Thetford 931 hob flame failure

[Bivers]

Hello All

i have a Thetford 931 3 ring hob problem, all 3 rings will light no problem but as soon as I let the gas valves out they go out, it was an  intermittent problem to start, I know this can be a thermocouple fault but I find it hard to believe all 3 would fail at the same time, any idea greatly received.

 

[Tracy D'arth]

Are the knobs touching the facia before they have fully pressed the gas valves open and engaged the electromagnetic solenoids? Try it with the knobs off.

[Bivers]

Thanks for response I’ll give it a try later👍👍

[MtB]

Agreed, this in theory is a fault that can't happen!

 

 

A quick look at the Thetford parts list shows the hob is of conventional design with three thermocouples and three thermoelectric valves inside the control valves for each ring, so yes it's got to be something common to all rings and probably basic like Tracy suggests.

 

[PeterF]

This has happened to mine, boat is not used over winter, it first happened in early December when I went to winterise the boat and then again when I visited to check in January. I need to sort it before spring. The first thought I have is damp if it got into the connectors as we got a fair bit of condensation on the winterising visit. The second thought is lower than normal temperature on the boat when last visited which would have needed the knobs holding down for longer. My plan on the next visit is to hold the knobs in longer and if still no joy then check the connectors. Both of these thoughts would affect all 3 burners. The only other thing is checking the knobs as mentioned above because I recall that my wife removed them to clean the hob after out last trip.

 

The thermocouples produce a very small voltage and anything impeding this could easily cause trouble. I found the following link with some info.

 

https://www.caravantalk.co.uk/community/topic/143848-thetford-spinflo-enigma-hob-flame-will-not-stay-alight/

Edited February 4, 2022 by PeterF

[nicknorman]

Good link! In summary, if the hob has a folding lid as many do, there is a (multi way) switch on the hob lid that shuts off all the burners if the lid is closed. If the switch sticks, all the burners don’t work…

[Slim]

Don't have any knowledge of the hob in question but do the individual knobs depress for ignition and then pop up when released. Is so are they popping up fully. I ask because I've got a similar issue with one knob on my Vannette hob. I think in my case I was a little to liberal with hob cleaner and the knob 'shaft' is slightly gunged up.

[PeterF]

13 hours ago, nicknorman said:

Good link! In summary, if the hob has a folding lid as many do, there is a (multi way) switch on the hob lid that shuts off all the burners if the lid is closed. If the switch sticks, all the burners don’t work…

The hob in question does not have a lid, the only commonality on the 3 burners / thermocouples /  valve assemblies / knobs is the gas supply which makes it weird to work out the issue. The thermocouples do not have a common power supply, the voltage generated by the thermocouples is used directly in the valves.

[Tracy D'arth]

Its the thermocouples that generate the power to hold the gas valve solenoids engaged.

[MtB]

As someone in the caravan thread comments, each thermocouple needs to be securely connected electrically to earth as well as to the coil in the on/off gas tap. It is common for gas bods not to realise this and not pay attention to securely tightening the physical fixing at the gas burner end. 

 

Anyway as also suggested in the caravan thread, the next step is to disconnect each if the three thermocouples and measure the voltage on their terminals with someone else holding the gas ring alight. Tell us the results. You should typically get about 23mVdc on each. 

 

If you're getting the right voltage then I'd move on to measuring the continuity of the coils inside the on/off valves. It should be near as dammit zero. 

 

(All this assumes you've checked the really obvious, that the thermocouple tips are nicely enveloped in th gas flame and getting hot.)

 

[Bivers]

Thanks Everyone for your help.

it was indeed the knobs, they weren’t pressing down enough, I packed out with a bit of tin foil and they are working perfectly now.

[Tracy D'arth]

Knew it would be. Its an old chestnut, clean the hob, push the knobs back on too hard and the hit the facia first. Or the hob top warps upwards with hot or cold and does the same thing.

[MtB]

42 minutes ago, Bivers said:

Thanks Everyone for your help.

it was indeed the knobs, they weren’t pressing down enough, I packed out with a bit of tin foil and they are working perfectly now.

 

Excellent news and thanks for the update. Much appreciated as not everyone bothers....

[PeterF]

Thanks for this update, i plan on visiting my boat in a few days so I will be sure to take some kitchen foil with me.

[PeterF]

Went down to the boat today, lifted the knobs up 5mm and all 3 rings lit straight away.

 

Common mode failure on 3 independent burners - cleaning the hob.

 

Thanks for the advice on this, which was well received before I started trying to do anything else to fix it.

[Bobbybass]

Is it very old ?

 

Thetford have a three year guarantee on much of their range.

 

It's not a ....return to base.....they will send an engineer canal side....they did with me.