Dometic
From VanagonWiki
Introduction and description of the Dometic refrigerator. Link to some good article on the ammonia absorption process. Usage note: "refrigerator" is often abbreviated either as "reefer" or "refer." Despite obvious connection to that fine film "Reefer Madness", "reefer" is the older and more common spelling.
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Theory of Operation
A good article on how refrigerators based on the ammonia absorption process work is here.
Another is here.
And Yet Another is here.
Testing
If your Dometic 182 is cooling poorly or not at all, there are several tests to perform. The simplest of which can be done with fridge installed in vehicle.
Checking fridge operation on AC mode is usually the most reliable mode to test that the various chemicals contained in tubes/components at rear of fridge, are circulating. Disconnect the AC plug of Dometic (rear LH side under sink) and connect it to a known working residential power source. This will eliminate the vehicles AC system as a potential source of trouble.
Cooling the fridge down on AC, may take up to a few hours. If no cooling takes place, try operating fridge on DC or LP ("liquid propane", "gas" or "propane") mode. If any mode tested is successful, then chemicals as a source of trouble can be eliminated for the time being. If none of the three modes work, proceed to testing the remaining DC and LP systems and their components while fridge still in vehicle.
On the DC mode, if 12 Volts is supplied to proper connections at fridge, and fuse, fridge switch and fridge relay are functioning, then the heater element should provide sufficient heat to chemicals, thus cooling the fridge down. Cooling on 12V mode may take longer than AC. If fridge doesn't cool down, then either a fuse, wiring, relay, fridge switch or heating element is suspect.
Testing a fuse is a simple affair; replace the fuse with a known working fuse of same size type. If fuse ok, check quality, proper connection and continuity of wiring. If wiring ok, test the fridge switch on front panel, then fridge relay. This relay may be installed in auxilliary battery compartment, or on fridge itself.
The fridge switch is a DPDT "On-Off-On" switch. If one probe of a voltmeter is connected to the middle contact and the other probe to either the lower or upper contact, then the switch should show continuity in either position. This is true for both physical sides (3 contacts per side) of switch. Be aware that it's in reverse of what you see. Switch pushed "up" should show continuity between the center contact and the bottom contact, etc. As for the relay, if a known working relay is available, simply remove the wires, one by one, and connect to working relay. Double check wiring against diagram supplied on relay itself and fridge or a Bentley manual. If second relay not available, connect a known working source of 12V power to proper pins of relay, set voltmeter to check continuity (ohms), and connect to other two pins. When 12V supplied to relay electromagnetic coil, the "switch" on relay should be "on", thus allowing VOM meter to show continuity. If relay is functioning, then the heating element is suspect.
Any heat source provided, in either of the 3 modes, needs to be sufficient enough to get chemicals to react and cycle throught the various tubes and components at rear of fridge. It may be possible to compare temperature of a known working AC heating element to the DC heating element. Compare with thermostat set to "Max" for AC. On DC mode, thermostat is bypassed and DC heating element receives max Volts/Amps. Obviously if DC element cold, and all components/wiring between power source to element are known working, then element must be replaced. If element is heating partially, the problem could be quality of wiring/components or heating element itself. Check for corrosion and/or poor physical connections.
Normally, the most difficult aspect of the Dometic is lighting it on LPG and getting it to stay lit. If fridge does not light, there are steps to take to determine why.
Try lighting the stove. If it lights, it's a safe bet that LPG is getting to connection at back of fridge. Determine if a spark is happening in combustion chamber. Lie down on your side, and in a dark setting, watch the view finder at RH lower corner of fridge. You may be able to see the spark. This should suffice as initial test for Piezo and associated wiring.
If there is a spark, then problem(s) usually exsist with the air pump, LPG supply lines, burner/burner jet, air intake tube, and/or exhaust tube. The various components installed between the LPG connection to fridge and the burner/jet in combustion chamber are usually not the problem, though with age, debris could travel through LPG system blocking any of these passages.
An initial test of the air pump could be done with two people. One listening at the exhaust vent (flue at exterior of van) while the other pumps. You may be able to hear air being pumped into chamber. A flexible tube put up against intake tube outlet and ones ear could help in this regard. If air is being pumped into chamber, and spark can be seen, but fridge won't light, one could suck out any potential debris from intake or exhaust tubes with a "Shop Vac" or equivalant. If this fails, air can be blown into combustion chamber via the drain tube at bottom LH side of fridge or through intake at flue at side of van. Either could help provide enough oxygen to help propane light. Blowing air into intake or drain tube may cause debris to be cleared and moved down intake tube into combustion chamber. This may interfere with future operation on LPG mode. Regardless, if none of this works, then fridge needs to be removed. Cleaning and inspection of combustion chamber is required at this point.
Once fridge is removed, and combustion chamber is open, one can clean and inspect the various parts. Even though initially visually tested, the piezo sparker should be operated while it is still installed. You should be able to see if a spark is being created between metal tip at end of porcelain and burner. If none, check the connection to piezo switch at front panel of fridge and quality of wiring especially at connection to porcelain piece. If clean and properly connected and still no spark, it may be possible to check the actual component contained in combustion chamber by connecting a standard BBQ piezo switch to other end of wire. If this provides the spark, then problem lies in piezo switch. If not, then porcelain part with metal tip is at fault.
With fridge removed, it is worthwhile to check the one-way valve at end of small rubber tube connecting to metal tube. It should blow one way and not the other.
If prior to removing the fridge, there was a spark, the jet and its' orifice should be inspected. Even if you can see daylight through it, it may need cleaning. Though proper precautions should be taken, one can clean the orifice with a mild mix of acid and water. A clean flexble rubber tube, with bulb (bulb similar to a turkey baster) on one end, and with the correct inner diameter to fit over jet, is needed. A battery acid testing tool should suffice in this regard. With open end of flexible hose connected to one end to the jet, the acid and water solution can be pushed through. This should be done in both directions. This will clean the orifice possibly to its' proper diameter. When several sessions of passing this solution through are completed, and part flushed with clean water, another clean flexible rubber tube can be attached and filled with clean water. One can then physically blow the water through the orifice. It should come out as a steady stream, not a drip. The burner itself, is usually not a problem source. It is simply a tube with a "grill" at on end the other end accepting the jet.
Once all components are tested and cleaned, then with combustion chamber assembled, the seal can be tested by plugging either the exhaust or intake opening with a rubber cork, then blowing air via a flexible tube into the other. There should be no leaks. Leaks can occur at flexible intake or exhaust tubes, combustion chamber gasket(s), join at intake/exhaust tubes, gaskets at intake/exhaust manifold, or intake or exhaust gaskets.
A standard BBQ tank and regulator will connect to LPG connection at the rear of the fridge. This should suffice to test if the fridge now lights.
Common Problems
Flame goes out when driving
A leaky combustion chamber causes hard starting and flame out when driving. To check for a leaking combustion chamber with the Dometic still installed, remove the chrome vent cover. Put a cork in the deepest of the two vents, and blow into the other. It should be an obvious seal. If not, got to find the reason, and retry. Is the drain cap off? Unsealed box? loose orifice in box? Cracked corrugated vent pipes (a good possibility)? Cracked vents are caused when removing and reinstalling the Dometic, especially with the 182b. Very brittle pipes. When bent too much, they will crack at a bend. If you have cracked vent pipes, remove the side grill and stick your hand inside and feel for the crack. Never done it, but muffler cement on the crack is the fix (Roger Sisler).
Troubleshooting
Poor cooling
- Check that you are not using the gas burner while operating on DC or AC. This overheats the boiler which will cause poor performance and permanent damage to the cooling unit.
The correct amount of heat is needed to ensure that chemicals in tubes at rear of fridge, circulate properly. Along these lines, if fridge is removed for servicing, "burping" the chemicals for a 24 hour period by turning fridge onto its' top, may help chemicals to circulate better.
If one mode cools fridge significantly better than another, this indicates poor function limited to this mode of operation. Each mode of operation can be considered seperate from each other.
If cooling poorly during use in "Gas" mode, then flame size/quality may be an issue. A partially clogged orifice in burner jet, burner, or restriction elsewhere in propane portion of system, will cause flame to be too small. Partially clogged intake or exhaust tubes, are also typical cluprits as is a leaking combustion chamber due to a failed/failing gasket(s).
In regards to AC, or DC modes, poor cooling can be attributed to poor quality connections, or failing heating units.
A poor door seal will also cause poor cooling.
If fridge is in good stock condition, here are some improvements to aid in cooling ability. Install CPU type fan inside fridge to help circulate cool air, replace exterior fan at rear with more robust computer case type (muffin) fan, install same type of appropriately sized fan and hardware at "City Water" hole on side of van to help remove hot air from rear area of fridge, reduce ambient temperature inside vehicle via pop top vent or equivilant, add insulation between exhaust/intake tubes and metal shroud, add tinfoil or equivilant to top of shroud/insulation to help keep heat in, renew thermal paste between "cold tube" and heat absorber (aluminum finned unit in fridge), and ensure that sensor at heat absorber installed correctly.
Dometic Factory and Other Documents
Before You Change That Cooling Unit (Bulletin R61/7A) (pdf)
Dometic RM182B manual (pdf)
Dometic RC160 instructions for installation and use (jpegs)
Blue LED flame indicator light modification
Things to watch out for
The refrigerator has to be within 8 degrees of level to work properly. Operating it more than than will result in damage.
Frequently-Asked Questions
How much "cold" is lost when the door is opened?
Not much at all. Air weighs approx .0755 pounds per cubic foot at sea level at 60 or so degrees F. At same temp, 50% saturation would be approx 7 grams or water per kg of air. Converting to same unit, 0.755 pounds is about 0.034 kg, so at 50% saturation, one cubic foot of water holds (very approx) (0.034 * 7.0 g), or, 0.238 g of water. 1 calorie is amount of energy needed to raise one gram of water by one degree C. To raise 1 gram 40 degrees F (very approx 22 C) would take 22 calories. To raise 0.238 g water same amount would take (0.238*22) = 5 cal. All this is VERY approx estimations, forgive me if i have dropped any decimal points. Bottom line is that there is not much water in the 1 cubic foot of air at 50% sat, really inconsequential compared to the amount of beer in the fridge (Alistair Bell).
