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Old 05-27-2013, 08:42 PM   #1
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1996 Dometic RM4872 Gas operation logic questions

Fridge doesn't light on gas. Check light comes on.
When the wife turns it on, I can light the flame with a match and it stays lit until it reaches the desired temp, shuts off, then later won't relight.

With a test light I found there's 12V power to the igniter when it's turned on, so I assume the igniter is bad. I even hooked the igniter up to the battery with jumpers and didn't hear any clicking noise. Is this a reasonable bench test?

I then looked at the thermocouple and noticed a bunch of crud on there, so I scraped it off. Then I turned on the fridge, lit the flame with a match, and I expected the power going to the igniter to stop once the flame was on, but the control board was still sending power to the igniter. Bad thermocouple too?

Shouldn't the board stop sending power to the igniter once the flame is going?

Is there a good bench test for the thermocouple? I have a multimeter and I'm not afraid to use it.

Thanks in advance,
Kevin
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Old 05-28-2013, 03:49 PM   #2
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Yes, if power is supplied to the igniter, it should make a snapping noise.

Found a good service manual here:
http://www.nwrvsupply.com/manuals/do...00_service.pdf

I guess I'll replace that part and see how the thermocouple works. My theory is that the thermocouple is either faulty and/or dirty, and it's wearing out the igniter prematurely because it's not sensing the flame and telling the igniter to stop.

-Kevin
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Old 05-28-2013, 04:26 PM   #3
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I would not depend on a test light for anything other than an indication of power present. It tells you nothing of the voltage present at that location....which is key. I would check it again with a digital VOM and get a specific voltage reading before you continue with your trouble shooting.
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Old 05-28-2013, 04:40 PM   #4
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The spark ignitor circuit is generally on all the time the thermostat calls for flame. What happens is this: When the flame is initiated something happens to the, lets call it "force field" and the spark cannot or does not make it through the flame. That is the flame acts as kind of an insulator (I am not clear on the physics here, but that is essentially what happens) and the spark no longer occurs even with power to the ignitor. So, once the flame is established, the sparking automatically stops. This is also true in your electronic ignition furnace and water heater. If the flame goes out, the sparking starts again automatically and re establishes the flame. This action has nothing to do with the thermocouple, which only keeps the gas going to the flame, and is the safety element for gas flow. If the thermocouple senses no flame, it shuts the gas off. It has nothing to do with the sparking. The circuit board does not turn the ignitor off until the thermostat says, stop heating unless it has a time out cycle on it and many do to keep the spark from continuing in an out of gas condition. The spark, however, does stop when the flame is established and will automatically resume when it goes out.

I don't know if that helps any. I think a likely problem is that the spark element electrode is out of place and is not establishing a proper spark. There is a very specific location of the spark electrode vs. the burner, both in gap and location. If it is not in the proper place, it will not spark reliably.
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Old 05-28-2013, 04:46 PM   #5
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10-4.

Lewster, you're the man.

I was out in the wild, and my tools were limited to a spare turn signal bulb and some alligator clips. I think I'm going to donate one of my 5 multimeters to the airstream for this kind of "in the field" trouble shooting next time. Not that I knew what numbers I would be looking for.

The service manual I found has all the specs for voltage I need to check too. I'll do a few more tests before I start ordering parts.

Thanks,
Kevin
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Old 05-28-2013, 04:54 PM   #6
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Idroba,
That helps a lot actually, and makes total sense.
I did move around the spark element thinking it was too close or too far away, but no dice. I think I'll check the continuity of that line and tip next. Maybe clean it up a little while I'm there.

So I took my little jumpers and hooked the ignitor straight to one of the batteries, and no snapping noise. Can I assume the ignitor is bad? or does it need the electrode and 3/16 gap to a ground to "snap"?

From your post, I wonder if I can borrow the igniter from the furnace or the water heater to test a bad part? I'll go check to see if they're the same RV 679 part?

-Kevin
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Old 05-28-2013, 05:41 PM   #7
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The electrode must have about that 3/16 gap to "snap". And it can be cleaned with an eraser from a pencil. The ignitor circuits that I have seen are all built into the circuit board of the appliance so I don't know how you could borrow one from another use, unless it had the same board. The electrodes might be the same, not sure about that. I know some have a clip on the electrode for the wire, and some the wire is part of the whole assembly.

I have an old Atwood flame keeper electronic ignitor that I used on a pilot light water heater for a while. It has a little encapsulated circuit board, and uses a 9 volt battery for power. You set it up so the electrode is close to the pilot light, and turn it on. It sends out a pulse (snap) spark every second or so. Punch the red button over ride and the pilot lights. The sparking stops automatically, you don't shut it off. The spark stops due to the insulating effects of the flame as I mentioned in my other post. If the pilot light blows out, the sparking resumes and re lights the pilot before the thermocouple cools enough to shut the gas off. It will work for an amazingly long time on one dinkey 9 volt battery, so I know it is not taking much power to spark. That is it is off mostly.
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Old 05-28-2013, 06:06 PM   #8
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So I think I have a bad igniter, and the fact that power was going to it continuously after it was manually lit was normal.

idroba, On mine, it has a green Dinosaur circuit board with a separate reigniter under a black cover. The RV679 reigniter is only $25. I'm going to take everything apart and make sure there aren't any shorts or broken parts once more. I'll probably buy a new electrode for the igniter too as it seems to be a consumable part. Fridge is 17 years old, so I don't want to invest too much $$ in it. but if I can fix it for $25 and it lasts another 5 years, I'll be happy.

I found some more instructions from Dometic that pretty much mirror what you two have told me:

59. The igniter reigniter, used on certain Dometic model refrigerators, operates on 12 volt current. On gas operation the igniter senses the resistance through the flame between the electrode and burner. When there is no flame at the burner, the resistance is high and the igniter begins sparking to light the burner. As soon as the flame is lit, the resistance between the electrode and burner drops and the igniter stops sparking. The resistance is monitored by the igniter, and, if for any reason the flame goes out, the igniter begins sparking until the burner is lit. This insures that the flame will always be lit when desired. Each time the igniter reigniter system sparks, a light will illuminate on the lower left front corner of the refrigerator.
[Edit: I found that after the 80's? the "L" (indicator light) connection wasn't used anymore.]

60. If the electrode does not spark first, make sure the igniter is receiving 12 volts. If the igniter is receiving 12 volts and produces no spark, it must be checked for operation.

61. Turn the refrigerator off and remove the wire between the electrode and igniter. Now turn the refrigerator to the gas mode. If no internal clicking sound is heard the igniter is defective. It is important to remove the high voltage wire that goes to the electrode from the igniter when you are checking the igniter for operation. The high voltage wire and the electrode can be shorted to ground causing the igniter reigniter to think that the flame is lit, resulting in no spark on gas operation.
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Old 06-06-2013, 11:13 AM   #9
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Just a resolution to this thread.

I received the igniter ("reigniter" is the official name) in the mail yesterday and installed it. Bled the propane line out and it lit first try. Igniter was $32 shipped.

All parts cleaned, serviced per the manual, and re-aligned.

I'm not sure why the original reigniter failed. I'm guessing it's just a product of a 17 year old fridge.

Hopefully the fridge lasts at least 5 more years.

Thanks for all your help.

-Kevin
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Old 06-06-2013, 11:57 AM   #10
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Kevin: I am glad to know the ignitor or re ignitor is a separate piece. I have not had enough of the electronic refrigerators apart to see that fact. I wonder if it is on both older ones like yours or even newer ones. Anyone know?
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Old 06-06-2013, 12:16 PM   #11
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The newer control boards include the ignitor. There is no separately replaceable component on them.
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