Cooker thermocouple failure - what to try next

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dovekie

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We have a Plastimo Neptune 2500 gas cooker.
The flame failure device on the grill has stopped working: that is, the grill will light and burn fine, but as soon as you stop pressing in the control knob, the burner goes out.
This is the type where you turn the knob to get gas flowing, but you have to press and hold the knob in for a few seconds after the flame has lit, in order for the flame-failure thermocouple to activate, and maintain gas flow when you stop pressing in.
So making toast require holding in the grill knob, even after 5-10 minutes. All the other burners and flame failures devices are working normally.

I have tried:
- cleaning the thermocouple tip
- removing the thermocouple cable from the control knob and cleaning the contacts
- swapping in a (brand new) thermocouple
All make no difference.

What should I try next? Any thoughts appreciated.
I have found this thread (http://www.ybw.com/forums/showthread.php?513356-Cooker-thermocouples&highlight=flame+failure+device) which tempts me to take magnet and see if I can sidestep the problem for now.
 
Ours also acts up every once in a while (different brand cooker, but they all work much the same way). They greatly benefit from a few semi-gentle taps with the long stem of the gas lighter on the burner whilst lit.

Like Ruffles says, this seems to also shake lose whatever dust/rust/debris may have gotten into the burner holes, flush it out and burn it off. Give it a try!
 
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Easy enough to replace, but don't pay more than you have to. When mine failed (Techimpex stove, although I suspect they are pretty generic on most boat cookers) I replaced with thermocouples for an 'Archway' doner kebab machine! £5 each or 3 for £12. Ebay is your friend
 
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A solenoid can be driven by the miniscule power of a thermocouple? That seems rather far-fetched to me, and even less likely that the solenoid would have the power to drive a gas valve in the absence of any other electrical power for the solenoid to activate it with.

Surely it is a gas-expansion device that actuates the gas-valve in a simple non-electric cooker like a Plastimo?
 
Old Bumbulum said:
A solenoid can be driven by the miniscule power of a thermocouple? That seems rather far-fetched to me, and even less likely that the solenoid would have the power to drive a gas valve in the absence of any other electrical power for the solenoid to activate it with.

Surely it is a gas-expansion device that actuates the gas-valve in a simple non-electric cooker like a Plastimo?
Click to expand...

Nope, was a surprise to me too, the gas valves are electric driven only by a thermocouple.
Means that old cookers are a neat source of electrically operated gas valves though...
 
Old Bumbulum said:
A solenoid can be driven by the miniscule power of a thermocouple? That seems rather far-fetched to me, and even less likely that the solenoid would have the power to drive a gas valve in the absence of any other electrical power for the solenoid to activate it with.

Surely it is a gas-expansion device that actuates the gas-valve in a simple non-electric cooker like a Plastimo?
Click to expand...
Electric as the Scottish man says!
 
Ruffles said:
This happens on our cooker occasionally. The thermopile/thermocouple drives a solenoid in the gas tap and that is probably sticking. I've always found a sharp whack on the cooker sorts it out! Seems to improve the gas flame as well. Or is that my imagination?
Click to expand...

Thank you all. I have replaced the thermocouple, with no effect - apologies if I was not clear.
I shall try whacking it and report back!
 
Same problem some time ago.
We thought that the probe was not in the correct place in the flame. lots of messing around trying to move the prob tip and cleaning the jets to get a 'better' part of the flame onto the probe end.
No idea which worked but it has been ok for years since! ( possibly our 'messing' acted like the sharp taps recommended above?)
 
Check that the tip of the TC is in a flame otherwise it will not detect the heat.
Check the resistance of the centre connection and the outer shroud, should be about 6 ohms
You may need to replace the gas valve as it could be a faulty solenoid
 
I shall check the tip is in the flame, the old one failed gradually and had not moved, I put the hew one at the same point.
I can check the resistance, indeed, presumably I can check the voltage of the thermocouple, in and out of the flame?
The electrical connection to the valve looked clean, but I shall double check.
Then I guess I move on to the solenoid and gas valve.

Thanks again.
 
dovekie said:
I shall check the tip is in the flame, the old one failed gradually and had not moved, I put the hew one at the same point.
I can check the resistance, indeed, presumably I can check the voltage of the thermocouple, in and out of the flame?
The electrical connection to the valve looked clean, but I shall double check.
Then I guess I move on to the solenoid and gas valve.

Thanks again.
Click to expand...

Cooker TCs are normally K type.
Ambient temp they will measure around 1 mV
600 deg around 28 mV
Use only a high quality digital multi meter
 
dovekie said:
I shall check the tip is in the flame, the old one failed gradually and had not moved, I put the hew one at the same point.
I can check the resistance, indeed, presumably I can check the voltage of the thermocouple, in and out of the flame?
The electrical connection to the valve looked clean, but I shall double check.
Then I guess I move on to the solenoid and gas valve.
I recall it was very frustrating !! Cant remember the exact facts gleaned about the hottest ( best ) part of the flame.
As the other bits are working can you swap parts around from the working burner(s) to eliminate what is working or not?
Thanks again.
Click to expand...
 
An update:
- Both thermocouples, the old and the new, no detectable voltage at room temp, 0.002V when the flame.
- Connection to the solenoid looked clean, but I have cleaned it anyway (no effect)
- Tapping the solenoid - i.e. assembly behind the control knob, did indeed work! In that the flame will now stay lit, after about 40 seconds.
- BUT it needs fairly vigorous whacking each time. If I don't whack it, the thermocouple + solenoid mechanism does not work.
I can live with that.
 
This happened on my oven a couple of times shortly after I got the boat. I was very dubious of the PO's advice to pull off the knob and put a few drops of WD40 down there, but it does indeed fix the problem perfectly.

Mine is a Dometic / Smev 300-series.
 
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