Another Refrig that Won't Cool on Propane

[Aironius]

The Dometic RM2452 in my '01 Bambi cools great on 110v but not on propane which is a new failure because it used to cool fine both ways. This is a 110/gas machine with manual piezo ignition and a gauge to indicate flame lit. The basics are good, clean flue, properly installed baffle, 12"H2O pressure at refrig inlet, etc. The burner lights and burns on what may be the pilot mode. It will stay lit after you hold the button for a bit but the flame indicator stays "off" no matter how long it is lit.
After removing and inspecting the burner (it was clean) I reassembled it and lit it with the button held down. Thermocouple voltage measured at the disconnected pigtail was .024v, flame was steady blue on all burner slots. Hooked the thermocouple back up and still no "high" flame, no deflection of flame indicator needle. Turned thermostat from Max to Min to Max w/ no change in flame and no "flame lit" indication.
So just before I waste money replacing a good thermocouple and then remove the machine from the cabinet to see what is really wrong, does anyone know how the "flame lit" indicator is connected to the thermocouple? And what is normal output voltage for the thermocouple? Should I throw one at it and see what happens?

[uncle_bob]

Hi

Best guess is you have something basic wrong with the burner setup. Bugs in the flue maybe. There are lots of threads here on digging into what may be wrong.

Bob

[CBWELL]

Clean the burner orfice. That is the usual problem. Easy to remove, and soak in solvent for an hour, then replace. Usually solves the problem.

[Aironius]

Thanks for the response, guys. I assure you the basics have been gone over more than once. I have to think that the fact the "flame lit" indicator does not move indicates some sort of electrical problem. I have discovered that the "thermocouple adapter" referenced in the wiring diagram of the indicator is the same thing as the "nipple, thermostat" shown in the parts picture and list. It is just a M/F coupling of the threaded type used on thermocouples but with a male spade connector sticking out of the side feeding the "flame lit" indicator. So absent any actual literature to the effect, I presume the indicator is simply tapped in parallel to the safety valve coils. The thermocouple shows up to .024v when disconnected with the burner lit, but the lack of any action from the gauge makes me suspect the voltage is not adequate under load. Anyway, I ordered a new thermocouple and will test it and install it to see what happens.

[JCWDCW]

0.024V sounds very low; surprised the burner actually stays lit! Thermocouples usually deliver about 0.5 volts for normal household gas appliances. If you have a clean blue flame, then the burner is (as you noted ) clean. That is the most common problem, and then a rusty plugged flue is the next most common. ASK about the expected voltage when you purchase the new T/C
JCW

[Aironius]

OK, a report is due. I pulled the refrig out far enough to access the control panel. The non-functioning "flame lit" indicator was a red herring. The meter movement was actually binding up. I discovered that if I shook and tapped the meter while the tcouple was hot, the needle would move. But it stayed where it was until I tapped the meter again to make it return to the peg. Moisture must have got in and gummed up the meter. I backed off a tiny bit on the bearing screw and now it is working, but it caused much confusion. So now the new tcouple puts out .033v no load. Hooked directly to the indicator, the needle rises to the top of the green zone. But with the safety valve coil connected the voltage drops to .009v and the needle only just barely gets into the green zone. The safety valve stays on but it must be drawing too much current to pull the voltage down like that. I need test specs to make sense out of it. After disassembling and cleaning the thermostat it seems to be working. Pressure at the test port on the burner is 11" H2O with tstat set to "off" and 11 3/4" at any other setting. Currently set to 6 and refrig is 36* this morning. I am waiting to see the temp stabilize and the flame go back to pilot level. So the machine appears to be working now. Did I fix something or unstop something? Or was it simply a matter of the tstat requiring a different setting to achieve the same temp on 110v and on gas? Anyway, I replaced the combustion seal around the unit and cleaned everything up so that can't hurt. I still think there is something wrong with the tcouple circuit. I plan to replace the indicator once I see that the unit is cooling properly.

[uncle_bob]

Hi

Most setups will give you 1/2 the open circuit voltage when you put the load on the thermocouple. The 0.009 V you are getting should be up around 0.015 to 0.018. Is it *always* that way? Unfortunately no. You have plenty of open circuit voltage so you do have a good thermocouple voltage wise. I'd re-check the wiring for bum connections.

Bob

[Aironius]

Thanks for the info, uncle_bob. The refrig seems to be working now, but the thermostat must be set to 6 for gas operation while 3 is just right for 110v operation. It probably was working all along but we didn't understand the calibration error. I have ordered a new flame indicator because the old one is still sticky and unreliable. We will see what happens when I install it. If the new meter shows a good solid green reading then I will assume the observed voltage drop is normal. I saw a thread from another forum during my online search for parts which said that as little as .005v under load might be normal. From experience with household furnaces using thermocouple powered gas valves, I expected to find a coil with many windings of very fine wire. I was surprised to see that the safety valve uses a cluster of four coils with just a few turns each of rather heavy gauge copper. So it looks like the magnet might well pull the voltage down a lot.