryer heater element every now and then, but only for about half a second, which seems kind of pointless and maybe strange. Is this normal, or could it point to an issue with the relays on the PCB? Or the transistors and associated circuitry that drives the relays? I am competent enough to replace those PCB mounted relays and can get hold of suitable replacements. I’m not competent enough to reverse engineer or diagnose other electronic faults on the PCB, but I have test gear and can follow instructions, and am competent enough to replace surface mount components.

It seems strange. Could the heater element have a fault that only shows after a while? Or maybe the thermistor is ‘apparently’ working but the readings don’t match what the system expects?

Does anyone have any ideas for me to try? I was going to just replace the relays to see if that helps. Trying to start with the cheapest parts. The NTC thermistor is a bit of a rip off at £15 for what it is.

thanks very much.
Carl

[electrofix]

could be earth leakage on dryer element

Dave

[carl0s]

electrofix wrote:could be earth leakage on dryer element

Dave

Hi Dave. Thanks for this.
I tested earlier, and I measured infinite resistance / open circuit between heater AC lines and earth.
However, after repeating the failure just now (another attempted drying cycle – not sure how long ago it failed.. could be 15 to 20 minutes ago) , I measured less than 0.2MOhm (a tenth of what is stated above), which was gradually increasing. 5 minutes later it’s up to 2.2MOhm.

I’ve got the element out and am testing it while watching telly, and readings are all over the place though. 9 – 10 meg one minute, 2.2 the next.

It’s a 1.2KW element and shows 43 ohms across it, which would be correct.

[electrofix]

sounds like the element resistance is reducing with heat

Dave

[carl0s]

Yes it did seem to be varying its resistance to earth with heat. The resistance across the two terminals was consistent at 43 ohms though. I had read that they could hold damp, which would explain varying resistance to earth with heat. I tried baking the element in the oven for an hour but this made no difference.
I bought a new element, and what do you know, it works.
I’m quite surprised that the element could appear to work, heat up properly, yet cause this fault.
I’m surprised the machine can sense something subtle like this. I sort of expected it would just switch mains across the relays without much finess or monitoring of the mains power to the element. I know the drum motor is 3 phase brushless induction with fancy control, but I didn’t expect a similar degree of control of the heating element.
I also would have thought an appreciable earth leak would trip the RCD in the consumer unit.
Anyway I did more tests and the bad element did show some conductivity between its terminals and outside metal bits, with less resistance as it got hotter. The new element shows open circuit / infinite resistance.

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