Thetford N100E fridge problems

[ChrisInNotts]

Hi,

I wonder if anyone could help as I've Googled but not found anyone else with a similar problem? Our fridge (Thetford N100E: manual select but with electronic ignition) cools pretty well on 12V/240V and gas and has been OK up until our last trip. Now, after a number of hours on gas the flame goes out (or at least the gas indicator drops into the red) and it just sits there clicking away to itself failing to re-light. It lights straightaway when you first turn it on (ie pressing and briefly holding the thermostat knob) so no obvious problems with the thermocouple/gas valve. It will also relight manually again by holding the thermostat button in and appears to be OK but then goes out again a number of hours later. The fridge is now sat on my bench and the burner and jet were both clear of any obvious carbon build-up so it appears there is no obvious reason for my problems. There is no pilot jet.

Thus, I assuming the problem seems to be happening when the fridge throttles back the gas when its down to its desired temperature. These are pretty simple fridges but I cannot tell from the information I have if the thermostat shuts the flame off completely or throttles it back to a low level. If the latter, then perhaps the thermocouple has partly failed so is unable to detect the reduced sized flame? This would also make sense if the automatic re-light is performed at a low flame level but the manual start uses full gas pressure. If the former then I am really baffled as I don't see why it lights manually but not automatically? From what I can see the part number for the (very expensive) gas control/thermostat valve is the same as in the manual ignition fridge so that suggests to me that the thermostat regulates cooling by flame size rather than cycling the burner on a fixed flame size. Can anyone confirm if this is the case?

I cannot see the flame in situ and its not quick and simple getting the fridge in/out so I really want to do as much as I can to trouble shoot this before I put it back. Thus, I would be grateful for any advice/tips.

Thanks,

Keith

 

[ChrisInNotts]

I’ve spent some time on the fridge so will share my findings in case it is of use to others. As always, get your reinstalled gas system double checked by a professional!

The fridge is a very simple design and a period of bench testing has allowed me to understand how it works and check the function. The thermostat inside the fridge cycles the gas valve into a low and high position as I suspected. Thus, the flame should always be lit. However, the fridge is designed to relight if the flame blows out though there is a fairly limited window for this to happen until the main thermocouple cools down and shuts the gas valve down completely. The electronic ignitor is an interesting affair in that it “auto senses” the flame and is permanently powered when the fridge is switched to gas. Thus, with flame lit the high voltage generated by the unit dissipates into the flame (presumably due to the ions generated by combustion). When the flame goes out, the high voltage can build up and it starts sparking. I tested by blowing out the flame on both high and low settings and it re-lit straight away – quite impressive. Blowing it out 5 times in quick succession allowed the thermocouple to cool down enough to trip the gas valve. It’s easy to test the thermostat operation by unclipping the internal thermocouple and dipping into iced water. The flame can then be observed to switch between the low and high settings as the thermostat knob is moved from a low to a high position.

So why did my flame go out when the fridge was cold? I think it was a combination of a slightly clogged burner (it was actually very clean) coupled with a thermocouple position that was slightly to one side of the flame. Positioning is going to be more critical when the flame is low as it is much smaller. Thus, I suspect that on the low setting the thermocouple was cooling too much and was tripping the gas out. It’s running OK on the bench now so it can all go back at the weekend

Keith

 

[ChrisInNotts]

Blimy, these fridges are frustrating! Its been fine since the above repair until very recently when it started refusing to stay lit - the flame has continued to light well but went out as soon as the gas valve was released. I took the bumper off to investigate and was a little surprised to find a nice clean flame. I then cleaned out the burner/jet/burner box/chimney which (as expected) made no difference. Thus, it seemed that the themocouple or gas valve was faulty. I was tending towards a gas valve problem as the flame indicator is driven by the current from the thermocouple and this has always gone well in the green region as soon as the flame lit.

I took the fridge out this morning and set it up on bench so I can troubleshoot. Oh how I laughed when it lit immediately and stayed lit (the flame is the same size so that seems to rule out regulator/gas problem). After a couple of hours the ice box is down at -15 so all seems well. Thus I now have no fault to troubleshoot :mad1: The thermocouple connection to the gas valve was nice and tight so I can rule that out. I'm not sure what else I can check? Thermocouples are cheap so I have ordered a replacement to try. The gas valves are not so cheap (£60) but I'm wondering if worth replacing?

Any advice would be appreciated!

Keith

 

[Channa]

I suspect the T/c is over sensitive or as you suggest incorrectly positioned ( you have rectified that) In either event allowing the GCV to close. Once that happens only a manual re start can occur. In effect the GCV doing its job correctly.

Face value I agree with your diagnosis , the thermostat seems to be doing its job, the GCV too. the problem seems to be the TC shutting down the GCV too early. Hopefully replacing it will cure your problems. It seems the other issue of loose connections or poor connections you have addressed

Possibly of comfort that merit in your theory is if you were to time the t/cs on a 4 burner hob they all click after a different time elapsed. That suggests that there is sensitivity issues and manufacture is acceptable but not consistent

Channa

I should add for the purists the TC is doing its job too, but it seems the way the fridge is configured to work it is a tad too sensitive

 

[ChrisInNotts]

Many thanks for that Channa!

Keith

 

[n brown]

thermocouple workings

here's a gas valve from an electrolux fridge. it's a simple solenoid with a plastic washer on the sprung shaft. when you hold in the gas control knob,you are pushing against the washer and the solenoid shaft , opening the valve and letting the gas through
lighting the gas causes a flame to heat up the thermocouple, creating [by magic] a small voltage which makes the solenoid an electromagnet- this magnet hols back the shaft, so when you take your finger off the gas control, the valve stays open.
within seconds of removing heat from the TC, the solenoid is no longer a magnet, and the spring forces the washer on the shaft forwards, closing the valve and stopping the gas flow. clever innit ? if you're fairly dextrous you can test the TC and valve manually by holding the TC end against the dimpled end of the solenoid,push the shaft in and apply heat to the TC

 

[Channa]

here's a gas valve from an electrolux fridge. it's a simple solenoid with a plastic washer on the sprung shaft. when you hold in the gas control knob,you are pushing against the washer and the solenoid shaft , opening the valve and letting the gas through
lighting the gas causes a flame to heat up the thermocouple, creating [by magic] a small voltage which makes the solenoid an electromagnet- this magnet hols back the shaft, so when you take your finger off the gas control, the valve stays open.
within seconds of removing heat from the TC, the solenoid is no longer a magnet, and the spring forces the washer on the shaft forwards, closing the valve and stopping the gas flow. clever innit ?View attachment 52672View attachment 52673View attachment 52674if you're fairly dextrous you can test the TC and valve manually by holding the TC end against the dimpled end of the solenoid,push the shaft in and apply heat to the TC

A wonderful explanation as to the mysterious world of the thermocouple and it working.

Fact is they aren't consistent ...go outside light the hob all four rings leave for a minute switch off and all will click off at a different time the click the electromagnet closing the GCV

It seems the T/C is sensitive. it is nothing to do whether it works or not that seems pretty obvious it does,

SO whilst your testing method is useful , it works so not as useful really as it seems my own thoughts are it is a sensitivity issue. Happy to be proved wrong though and expecting the village idiot along soon

Channa

 

[n brown]

A wonderful explanation as to the mysterious world of the thermocouple and it working.

Fact is they aren't consistent ...go outside light the hob all four rings leave for a minute switch off and all will click off at a different time the click the electromagnet closing the GCV

It seems the T/C is sensitive. it is nothing to do whether it works or not that seems pretty obvious it does,

SO whilst your testing method is useful , it works so not as useful really as it seems my own thoughts are it is a sensitivity issue. Happy to be proved wrong though and expecting the village idiot along soon

Channa

i just thought ,with all this talk of valves and thermocouples, that there might be a few people who have never seen either and didn't know how they worked. as for the test, it will show something's wrong, but not which part- or it can show they're both ok , better than nowt

 

[ChrisInNotts]

A wonderful explanation as to the mysterious world of the thermocouple and it working.

Fact is they aren't consistent ...go outside light the hob all four rings leave for a minute switch off and all will click off at a different time the click the electromagnet closing the GCV

It seems the T/C is sensitive. it is nothing to do whether it works or not that seems pretty obvious it does,

SO whilst your testing method is useful , it works so not as useful really as it seems my own thoughts are it is a sensitivity issue. Happy to be proved wrong though and expecting the village idiot along soon

Channa

Ah...Village idiot reporting in! There are some clear crush witness mark on the thermocouple tube just down from the gas control valve. When I wobbled the tube it changed the indicated level of the flame indicator so it seems there is some damage there from the hob above pressing on it. Thus, a new thermocouple and (more) careful routing of the tube should hopefully cure the problem!

Thanks!!

Keith

 

[Channa]

Ah...Village idiot reporting in! There are some clear crush witness mark on the thermocouple tube just down from the gas control valve. When I wobbled the tube it changed the indicated level of the flame indicator so it seems there is some damage there from the hob above pressing on it. Thus, a new thermocouple and (more) careful routing of the tube should hopefully cure the problem!

Thanks!!

Keith

For purpose of clarity you are not the VI !!!

Channa

 

[ChrisInNotts]

For purpose of clarity you are not the VI !!!

Channa

Seems you are correct The new thermocouple had a similar crush mark so it appears to have been a manufacturing thing. Regardless the new thermocouple has done the trick and there is a nice stable flame indication. The fridge behaved itself over the last couple of days and relights easily so it appears all is well again!

Keith

 

[groyne]

Just back from our 3 week mini European tour, all was fine with the fridge gas, 12v and 230v till the last couple of days when the gas refused to stay lit. I managed to get the thermocouple wedged further into the flame which got the fridge going. Today I cleaned the end of the thermocouple that goes into the gas valve with some emery cloth and put the thermocouple back to it's original position. This seems to have done the trick.

 

[ChrisInNotts]

Seems you are correct The new thermocouple had a similar crush mark so it appears to have been a manufacturing thing. Regardless the new thermocouple has done the trick and there is a nice stable flame indication. The fridge behaved itself over the last couple of days and relights easily so it appears all is well again!

Keith

I spoke too soon and the saga continued. All was well for a bit but it failed to stay lit last weekend again. Extensive testing followed! I even took the gas valve out and checked it on the kitchen stove with the old thermocouple which revealed a reluctance to "hold" in even with the thermouple in a really hot flame. Thus, the fridge now has a shiny new gas valve to go with the new thermocouple and it seems to have done the trick. I now only need to hold the gas valve knob in for a couple of seconds once the fridge is lit so that looks promising. There is not much else to change now anyway so I am hoping that finally is the fix!

In case it helps others here is a list of things that could go wrong and cause the flame to go out on the “manual” Thetford fridges;

1. A clogged burner or jet. Obviously the first port of call! Bear in mind that these older Thetford fridges throttle back the flame when the fridge gets down to temperature. Thus, if it goes out when cold a poor flame pattern may not be producing enough heat on the thermocouple to keep the gas valve open.
2. Misplaced or failed thermocouple. If the tip is not correctly in the flame then the thermocouple may not produce enough juice to keep the gas valve open when the fridge is cold. If you have a multimeter then the thermocouple should generate about 25mV when heated in the flame.
3. Faulty selector switch. The selector switch disconnects the thermouple from the gas valve when the gas setting is not selected. Thus, if the fridge will not stay lit when the gas valve knob is released then ensure there is appropriate continuity with a multimeter. This can also be tested quickly by bridging the two terminals together (top of gas valve). Interestingly, the way our fridge is wired means that the little flame indicator will work when there is heat on the thermocouple even if the selector switch is faulty.
4. Check that gas valve knob ifully opens the gas valve when pressed and is not fouling. If not, the gas valve may not hold in when the knob is released. Basically, the thermouple only produces enough current to hold the valve open and not to actually pull it fully open. I know that others have had this issue and have packed the knob out on the spindle.
5. Failed gas valve. This was ultimately my problem and was pretty much ruled in as all the above were OK. I stripped it down for curiosity and the solenoid part was clean so could see nothing obviously wrong.

Hope that helps!

Keith