You should read the section on gas safety before attempting any repairs. See Ignition Devices for problems igniting the burner.
Please note that the same graphic is repeated on this page to help in understanding the system.
How it Works
The purpose of the propane system in an RV refrigerator is to safely provide and regulate the gas heat to the cooling unit. The components shown below, along with the correct gas input pressure of 11" water column, will do that. The drawing below represents a common gas system. The components are drawn for easy understanding and not mechanical accuracy. Although various brands and models place their components and modify their components in vastly different ways, the drawing below represents all standard refrigerator (non-electronic) propane systems with the exception of a few older ones that used the Klixon valve and the Teddington burner. The Klixon valve and Teddington burner were different safety mechanisms, otherwise the propane systems of those refrigerators worked the same.
Propane at the correct pressure (11" water column) passes through the gas valve and the gas filter (optional) to the thermostat. Correct input pressure is critical. If the input pressure is not correct, the resulting flame at the other end of the propane system will not be correct. If you have any doubts about the gas pressure, buying a new LP regulator for your propane tank may be the simplest solution.
Thermostat. The gas thermostat controls the gas pressure to the orifice and therefore the size of the flame at the burner. By using a capillary tube (sensor) mounted in the lower box of the refrigerator, the thermostat directs the gas flow through two different paths, depending on whether or not the refrigerator has reached the desired temperature. One path allows full gas pressure through to the orifice for high flame and cooling, while the other path reduces the pressure creating a low flame. The low flame should be sufficient enough to stay lit and to power the safety mechanism, yet low enough not to generate cooling from the cooling unit.
Safety Valve. The propane passes from the thermostat through to the safety valve. The safety valve does not have to be placed after the thermostat; it just has to be in-line in the propane system so that it can shut off the propane flow if necessary. As partially inferred from the last sentence, the purpose of the safety valve is to shut off the flow of propane if the flame is extinguished for any reason.
The safety valve contains an electromagnet which holds a plunger open to allow gas flow. The thermocouple generates millivolts when heated by the gas flame and activates the electromagnet. As long as the flame stays lit and the thermocouple generates millivolts, the safety valve will stay open. If the flame goes out for some reason, the thermocouple will cool and quit generating millivolts. The electromagnet will then de-energize and the plunger will snap shut, cutting off the gas flow. See the Safety System section for more information on flame outage.
After the safety valve, the propane passes through the optional test block to the orifice. The test block is an in-line piece that contains a plugged 1/8" fpt connection for testing. When testing the gas pressure to the burner, the plug is removed from the test block, an adapter is inserted, and a manometer is connected to the adapter.
Orifice. The gas orifice is a fitting with a very precise, small hole at the end of it and is the heart of the propane system. If it weren't for the need for safety and the need to control the temperature of the refrigerator, a gas line straight to the orifice could be the entire propane system up to this point. The size of the orifice coupled with the correct gas pressure (11" water column) passing through it set the BTU output of the flame. If the orifice is dirty or, even worse, enlarged, the flame will not be correct.
Gas leaving the orifice shoots pass the air holes in the burner, picking up air for a mixture, and exits at the burner head. Here, once ignited by an ignition device or match, it should burn steadily as a vibrant, blue flame.
Above the burner, inside the chimney, is a hidden and little known component called the baffle. Many people who stumble upon it think it is a chimney cleaner. The baffle is a metal spiral device that hangs at a certain point in the chimney. Its purpose is to slow the heat rise from the flame and to concentrate that heat on an area of the cooling unit boiler system. The baffle is usually not a problem, unless a situation has occurred where the baffle was removed and may not have been replaced, such as in the changing of a cooling unit. A missing baffle will cause intermittent cooling problems on the gas side.
Usually the best way to troubleshoot the propane system is by using a manometer. A manometer is a gauge that measures gas pressure, but unfortunately most people don't have one, which means they're always working somewhat in the dark, but a lot of problems can be solved by logic and the process of elimination
All gas problems, except for leaks or a blocked chimney, show up in the burner flame. Either the burner is not putting out enough BTUs or is putting out too many BTUs for the situation. Although we've used the term BTU loosely in this section, most mortals will never be able to measure the BTUs of the flame. Just think of BTUs as "the heat output".
Inadequate or No Cooling. If the gas problem is inadequate or no cooling, and electricity is available, always test the refrigerator on electricity to eliminate the cooling unit and venting as possible problems. In other words, if the refrigerator works good on electricity, the cooling unit is good and you can concentrate on the gas problem.
Assuming the cooling unit is good and its requirements are met, the baffle is in place, and the chimney is unobstructed, poor cooling on the gas side is an incorrect flame. That simple. The most common incorrect flame is one that is too small, which is usually caused by a dirty orifice and/or burner. Remove the orifice from the burner assembly and clean it with air pressure or by soaking it in alcohol or paint thinner. DO NOT poke something like a wire through the orifice. This could easily enlarge or ruin the orifice. An enlarged orifice will put out too much heat and eventually damage the cooling unit. If there is sign of sooting or loose rust build up on the burner, blowing air pressure up the chimney and onto the burner area will clean them (protect your eyes when doing this). Don't let any loose particles blow into any open gas connections.
If, after cleaning the orifice, the problem is not solved, the orifice is still not eliminated as a possible suspect. Oil from the propane tank can accumulate around the hole of the orifice attracting dirt and making it very difficult to clean. This is a point where a manometer would come in handy. If you knew for sure that the gas pressure was correct to the orifice, you would know that the orifice is still the problem. Since you're probably working without a manometer, you'll need to make a decision. First, check out the thermostat and filter sections, then, after eliminating those two items as suspects, make a gut decision as to whether or not you want to replace the main LP gas regulator to insure proper input gas pressure or want to replace the orifice. Replace one, test the refrigerator, then replace the other if the first replacement didn't solve the problem. When working without a manometer, this is the logical way to proceed. Also, don't forget the assumptions you made at the beginning of this section and double check them.
A bad thermostat (usually one stuck in by-pass mode) will also cause no cooling.
Overcooling. Overcooling is applying enough heat to the cooling unit to create cooling at a time when there shouldn't be cooling. In other words, what should be a low flame isn't a low flame or isn't low enough. This will either be caused by too high of an input gas pressure or a defective thermostat. Check out the thermostat section for troubleshooting the thermostat, or replace the LP regulator on the main propane tank if high gas pressure is suspected.