While all RV gas/electric refrigerators use the same principal to cool, the the method used to control the cooling, in addition to energy source selection, has evolved over the years.
In its' most basic form, the control simply applies heat to the cooling unit when cooling is required, and removes the heat when it is cold enough. In the early years, purely mechanical controls were used, requiring the user to go outside, open the exterior access door, and flip a switch to turn on the electric part, which was controlled by a mechanical thermostat that turned the heating element on and off, or light the burner, which was controlled by a mechanical gas thermostat- because the flame had to be lit by hand, the gas thermostat would not shut the flame off when the refrigerator was cold enough, rather they had a "bypass" in them which would let the flame keep burning at a very low level.
It is worthwhile to note that the smallest modern gas/electric refrigerators still operate this way.
The electric side of these refrigerators is pretty straight forward, consisting of a switch (which should have some sort of interlock to prevent the electric and gas from operating at the same time), the thermostat (which simply turns the electricity to the heating element on and off as required), and the heating element itself.
The gas side is slightly more involved, simply because it will contain a safety valve which will shut the flow of propane off if the flame is extinguished- other than that, it consists of a valve (with the interlock), a thermostat (which closes when the refrigerator is cool enough- but has a bypass in it so as to keep the flame burning on low), and the burner itself, which consists of a precision drilled orifice (to regulate the flow of gas on high), and the burner head.
On purely mechanical refrigerators, there have been a few different types of safety valves- the first being a "Klixon" valve which consisted of a bimetal disc, which when heated would pop up and open the valve- if the flame went out, it would cool down, snapping closed and shutting the flow of propane. Because these have been obsolete since the mid 1960's, you probably will not ever see one, unless you are interested in vintage rigs. The next type of safety valve, which is still in wide use today, is the thermocouple/mechanical valve setup. In this type, when the thermocouple is heated, it produces a very small electrical current, which is used to hold an electromagnetic valve open. The tell-tale sign of using this type of safety is that you will always have to hold the valve open (either by pulling, pushing, or turning something) until the thermocouple is heated enough, at which time the current will hold the valve open. Again, if the flame blows out, the thermocouple will cool and stop producing the current, which will allow the valve to close, shutting the flow of propane.
Something to keep in mind is that even though it looks like a small tube, the thermocouple is actually an electric device. The tube like "lead" simply contains some insulation and an inner wire- the outer sheath is the ground, and looking at the end that screws in to the safety valve, you will see the outer nut, and the contact on the very end, which is separated by a small piece of insulating material.
Igniting the flame in the first place has evolved from lighting it with a match, to having a flint spark lighter, to a piezo electric lighter, to the electronic re-igniters that are still in use today. The benefit of the electronic igniter is that it will stay active (though not sparking), and be ready to re-light the flame in the case of a blow out.
If you have already performed the troubleshooting for the cooling unit, you will know that the cooling unit and heating element are OK. The next question is whether the problem appears on electric operation, or LP gas operation.
Electric operation testing is fairly straight forward- with a volt-ohm meter check the outlet the refrigerator is plugged in to. Then check the power coming out of the selector switch (note that on some models the selector switch is not easily accessible, so it is easier to check the power in to the thermostat )- then the power out of the thermostat. A tip- be sure to check across the two leads- on some later model refrigerators, both the hot and the neutral lead are switched, and if you are checking with a grounded lead on your meter, you might miss a bad neutral side connection in the switch (I know I have).
These tests, along with the resistance tests of the heating element (which would have been done in the cooling unit test) pretty much cover all of the 120 volt electric side of the refrigerator.
The LP gas side, as far as troubleshooting is concerned, isn't much more difficult, though different techniques are used to test, as well as different testing tools.
If a refrigerator works fine on 120 volt ac power, but doesn't cool on gas, I generally work backwards from my electric troubleshooting- making only a rough test of the LP supply first- by simply lighting three or four burners on the range and visually "guestimating" whether the LP pressure is good by seeing if the range burner flames look good, and do not decrease in size when all burners are lit. Note that this is not the "book" way- but it is the first thing that I do.
After doing a rough test on the LP supply, I do a visual inspection of the flame. If the flame will not ignite, I will light it manually by having a helper hold the safety valve (on most refrigerators you need someone on the inside to engage the safety, while you light the flame- but not all). If the flame lights and burns well, you know that the problem is in the ignition system. Troubleshooting these is usually fairly straight forward- if it is a push button system which snaps when you push it, it is a piezo electric system. These are really very reliable, and if they still "snap", the problems will usually be either in the spark probe/ gap, or in the spark cable itself. Problems in the spark probe or gap are generally confined to an improper gap or rust flakes shorting out the gap, or a broken porcelain insulator around the probe. Problems with the spark cable are generally caused by the cable having been damaged in some way- either burned or cut. Many times these can be found by waiting until dark, and having a helper push the sparker button while you look at the cable- just like a spark plug cable in an auto, you will usually be able to see where the cable is shorted. For electronic ignitors, a simple test of 12 volt power going to the electronic ignition module will be first- if that is good, then inspect the cable, probe and gap as above- if these test good, the module is probably bad.
Once the flame is ignited, you can do a visual check. The flame should be clean, "hard" and blue. If it is "lazy" (not a well defined flame), or yellow- I will generally carry out the standard maintenance procedures, which should be done once a year, but usually are not. This consists of cleaning the burner and orifice by removing them and soaking them in alcohol, then letting them air dry. While you are checking these things, something else to watch for is whether the flame is getting a good draft- whether it is going up the flue well. If the flame has been burning poorly for a while there is a good chance that the flue a baffle could be covered in carbon soot. While the "proper" way to deal with this is to remove the refrigerator and physically remove the baffle and run a brush down the flue, many time compressed air blown up the flue will get the job done.
A couple of points here- until the past few years (everything up to the 9XX and N series), Norcold has used a very inexpensive orifice together with a copper sealing ring. If you remove the orifice in one of these, you need to have a new copper ring to reinstall, and because the ring/ orifice together cost less than $5.00, I don't bother cleaning them, I just replace.
Dometic orifices are somewhat more expensive, consisting of a synthetic sapphire that has been laser drilled. The laser drilling operation actually leaves "rifeling" in the orifice, which spins the LP gas stream. These rifeling slots are easily clogged, so it is good practice to clean these whenever maintenance is performed on the burner.
Once these procedures have been done, I move on to actually testing the LP pressure at the burner- which most manuals call for earlier in the process, but I usually leave this far later, simply because the cleaning problems seem to be more prevalent. Checking the LP pressure is not terribly hard, but it does take an instrument called a manometer . While this sounds expensive (and can be), a perfectly accurate manometer can be built very inexpensively. Rather than duplicate the plans here, check out Les Dolls' website - RVer's Corner- here.
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