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Old 12-01-2008, 03:55 PM   #1
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what is the thermacoupler?

Hi We have been studying the threads here trying to troubleshoot our oven woes. Our '70 landyacht oven looks not to have been used much at all. We can get the pilot to light and will increase in flame size until 350+ degrees. It stays lit for 1-2 minutes and then goes out without ever lighting the large burner. We have tried to manually light the burner as well, no luck. Does this sound like the (mystery)thermacoupler? If so is it the part inside the oven on the side wall which attaches to the burner? And where would we buy a replacement part? We have taken that part out and cleaned it and the spring part seems to let airflow when it pushed manually. Also is it the thermostat that controls the operation of the the thermacoupler, and this is more likely the problem? Thanks!!!
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Old 12-01-2008, 04:11 PM   #2
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When you light the pilot and while holding the valve in against the spring note the size and shape of the flame. Hold the valve in for a minute to be sure that there has been an attempt to heat the thermocouple. When you release the valve again note the size of the flame. If the flame gets smaller when you release the valve and the flame goes out in a minute or 2 I would suspect the flame is not keeping the thermocouple hot enough to satisfy the gas valve. A reduction in flame size may indicate a blockage in the main gas valve.

If the flame goes out the instant you release the valve after more than 45 seconds of pilot flame while holding the valve in against the spring than I would consider the thermocouple.

The thermocouple should heat up in about 30 seconds to a point that it feeds a low voltage to the main gas valve allowing the pilot to stay light.
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Old 12-01-2008, 04:14 PM   #3
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Yes, it sounds like you have a thermocouple problem. The thermocouple is the part that the pilot light heats up, and it produces a voltage (I think) that will allow the main burner to come on. I'm sure it's more technical than this, but this is what I know. Also, if it does not work, you need to replace the thermocouple as they are not servicable.
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Old 12-01-2008, 04:35 PM   #4
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Originally Posted by cnsilver View Post
We can get the pilot to light and will increase in flame size until 350+ degrees.

I'm not sure what you mean because the pilot has nothing to do with the temp setting for the oven.

It stays lit for 1-2 minutes and then goes out without ever lighting the large burner.

Sounds like the thermocouple. The thermocouple consists of two pieces of different metals that generate a small amount of electricity as they heat up and it's the pilot light that heats it. As more electricity is generated, the main gas valve is opened so the burner lights.

We have tried to manually light the burner as well, no luck.

Sometimes, depending on the appliance, lighting the main burner directly will heat up the thermocouple and light the pilot; if the thermocouple doesn't work, the burner will go out.

Does this sound like the (mystery)thermacoupler? If so is it the part inside the oven on the side wall which attaches to the burner?

Sounds like the sensor for the thermostat, but maybe not.

And where would we buy a replacement part?

Try hardware stores or RV stores.

We have taken that part out and cleaned it and the spring part seems to let airflow when it pushed manually.

Beyond me what you are describing.

Also is it the thermostat that controls the operation of the the thermacoupler, and this is more likely the problem?

The thermostat only controls the temp of the oven by turning the main burner on and off. The thermocouple is a safety device to prevent the main gas valve to the burner from staying open after you turn the oven off. There will be two small objects sticking upwards (usually) next to each other. One is the pilot and can be identified when it's lit. Next to it where it can be heated by the pilot is the thermocouple. It is usually a cylinder an inch or less long by about a quarter inch in diameter with a 2 wires (often sheathed, so it looks like one) attached to it that go to the main gas valve. Nearby is the small (about 1/4 inch in diameter) gas line to the pilot which also goes to the gas valve. You can remove the thermocouple, not necessarily easy to do and may require triple jointed fingers and small wrenches, take it to a store and try to match it. It's the first and simplest part to replace and the most common one to fail. Once you figure it out and get it to work you will feel a sense of mastery and accomplishment. You may reward yourself with beer or other libations. If it doesn't work after you replace the thermocouple (cleaning probably won't help the old one), reward yourself with beer or other libations.

Thanks!!!
Good luck, flowers. I see Howie and Steve beat me to it and they have some good advice too.

Gene
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Old 01-15-2009, 11:06 AM   #5
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A thermocouple is a safety used in ancient design systems that our beloved mother ship continues to sell.

it is aN old invention that has 2 dissimilar metals that when heated by the pilot flame generates a small amount of voltage that tells the main gas valve it is ok to open.

You do not find them on modern heating systems, they use control boards to confirm flame operation and the t/c is a relic from the past.

Really, time to move on a/s. I still feel an all electric unit with inverters and a small built in generator/fuel tank would be the way to go. No propane, after 30 years in the HVAC equipment business, I have seen my share of disasters with the stuff most in the industry call NASTY!!!
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Old 01-15-2009, 11:11 AM   #6
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I agree that thermocouples are safety devices, but there is no electricity, high or low voltage, involved. Heat of the pilot causes expansion which physically opens a small valve.
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Old 01-15-2009, 11:36 AM   #7
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I agree that thermocouples are safety devices, but there is no electricity, high or low voltage, involved. Heat of the pilot causes expansion which physically opens a small valve.
not so. there most definitely IS electricity involved, but its a very small amount (millivolts...but thats all you need).

I don't agree that they are "relics" that should be eliminated from travel trailers; quite the opposite, actually. I think it is absolutely asinine that anything in a TT "require" electricity to run, that doesn't absolutely have to. Even with today's advances in technology, no one has been able to make it "easy" to store a large amount of electricity in a TT. Batteries fail or get drained down very easily, and when they do, they can't be quickly recharged. If I run out of gas, I can much more easily and quickly replenish that, even in remote areas. "So, carry a generator". well, ok. that's a $1000 solution to a $3.98 problem. Why manufacturers insist on putting a computer in "everything", even things that don't benefit from one, is just beyond me. It only adds needless cost and complication, and another point of failure (a very likely one, at that) to what could otherwise be a simple, elegant solution.
In a house, its a different story, because the electric supply is much more reliable, and also, the fact that the appliances are running 24/7/365, there is a real cost to an idle pilot flame. But most TT's are used so little, that cost is so small, its insignificant.
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Old 01-15-2009, 12:11 PM   #8
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Our ovens in the trailers are different that the ones in our houses. The older ones use a Robert Shaw valve that depends on the heat from the oven causing the gas (in the small balloon shaped sensor mounted to the side of the oven) to expand and turn off the gas when the oven reaches the temperature selected by turning the front knob. I had to replace mine when it failed to work. They are about $50. There may be a secondary safety valve down in the oven close to the burner which will have two wires leading to it from the thermocouple. This valve is controlled by the thermocouple. It will not allow the gas to flow to the burner on unless the thermocouple has been heated by the pilot light. There is no outside source of electricity used. The thermocouple develops enough voltage (and therefore current) to cause this valve to open.
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Old 01-15-2009, 12:15 PM   #9
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I agree with Chuck about the use of thermocouples in TT's. It's a simple way to insure the stove can be lit. It causes no drain on the batteries. The problem with the oven in ours is that the pilot is at the back of the oven and is hard for people to light. That's just awful design. There are a number of things draining the batteries in newer models and we don't need more, especially since Airstream puts the cheapest batteries in their trailers as original equipment. I just put a switch in the 12 v line to the subwoofer so that thing isn't hot all the time and draining the batteries—another poor design.

In recent years we have found how unreliable electric service is to homes so even there mini-computers may be unwise. When people don't have power for weeks or longer, we can no longer rely on power companies. But that's another issue….

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Old 01-15-2009, 12:36 PM   #10
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Well, I don't know about youse guys, but my oven has a thermostat and no thermocouple. Granted, it's a '59, so this doesn't apply to the original poster, but I like the way it runs.

The pilot is located way in the back, but there is a lighting tube right in front so it is easy to light.
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Old 01-15-2009, 12:46 PM   #11
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Where can I get a Harper-Wyman oven safety valve from a 1964 Magic Chef oven tested & properly rebuilt with perhaps new thermocouple installed? Or a good working used one.

It’s the odd H-W model with stationary pilot outlet.

Also need H-W oven thermostat with heat sensor tested/rebuilt.

I know the Harper-Wyman parts are obsolete, (like many of us) but there has to be some old home stove/oven shop somewhere that can test/rebuild them. They were used in many home ovens ‘50s-60s.

Stove works great after rebuilding gas valves….now I want fresh baked cookies!
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Old 01-15-2009, 01:13 PM   #12
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Well, I don't know about youse guys, but my oven has a thermostat and no thermocouple. Granted, it's a '59, so this doesn't apply to the original poster, but I like the way it runs.

The pilot is located way in the back, but there is a lighting tube right in front so it is easy to light.
Mark, there's probably a date before which thermocouples weren't required. We have a circa 1930 Magic Chef stove in our house without a thermocouple in the oven. It works great and we have been using it as our kitchen stove for 20 years. On the other hand, I have seen some pretty old propane space heaters in mountain cabins and they all had thermocouples when someone asked me to fix them, so they may have been required in them earlier than ovens. Magic Chef started around 1930 and was the premier line of the Quick Meal stoves.

So, if you don't want to mess with thermocouples, buy a really old trailer.

Gene
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Old 01-15-2009, 04:32 PM   #13
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Where can I get a Harper-Wyman oven safety valve from a 1964 Magic Chef oven tested & properly rebuilt with perhaps new thermocouple installed? Or a good working used one.
You can try this shop. I decided to go with a new replacement for my Robershaw valve because I thought the price for a rebuild was too high ($140).

There's another outfit that does rebuilds, but I've lost the link.
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Old 01-15-2009, 11:24 PM   #14
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Well, each his own, but propane is not my choice in a vehicle that gets the abuse such as I give to the a/s. 2 cross country runs/year in the winter.It seems crude to have propane lines and connections hanging on the bottom just ready to be hit by something. Not to mention all the flare fittings and connections to the ref, stove, water heater etc. Potential with aging for a leak is indeed an issue. I would like the following.

1. All electric, furnace, ref,instant water heater and flat top stove. It would need 4 deep cycle batteries matched to 4k inverter. Going down the road t/v would charge enough to run ref and furnace on a low setting. Mounted where propane tanks are would be a 3.5k gen/ super quiet fed by a chassis mounted fuel tank, similar to many toy haulers. Could be diesel if quieter and like what I have heard on many new semis not idling at truck stops. Many have installed these with independent a/c to stop overnight main engine idle. When boon docking it would run when demand is there, a control monitoring demand would kick it on when needed.

When at the park, 50 amp or just be careful what you run. I can hear the criticism already, but guess what. Most, high end motor-homes such as prevost and the like are going all electric. I suspect reliability and safety are just a few of the reasons.
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