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Old 03-12-2014, 05:49 PM   #1
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Dometic 7732 won't run on propane

I have a 1996 motorhome with a dometic 7732 fridge. I bought this as a project and don't know the history. The refrigerator would run on ac sometimes and sometimes not. Sometimes it would run on gas and sometimes not. I put in a dinosaur 711 board and now it runs perfectly on ac always, but when it goes to propane it would run for 45 seconds. I put in a new thermocouple and now it will run on gas for about 55 seconds and then it shuts down. It will light every time but just won't stay running. What are the possible problems. Any suggestions as to do next? Any help would be greatly appreciated.
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Old 03-12-2014, 06:55 PM   #2
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Check to see if the propane regulator is working and if it has the correct pressure (10.5 inches of water. Another possibility is dirt, rust on the burner tip.
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Old 03-12-2014, 07:09 PM   #3
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When it ignites, does it keep sparking?
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Old 03-14-2014, 06:58 AM   #4
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I will try and check that tomorrow, but I will say that when it is burning it appears to be a very hot and steady flame. Also it does not keep trying to light after it has a flame.
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Old 03-14-2014, 07:13 PM   #5
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I cleaned the burner tip and the flame is constant and all blue. It looks great. I put my old board back on and it does the exact same thing. I put the old thermocouple on and it does the same thing. When you change the fridge to propane it lights quickly and burns for 52 seconds shuts down and the check light comes on. I tried holding the thermocouple further down in the flame with the same result. I haven't checked the regulator yet but all my other propane appliances work just like they should.
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Old 03-15-2014, 06:38 AM   #6
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There could be a few different things at play here....

1) Have you checked the DC voltage right where it connects to the circuit board and the unit is attempting to run? Checking it at the batteries is frivolous as there can be wiring damage, corroded connections or a bad ground right at the control board connections. The absolute minimum voltage would be at 9.6 volts DC right at the refrigerator control board. Keep in mind that the front panel lights will continue to work all the way down to about 5 volts. Inadequate voltage would throw off the readings being supplied by the flame sensor, thus causing it to shut down as it could not properly detect a flame. You must check this voltage while the wires are still connected to the control board and the unit is attempting to run. We need to see the voltage while there is a draw on the circuit. Many times connections that are corroded and while unplugged from the control board will read 12+ volts until a load is applied, then it can fall to almost 0.

2) Is this problem only occurring while plugged into shore power or running the generator? If so, here is another test. Your converter, which charges your batteries while plugged into shore power or using the generator, can go bad causing a LOT of noise on the DC side. This is best checked with a standard Volt/Ohm meter. Set your meter to AC volts and probe the DC + - connections right where they connect to the refrigerator control board. If you see any AC voltage over .5 volts you have a bad converter. Unplug the shore power or if you were running the generator shut it off and re-test for AC voltage. Repeat Step 1 above too.

3) The LP gas valve is a 12v DC controlled device. Albeit rare it is entirely possible the coil inside the gas valve is bad or failing causing it to shut off. Using your volt ohm meter, unplug the 2 wires from the gas valve. Set your meter to OHMS and probe across the two terminals on the gas valve. Post your ohms reading. It should be between 45-55 ohms.

4) If memory serves me correctly, the 77xx and 70xx models had an LP gas ignition system that was tied to the vehicle's ignition. When the vehicles ignition was turned off the refrigerator began a cycle which prevented the unit from operating on LP gas for 25 minutes or so. This was done to avoid having an LP flame burning while refueling the chassis or even having your propane tank filled. They referred to this as their D+ circuit. If you have selected gas only mode and this feature is failing it will set the CHECK light. On the circuit board will be a terminal marked either IGN or D+. If you have a wire connected to this terminal, unplug it. Turn the unit off for 30 seconds then back on again. Same results?

Jesilvas asked if it is attempting to spark once the flame goes out. This is an important thing we need to know. IF the unit has been shut down the ingition/LP lockout it will not attempt to re-light itself by continuously sparking.

Keep in mind also many times while attempting to troubleshoot these things, if all else fails, you can completely disconnect the unit from the coach's own 12v power and wire up a completely separate 12v battery source and see if problems disappear. This can certainly help eliminate the most common problems associated with voltage oddities - which can make you want to pull your hair out :-)

Good luck and keep us posted.
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Old 03-15-2014, 08:12 PM   #7
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Thanks for all the info. Checked everything and found the gas valve ohms open. My only question now is how does it ignite at all if the valve is bad?
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Old 03-16-2014, 04:06 AM   #8
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Well, thats a good question. If you cleaned the spade terminals real well and still have 0 ohms, then I have no explanation as to how it's lighting at all. Can you post a pic of the gas valve? Perhaps yours have been changed over to something different.....?
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Old 03-16-2014, 11:13 AM   #9
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First of all it is the factory valve. I checked and when you pull the ac power you have 12.8 volts at the terminal on the gas valve and it lights. After about 50 seconds you loose power at the terminal and it reads 0 volts. I checked the solenoid and it ohms at either o open or 0.01 ohms. I jumped the solenoid with 12 volt power and manually lit the flame and it will stay running as long as you leave the power connected, so no matter why it is ohmming open it is in fact working. After a few minutes of the flame burning I checked the factory input wire to the solenoid valve and it reads 8.5 volts. I checked the output on the thermocouple leads and with my voltmeter set on 2 volts it reads 0.033. The bottom line is it is losing 12 volt power to the solenoid valve after 50 seconds from the board. This is driving me crazy.
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Old 03-16-2014, 11:27 AM   #10
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Im interested in the slow loss of power to the solenoid on the gas valve. The .033 is within the proper range of 25-35mv, let me think on this.. im baffled too.

EDIT: Back to an earlier question while Im thinking on this... when it fails does it try to re-light after a few seconds or not?
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Old 03-16-2014, 11:32 AM   #11
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I assume you have this, but ....

http://www.beamalarm.com/foretravel-...030_7732SM.pdf
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Old 03-16-2014, 01:28 PM   #12
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No, it does not try and relight. The flame goes out and the check light comes on. You can turn the fridge to ac and basically reset it and then remove the ac power and it will light again and do the same thing. You can do this all day with the same result every time.
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Old 03-16-2014, 01:36 PM   #13
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Ok. An interesting test!
Leave the thermocouple connected to the control board.
Disconnect the 12v from the control board to the gas valve.
Apply your own separate 12v source to the gas, and light it manually just like you did in your testing above.

Once it's lit and running remove either of the thermocouple wires from the control board and see if it attempts to spark again. It could take 5-10 seconds before trying.

If it does not try sparking then disconnect the spark module from the control board. Using your meter check voltage on the terminal connections for the spark module while re-doing the test above. If it attempts to re-light when pulling the thermocouple wiring loose you should see 12v being applied to the spark modules terminals on the control board. Im just wondering if something is futzed in the spark module causing this whole mess since both the control board and thermocouple are new.

EDIT: Another thought... are your thermocouple wires on J3 and either J9/J10?
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Old 03-16-2014, 03:12 PM   #14
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I went out to try it and now the spark igniter is not working at all. I did however light it manually with a jumper wire and checked the voltage on the factory lead to the solenoid. It starts out with over 12 volts and after the 50 seconds the voltage begins to leak down slowly over a couple of minutes until it is zero. This was with the spark igniter disconnected from the lead. As far as the connections from the thermocouple to the board it is a dinosaur board and the leads are labeled for the thermocouple. I am at a loss so I guess I will call board tech people tomorrow and see if they might help me trouble shoot it over the phone. I really appreciate your help, but as always if it belongs to me it has to be some kind of crazy problem.
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