Next, we wire the DC side (if you want to implement it). Things are different over here, because one Power Path is made independent of the Norcold selector position. An "ON/OFF/ON" switch, just before the heater input +12V wire, is used to select from Relay and Norcold output. (It's after the relay, whereas the optional AC switch was placed before the SSR. It's also mandatory to have some kind of switch either ON/OFF or ON/OFF/ON, in order to turn off DC power.)
We create an entirely new path to "power up" the DC Heater, and it starts with with a new "Y" connector (Red twist connector), inserted immediately after the big fuse which Norcold installs on "their" +12V input power line from the 8-pin "straight through" set of connectors (4 by 2) between Norcold and TM wiring. Use this wire as the 12V source, because it is protected by double fuses (one at the TM power distribution panel, and another immediately before your Cut).
3 wires in the red connector are: (1) "Hot wire" from Norcold interconnect bus, with the fuse; (2) The other end of the wire you just cut, leading up to the Norcold selector switch (and other Norcold wiring); (3) A new wire, connecting to a "load" terminal on the automotive Coil-based Relay. (This is the kind which makes a little "click!" noise when it turns on or off.) Go ahead and make these connections now.
Next, Now cut the Norcold DC heater +12V lead, quite far away from the heater unit, and rewire into the ON/OFF/ON selector switch as follows:
- Pin 1 connects a new wire from the other power terminal of the Relay;
- Pin 2 is Common, the cut wire which goes to the DC heater coil;
- Pin 3 is the Norcold +12V "Power to Heater" wire (The other end of the wire which you just cut).
Whenever the Selector switch has the Relay circuit active, you
WILL have DC power going to the fridge heater (even with the Norcold selector switch set to "Off", or "Propane". Even in storage.
And even in an RV park where you have turned the interior selector switch to "AC" - you will be running both heaters, and that is a really bad thing(tm).
But it has big benefits. First, and maybe most important, it gives some Boondockers, with lots of Solar power available, a chance to use propane via the Norcold switch, and augment it with Solar-sourced DC power to achieve perfect temperature control. If you mount the switch in place of your CATV input port, then you can also take care of the "I'm towing all day and forgot to turn the Fridge to DC!" scenario by popping the front shell up and opening the switch cover - that's a lot less work than opening both shells, assembling the door, and then walking in to change the fridge panel setting. But most important of all:
No more frozen beer, even on DC. Because it's controlled by the same PID, 12VDC power achieves the exact same temperature management as 120VAC: in my case, 34.6 to 34.9 degrees in the coldest part of the fridge.