Dometic 3-way fridge grounded only though AC cord

[reilym]

All,
I am in the process of implementing thermostatic control the 12V operation of my Dometic RML8330 fridge following the very slick instruction of Tim Terry (see URL at bottom of this post). I'll follow up with a separate post on how that goes once the project is complete.

For now, I just have a question for those more knowledgeable. With the fridge on the RV floor, I ran a temporary wire from the 12V+ and black DC ground wire to the corresponding wires on back of the fridge so I could test everything. The lights worked, but nothing else. After a lot of head-scratching, I figured out that the fridge chassis was not grounded through the DC wiring. As soon as I plugged in the AC cord (not on shore power), everything worked. So the chassis ground, which most of the DC electronics rely on, is only made when the AC cord is plugged in. It turns out that the lights are connected directly to the DC negative, but everything else is grounded to the fridge chassis. Is there anything wrong with me also grounding the DC- feed directly to the fridge chassis so it will operate in DC mode whether or not the AC cord is plugged in? I know that both the AC and DC are vehicle chassis grounded, so I suspect that I can.
TIA!

https://www.youtube.com/watch?v=jUb1ebA8Qs8

[booster]

There is something wrong their, I think, as the chassis is almost always tied to and used for the negative 12v wiring. The AC ground is also tied to the same in most cases, but you shouldn't need it to run 12v power in the frig.



It sounds like the black ground wire to the chassis is not connected to the chassis well enough. Check for continuity from that black wire to the chassis with a multimeter to find out.

[reilym]

Booster, I might not have been clear. The fat black DC wire feeding the fridge is definitely grounded to the vehicle chassis. What it's not connected to is the refrigerator chassis. Ergo, when I was testing the electrical with DC connected and the AC cord unplugged, the circuits grounded through the metal refrigerator frame failed to function. Plugging the AC cord into a dead socket restored the ground path to the vehicle chassis and all was well. It just seems to me that the DC- should also be connected to the metal frame on the fridge.

[booster]

Originally Posted by reilym

Booster, I might not have been clear. The fat black DC wire feeding the fridge is definitely grounded to the vehicle chassis. What it's not connected to is the refrigerator chassis. Ergo, when I was testing the electrical with DC connected and the AC cord unplugged, the circuits grounded through the metal refrigerator frame failed to function. Plugging the AC cord into a dead socket restored the ground path to the vehicle chassis and all was well. It just seems to me that the DC- should also be connected to the metal frame on the fridge.

That makes more sense, but not from the frig design if it is built like that intentionally. If they need the AC line to ground the DC then there is no need to have a 12v negative at all.


It would be very interesting to get a hold of the factory and see what they say about it, as it just doesn't seem likely it would not be an error.


When you have the 12v plus and minus hooked up to the frig and powered, does the case get 12v+ on it referenced to the chassis of the van? If it does that would reinforce that the chassis needs to be externally grounded to work. If you can find the install instructions for the frig model, it may show one if they really want it that way.

[reilym]

Originally Posted by booster

That makes more sense, but not from the frig design if it is built like that intentionally. If they need the AC line to ground the DC then there is no need to have a 12v negative at all.


It would be very interesting to get a hold of the factory and see what they say about it, as it just doesn't seem likely it would not be an error.


When you have the 12v plus and minus hooked up to the frig and powered, does the case get 12v+ on it referenced to the chassis of the van? If it does that would reinforce that the chassis needs to be externally grounded to work. If you can find the install instructions for the frig model, it may show one if they really want it that way.

Yeah, I took a look at the schematic and the only cabinet ground comes from the 120V AC line. The DC- appears to be limited to providing a path for the lighting and the DC heater and shows no connection to the cabinet. It explains why the lights worked but the control circuits did not until I plugged in the AC cord. Although it seems that a redundant chassis-cabinet ground would not hurt anything, I guess I'll just leave things they way they are since the fridge has it's own AC outlet that is normally always connected (and it works!).

In any event, I will soon have thermostatic control on the 12V heating element. This should eliminate freezing on long road trips and make more alternator current available for charging the battery. To my mind, this will correct the main shortcoming of this system.