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Tumble Dryer Electrics

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Hi Guys

Can someone out there put me straight, I’ve got an old Hotpoint TDC 60N tumble dryer and it keeps blowing the thermostats at the back of the machine where the heating elements are and I'm trying to find out what’s causing them to blow, they are the one shot type.
I'm only guessing here but could the cause be the heating elements themselves, I have seen on a website it said for a working heater the resistance should be between 20 and 50 ohms.
Can someone tell me how to perform this test using a multimeter, I’ve included a sketch of the wiring coming from the elements but I'm uncertain where I should place the test leads in order to perform the test, if someone could mark on my sketch where I should place them that would be great, well at least I will be able to eliminate the heating elements.
Thermostats & Heater Wiring.jpg

Thanks to anyone who can help

If the dryer is overheating, chances are it's an airflow problem - plugged or restricted exhaust vent tubing being top on the list.

Also inspect the blower to be sure it's clear and able to spin freely.
exhaust vent tubing being top on the list
Yes. Most often,

Last month: The thermostat failed closed. This caused the over temp fuse to pop. I replaced the fuse and it pop again. Then I fount the bad thermostat.
To test a thermostat I place just the case of the thermostat in my coffee (hot water) and watch it open/close. It makes a tick on open/close and the resistance changes.

One more thing to test. Some times the heating element shorts to case. Current flows through the heating element going around the thermostat. Test to see if there is current flow (resistance) to case. Unplug the heater and measure to case.
Hi Fellas

Thank you both for your replies,

JonSea there are no blockages as the machine has recently been completely striped down because the steel back panel had corroded through as it’s a condenser dryer not a vented one and the steel had become porous where the condenser contacted the back panel to which I’ve bonded a thin brass plate to the inside see photo, also the fan is working fine which is driven by the drum belt.

Ronsimpson, The way I’ve tested the thermostats was to do a continuity test on them having said that two new ones were fitted only to have one of them blow after about two times of using the machine since they were fitted.

The door of the drier is not opened until the cycle has finished and the buzzer sounds as I do know if the door is opened part way through the cycle the sudden rush of cold air into the machine can cause the stats to blow.

I know the problem may well be the heating module but at a cost of £111.00 which is the cheapest I have found, I want to try and eliminate any other causes first.

Which brings me back to where do I need to put the probs of the test meter in order to check the heating elements are within the correct resistance range.

I know I’ve not shown the wiring circuit for the complete machine, only the wiring from the two thermostats and the heating elements which consists of four coils but I still need to know where I need to attach the multimeter probes in order to perform the test.

Thanks again guys for your time.
Back Assy..jpg
Brass Plate.JPG
Thermostat Wiring.JPG
Inside Front Panel 3.JPG

Sorry I got that wrong, the fan is driven by the motor and on the same shaft is a belt which rotates the drum, I got mixed up because I'd got the top off the machine and gave the drum a turn by hand and could see the fan on the back turning at the same time.
I can;t tell what's going on, BUT you have three wires going to the heating element. take the resistance to all three.

You will likely find 1-R-C-R-2

call it 1 for element 1, Some R, Common, Some R and 2 for element 2.

So, if the element's resistance is too low, your going to ruin a contact (thermostat - whatever).

So, i would EXPECT both elements to be the same R and it's possible that only one element is used sometimes. e.g. A high/low heat setting.

Look also for conductive paths to ground. e.g. a discoloration that gets wet.

Loose wires will kill contacts too.

Gas dryers, era 1960 i what I'm familiar with. There is an overtemp safety and a copper tube regulator and some safety stuff.
If the burner ignites, then the temp is regulated by the tube. If it gets too hot, the burner shuts doen.
Usually if it shuts down, it's due to lint on the sensor or a blocked vent (usually outside).

One trick that's probably not understood, is you can measure the VOLTAGE across a contact you know is closed. You may have the line voltage, some small voltage or zero. Line voltage or 0 voltage may mean open. Voltages like 10V means the contacts are bad. You basically check the health of a contact by reading the voltage across the contact. You can even do that with those cliop.
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