Basic 12v wiring question

[Stereo]

I have a Dometic 2193 3-way fridge in my Northstar popup camper. The 12v is not working. (I know the caution around running down my house battery if I run it on 12v for long.) I suspect it's the wiring but I'm a newbie when it comes to 12v wiring.

Here's the wiring diagram:

Strangely, the diagram doesn't show the wires coming into the block, but of course they're the main positive and negative wires as pictured here:



I'm not familiar with this type of terminal block but I would think there are internal connections only from left to right (not diagonally) so it appears I don't have a complete circuit since the black wire going to the switch is not opposite the (main) negative wire. Perhaps someone disconnected the 12v circuit to avoid bumping the switch (easy to do) and running down their battery. If I'm right, I assume that I need to move the black wire up so that it's opposite the negative wire to complete the circuit. Please correct me if I'm wrong. I don't want to short anything out.

My multimeter reads 12+v at the top of the toggle switch but there's no voltage on the wire at the bottom of the switch with the switch on or off which reinforces to me that I have an open circuit (or the switch is bad).

I also need an education on electricity flow. Based on the wiring, it looks like power first goes to the heating element and then to the switch. Of course, the heating element won't turn on until the switch is activated to close the circuit no matter which way the power flows, but why don't the designers send power to the switch and then to the heating element?

The same mystery-to-me is evident on the 120v side: The power appears to go through the switch to the heating element but the thermostat is wired into the neutral side of the wiring (though there's a tandem switch for both the power-in side and the neutral return side after the thermostat). I would have thought that power would go to the switch, then the thermostat, then the heating element and then return through the neutral side. Why is it wired in the sequence that it is?

Thanks for your help!

 

[burleyman]

Yes, sometimes the switch(s) are placed in the neutral or negative wires. Old vehicles points systems and non-USA things were my first exposure to that decades ago.

Yes, it does seem that you need to move the mismatched wires directly across from each other. If you remove the two mismatched wires and brush them together with the switch on, you should see slight sparking. If so, you're good to go.

The 12v heating element fridges I've encountered draw about 10 amps when on. You would need a DC ammeter to read that, or the sparking mentioned above.

A 12v LED bulb connected to the wires to the heating element would indicate when turned on.

 

[DaveInDenver]

A lot of circuits switch the return/negative/neutral.

There's electrical reasons for this but one reason designers avoid doing it is leaves the load/device/application on a hot circuit open waiting for something to get it back home. In mains wiring that can be a problem if you or something else is touching ground. On a vehicle it's a problem since the whole vehicle is waiting to rub through a wire or other fault.

So switching on/off the positive/hot side is generally preferred. But most circuits don't care electrically where the switch is and as a wiring technician you need to be aware that it could be either way and roll with the punches troubleshooting.

One such thing is heating elements for example, which is why it has two browns and is otherwise indifferent to which side is which. It'll work just the same regardless of polarity and which side gets switched. You could reverse the brown wires or which side you switch and nothing would change functionally.

In electrical engineering the concept is known as high-side vs low-side switching. High-side being that "high" voltage side, e.g. the hot side, is switched. Low being the ground or low, as in zero voltage, side. Not to bore you with too much theory but when you do solid state switches you many times have to do low side, e.g. switching the ground on and off. With mechanical switches and relays you are flexible so might as well do high side, e.g. switching the power side.

If that thermostat happens to have a solid state switch it may have to be on the low side to work (this delves off into explanation of transistors and specifically MOSFETs, so just know it's "because magic" of electronics. You'll also see that as a requirement on some solid state relays, too.

A only reason you might see a low side switch on a relay, for example, is the designer wanted to save a wire and perhaps shared a wire or connector for both the coil and contacts or with other circuits.

Those wire terminal blocks are colloquially known as "euro" style since you see them in Europe and Asia often. They're not as popular in North America but that's changing with globalization. They are usually straight through, one termination on each side. To get multiple ones you'll put in a bus bar or jumpers.

 

[Stereo]

Thank you both for the education. Much appreciated.

I successfully got the circuit working but it took a bit of work. I removed the negative wire from the terminal block to close the circuit with the black wire connected to the switch. I got the connection spark. I then tried to return the negative wire to the block but couldn't seem to get the wire in. I removed the block from the back wall so I could see into the block and discovered that the wire gauge was too large for the receptacles in the block. Both wires coming in from the left had about a quarter of their conduits cut off and many more were smashed around the block. The manufacturer thinks this is OK?

I removed both wires on the left side of the block, snipped off the bad ends, connected them to a pigtail wire of smaller gauge (but larger than the wires on the right side of the block), soldered the other ends of the the pigtails and inserted those ends back into the block.

The heater works. It's pulling about 9.6 amps as predicted by burleyman. My battery regen wiring shows 12.6a through the positive cable at idle with the fridge on so I assume I can drive with the fridge running on DC without depleting my battery. Right? We've always driven with the propane on but that's obviously less safe.