Dometic RML-8330 3-way refridgerator modification

I generally like the Dometic RML-8330 that came in our 2016 Pleasure Way Lexor TS. It’s large, looks good and keeps food cold. The biggest issue I have with it is the lack of thermostatic control in 12-volt mode. This inexplicable design omission results in a constant ~15 amp draw while travelling and during short stops when it does not make sense to switch over to LP or 120V, both of which are modulated by a thermostat. Another engineering blunder on this model is that the manual controls are hidden behind the top 2-1/2 inches of the door, so any adjustments require that the door be open. Not good for RV refrigerator discipline. Finally, there is no fan inside the unit and I understand that having one is good for efficient operation. Fortunately, smart people have already addressed these shortcomings and I blatantly pilfered their ideas. Below is my spin on these mods.

Adding thermostat control to 12V operation

To fix the constant draw in 12V mode, I installed a Chinese temperature controller (XH-W1308) based on Tim Terry’s YouTube post: https://www.youtube.com/watch?v=jUb1ebA8Qs8. Here is where I got mine for $9: https://www.ebay.com/itm/371583683212?hash=item56841d0a8c:g:3OIAAOSwoudW9JF2. The installation was pretty straightforward. The hardest part was removal of the fridge and mapping the schematic to the actual wiring.

Wiring was not complicated and mostly from terminals on the mode selector. I pulled the 12V DC from the light circuit (terminal 5A), so the unit displays the internal fridge temperature whenever it’s on in any mode. The relay circuit was inserted into the line going to terminal 2B which is +12V when in DC mode and feeds the 30-amp relay that drives the 12V heating element. Now, the relay is only actuated when called by the XH-W1308.

With a little bit of creative cutting, I was able to install this into the OEM control panel for a fairly clean look.

Installing a fan: While I had things torn apart, I figured that I’d re-deploy an old computer fan I had laying around which only draws about 50 mA on low speed. I fabricated a bracket out of some aluminum C-channel I had.

Fortunately, I discovered that the stock light switch interrupted ground to the lighting circuit and had a 3^rd terminal that was normally open when the door was open and closed when it was shut. I had to swap the red and black wires and then solder some 26 gauge leads to the 12V and the center terminal for the fan. This enabled the lights are on and the fan is off when the door is open and the inverse when it’s closed.

Cutting the top off the door

The final step was inspired by James at FitRV (https://www.thefitrv.com/rv-tips/why-i-took-a-hacksaw-to-our-rv-refrigerator/). The door was easily removed by unscrewing the top hinge pin. I followed his instructions; except instead of a hack-saw, I used a jigsaw with a 20 tpi blade to do the surgery. Getting the top trim piece off was easier than James made it look using the Dremel tool with a cutoff wheel. Reattaching it to the cut top with a little JB weld worked nicely.

Here is what the final result looks like:

Performance observations

As I noted above, when in 12V mode, the fridge drew a constant 14-15 amps that kept the 175 watt DC heater running constantly. This translates to a maximum run time of about 13 hours before my two 100 Ah LiFePO₄ batteries would be dead. After this modification, I decided to test the performance and the capacity of my 6-year-old batteries by first fully charging them using shore power. I then unplugged and let the fridge go on 12V with the thermostat set to turn off at 3C (37F) and back on again at 6C (43F). The ambient temperature in my barn was about 55F throughout. To make a long story short, The BMS shut the batteries down somewhere between 23 and 24 hours after the experiment started, with my Victron BVM reporting a total of 182 Ah used over that period. I noticed a little hysteresis during warm up, where the fridge would briefly overshoot to as much as 8C (46F) after the DC heater came on at the 6C setpoint. Under these ideal conditions, the net current usage in DC mode was 182Ah/23.5 = 7.7 amps, or about ½ of what it would have been otherwise. Of course, the savings will be less in the real world, but still significant. On a recent 7 hour road trip, where ambient temps got into the mid-80s, although I did not quantify things, the fridge definitely cycled on and off, leaving more of my DC-DC charger output to feed the coach batteries. Once the batteries were full, the DC-DC only drew 18 amps of alternator current when the fridge was cooling and 0-2 amps when the fridge cycled off. The other thing I learned was that the actual capacity of the batteries has degraded to 182 Ah (a 9% loss), which seems reasonable for a six year old pair of OEM batteries that until recently were not managed under ideal conditions.