Dometic RM2453. LP good, AC freezes, nothing on DC.

[pjw73nh]

Dometic RM2453. Works fine on LP, freezes on AC and does not appear to do any cooling on DC. I know from what the previous owner told me that the AC heater was replaced last year.

I am using the following diagram for troubleshooting.I am also using the Dometic troubleshooting manual.

Schematic

I am just getting around to writing this now. But based on my observations from the summer (and if I remember them correctly), the fridge operates as mentioned above.

I am pretty sure I have a shorted thermostat but would like a second opinion. Not only is the part quite expensive, but being installed a Roadtrek, this fridge is a real PITA to remove to gain access to the thermostat. If in fact, I need to remove to to get to the thermostat.

It's rather cold up here in the northeast today (11/11/2-17), but I figured it's a good environment to test for a thermostat short, because if anything, it would be too cold and the thermostat would be satisfied (open). I made two sets of tests today.

The first set was done with the inside of the fridge at about 25 deg F. Then I turned to fridge on AC for a few hours and brought the inside temp down to 5 deg F and did the second round.

Outdoor ambient is around 35 deg F, internal fridge (not freezer) temp is about 25 deg F. In AC mode, with the thermostat knob all the way counterclockwise (off) with an inductive clamp-on ammeter directly on the AC heater lead, I measure 1.5 amps flowing to the AC heater. Turning the thermostat knob full on makes no difference in current flow. Turning the selector switch from AC to off reduces current flow to 0 amps as expected.

Performing the same test with the same environmental conditions but in DC mode and measuring the DC heater, yields similar results. The only difference is that the current flow when on DC is 15 amps. Again, the thermostat control has no effect and switching the mode selector from DC to off kills all current flow to the DC heater.

As mentioned, I repeated these same procedures with the internal fridge temp at 5 Deg F and I received the exact same results. The current NEVER stops flowing to the heaters in either mode at either internal fridge temp.

If I recall correctly from the summer, gas operation works properly.

I don't see any schematic information for the gas operation. The manual mentions that the thermostat is "calibrated by the manufacturer to maintain a 40 deg temp" in the fridge. But at the same time it says that temp can be controlled by the thermostat when is gas mode. What is the mechanism that turns the flame on for cooling, and off for pilot (standby/bypass) operation?

Summarizing my issue:

1. Does it sound like a defective thermostat? Do they go bad? Often? In 30 years of RVing, in 6 different rigs, I've never had one go bad. From the schematic, I can't see it being anything else.

2. If it IS the thermostat, is there any means of accessing it without removing the fridge? What about the little capillary sensor? How is that routed into the fridge?

3. Why might it not cool on DC mode, when in fact, there's current flow? I am thinking when they replaced the AC heater, they might have mis-positioned the DC heater, and it just doesn't get close enough to the boiler for effective heat transfer.

Any thoughts or suggestions welcome.

Thanks.

P.../NH

[pjw73nh]

The 12 volt element is NOT using the tstat control.... When you switch to DC, the 12 volt current is applied direct to the element and not thru the tstat control system.

Hi Doug,

According to the schematic it appears that 12V DOES go through the tstat. See link in my original post for schematic. Do you know if there is a newer schematic for this unit that shows the direct 12V application you speak of?

As far as the wires go, the 12V supply for the fridge is directly from the coach battery, is very close to the fridge, about 3 feet as it is woven, and is fused at the fridge, NOT at the converter or distribution panel. . I didn't measure their gauge, but the conductors look to be 8-10 ga. See here.

Heater wiring

Further, at 15 Amp current flow, if these wires were at all inadequate, they would heat up and possibly melt. This has not been the case.

The charging system in the motorhome is such that when the vehicle is running, BOTH the coach (house) battery as well as the chassis battery is being charged from the vehicle's alternator. When the vehicle is NOT running, the chassis battery is isolated from the coach/house battery.

I was not aware that there were "pockets" for the heaters welded on to the boiler portion of the fridge. I have never been inside the boiler area. That is good to know. I can't seem to find any diagrams of the refrigeration unit. Probably because there is not much servicing that can be done to it.

[BFL13]

Here is a link I have handy, that might be the one you already have. Anyway:

http://rvrefrigeratorrepair.com/wp-content/uploads/2016/07/Dometic-Service-AES.pdf

[dougrainer]

pjw73nh wrote:
Doug, Thanks for the reply. I have tried this several times. Used AC or propane for a few days while "camped", then switched to 12V while driving. Did not maintain temp. Even on moderately cool, northeast spring days. 60 deg or so.

From the operators manual. It all depends on how fully charged the RV battery is and the Gauge wiring to the refer. Also the gauge CHARGE line from the chassis system to the Coach battery. MOST OEM's require at least 8 gauge wire from the BATTERY to the refer for 12 volt DC operation. NOT from the Converter/Fuse panel. Doug

DC electric should not be used to initially cool the refrigerator. Only use DC when the other
modes are unavailable (for example; while in transit).

[pjw73nh]

Doug, Thanks for the reply. I have tried this several times. Used AC or propane for a few days while "camped", then switched to 12V while driving. Did not maintain temp. Even on moderately cool, northeast spring days. 60 deg or so.

[dougrainer]

DC(3 way refers), are NOT supposed to use DC element for cooling down a refer. The 12 volt element is a sustainer of the existing cold temp and is used while in transit. Doug

The 12 volt element is NOT using the tstat control. There was no reason to remove the 12 volt element when they replaced the 120 element. The tubes are welded to the flue. When you switch to DC, the 12 volt current is applied direct to the element and not thru the tstat control system.