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Old 08-04-2006, 08:49 PM   #1
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Question Should I be able to hear my fridge on 110V?

I've just burped an old dometic ('79 vintage), don't have the LPG system hooked up, and want to test if it works on AC. I've plugged it in, but I'm not convinced that the switch to toggle from gas to AC is working, in part because I don't understand the purpose of the various bits in the back. I'm surprised that I'm not hearing anything when the fridge is plugged in (direct to a known live extension cord) and set to AC. (I'm used to regular fridges, where you can hear the compressor and various bits of humming). Is the heat inverter technology noiseless?

If anyone can explain the purpose of the black bakelite unit which receives 110V and seems to be some sort of switch, directly in line with the AC/gas knob, and the purpose of the black device in the corner that also gets AC and looks like it connects the ground to the frame, or anything else that I can poke at with an ammeter to see if the bits supposed to be live are live, that'd be great.

I'm happy to leave it on for a while and see if it cools, but I'm not convinced I have it "on" yet...

--david
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Old 08-04-2006, 09:25 PM   #2
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The RM100 Dometic fridge in my '77 didn't make any noise either. I don't know what type of unit you have but my controls were inside the refrigerator at the very bottom. One knob set the thermostat temp (instructions said set it to 4) and the other knob set the refrigerator to gas or electricity. It took over 6 hours for the refrigerator portion to get cold on either electricity or gas.
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Old 08-04-2006, 11:33 PM   #3
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Quote:
Originally Posted by davida
...If anyone can explain the purpose of the black bakelite unit which receives 110V and seems to be some sort of switch, directly in line with the AC/gas knob, and the purpose of the black device in the corner that also gets AC and looks like it connects the ground to the frame, or anything else that I can poke at with an ammeter to see if the bits supposed to be live are live, that'd be great.
...
--david
If your fridge makes a noise when running, you should run...

These are amonia refrigerators--they work by boiling an amonia solution--through a process of evaporation, condensation, and recombination of the working fluids within the pipe, heat is extracted. The propane boils the amonia when the fire is lit, the 115V AC boils the amonia with a 75 Watt or so heating rod when the power is on. The dohickeys you are describing are the devices which switch between heat sources so that only one can be on at a time.

It is beyond the scope of this post to speak further regarding how the heat is turned into cold. BTW, you know that there is only heat--there is no such thing as cold. It's all relative. A trace of the circuits and pipes is left as an exercise for the student.

You can check the AC heating element by detaching one lead, then connecting an ohmmeter across the leads to read resistance--it will be in the vicinity of 1.5 ohms. Make sure the fridge is unplugged or you may experience a wakeup call from the local utility. The other check is to see that you have 115V across the heating element when the fridge is on and set to the AC mode. Both these checks are necessary. If you don't have an ohmmeter, CAREFULLY touch the vertical insulated enclosure above the propane burner, it should we warm (sometimes HOT) if the fridge is on electrical and is powered up. If it is hot and the fridge is not showing signs of cold in 2-3 hours, you have a problem in the amonia system. Other threads will advise how to attempt to fix this problem, but in general it is seldom fixable.

Yes, it usually takes a few hours for the system to get working and to suck enough heat out of the fridge for you to notice. It is common practice to start the fridge on AC the day before you leave on a trip.
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Old 08-05-2006, 01:45 AM   #4
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Zeppelinium -- thanks. I'll go out with an ohmmeter/voltmeter and check what I can. Something is defintely not right with the circuit, as the heating unit is not hot at all, after several hours plugged in, and I know it was heating when running on propane (well duh =).

I'm hoping I can identify the heating element to measure its resistance. I'm assuming that as with most heating elements I should be able to detect the thermal output quite quickly as soon as I figure out why current isn't flowing through it.

When I last looked I could count three different cables connected to the AC in put. One pair of wires went in a metal tube up, one set went down to an unidentified unit in the bottom left, and one went to the doohickey which seems to be activated by the knob, which I assume is the primary AC switch. The plastic bit connected through a rod to the know which should actuate the switch seems fairly flimsy and out of alignment, so I'm wondering (hoping!) that all that's wrong is that the AC system isn't ever turning on. If I'm right and it is a switch, how does that switch work? Should it be pressed in (towards the outside of the trailer)? Pushed aside?

Thanks for all the help folks,

--da
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Old 08-05-2006, 02:17 AM   #5
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I figured out the circuit (AC to switch to thermostat to resistor), and I think the switch or the thermostat are broken (or the coupling between the knob and the switch isn't working), so I bypassed both, and connected the resistor straight to the AC (there isn't a simpler electrical circuit in the world! Amazing what a simple resistor and thermodynamics can do...).

At this point the heating unit is heating, and I'll check it in the morning to see if the inside is cooling. Then I'll know whether to get rid of the whole unit or investigate what's wrong w/ the remaining two parts, which I'm sure I can source if necessary.

Will update this thread in the morning...

--da
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Old 08-05-2006, 10:33 AM   #6
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I don't know if you were lucky or not. I've always found that the heating element is difficult to find, but the switch may be impossible. If Airstreams were black, I'd call mine the tarbabies--once you open something up, you just keep finding little (and big) things to fix.

Good luck.
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Old 08-05-2006, 10:50 AM   #7
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Success! The fridge is quite cold this morning. Now to figure out the details of what's wrong with the switch and thermostat.
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Old 08-05-2006, 10:52 AM   #8
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Zeppelinium: As I'm planning on almost always using the fridge off of AC, I don't need to find an original replacement switch -- any switch will do. I read in the manual that it's best to operate the fridge on LPG every now and then, though -- why's that?
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Old 08-05-2006, 11:02 AM   #9
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probably to keep the burner clean and bug nests out of the chimney.

I applaud your willingness to replace the switch with something generic. However, you really really really have to be careful that if you bypass the "exclusion" nature of the orginal design, that you never wind up with both heating methods operating at the same time. Maybe a risk of fire, but you will certainly dry and harden the crystals inside the tube--once that happens the fridge is permanently dead.
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Old 08-05-2006, 11:19 AM   #10
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Noted. I'm actually fine replacing the switch with an original-style switch, but I'm pretty sure that switch will be hard to find before I need to use the fridge. Anyway I'll put a notice on the gas valve, so that if anyone else tries to turn it on they'll be told clearly about the modification, until I find a proper fix.

One more question: testing the thermostat should be as simple as checking the resistance when hot vs. cold, right?
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Old 08-05-2006, 11:31 AM   #11
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Try "vintageairstream.com" under refrigerator . They have a list of folks that repair them . I would think that at least one of them would have the parts you are looking for . Good luck .
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Old 08-05-2006, 01:18 PM   #12
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Quote:
Originally Posted by davida
...One more question: testing the thermostat should be as simple as checking the resistance when hot vs. cold, right?
Yes, but I've never had any luck figuring out if the resistance is doing the right thing.

I think (help out here, experts) that the thermostat only works on the electrical side. I've never been able to keep from freezing my letuce when on propane.
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Old 08-05-2006, 03:03 PM   #13
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Quote:
One more question: testing the thermostat should be as simple as checking the resistance when hot vs. cold, right?
yes, when you turn it up to a higher (warmer setting) it should open interupting the circuit.

when you turn it down it should close completing the circuit.

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