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Power for Demco Wireless Coachlink
The “check entry step” light is right next to my Demco brake controller. Power is supplied to the step light when the key is in the on position.  Could I tap into the light wiring to provide power to the brake controller?  Right now it plugs into a 12V outlet but I prefer it be hardwired.  Actually, it’s not really a controller; just a remote warning system to tell me the toad brakes are being applied.

Wireless Coachlink for Braking Systems | Demco Products
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Brake Controller
Reply #1 –
Assuming I can tap off of the entry step light, how can I tell which wire is negative and which is positive (electrical dummy here)? One wire is red, the other white.
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Brake Controller
Reply #2 –
"...how can I tell which wire is negative and which is positive..."

Normally it's a safe bet to say the red wire is positive!

You need to acquire a good Multi-meter and learn how to use it. They can be had on the cheap but mid-price will get you a better instrument.

Place Multi-meter's red lead to red wire and it's black lead to the white wire (or any convenient chassis ground point) and you'll know which one is 'hot'.   :o    ::) 
Steve S.
Lazy Bones & Cedar
2004 30'IB (Island Bed)
Yesterday is History, Tomorrow is a Mystery
Live for the day!
Re: Power for Brake Controller
Reply #3 –

In RV 12 V wiring, white is usually negative/ground, and any other color (even black) is usually positive. But I have heard of exceptions (new guy at the factory, maybe?), so never take that for granted. I strongly second the suggestion to get a multimeter. They're good for all kinds of things around the house--you'll never regret having one. For a beginner's guide to using a multimeter, see this article on my website.

If you're on a budget, I suggest getting the least expensive digital meter that has autoranging. That feature means the controls will be simpler and less confusing. For example, avoid meter like this one, with dozens of settings:

X

It'll work, but it's hard to figure out which setting to use, and easy to get the wrong one. An autoranging meter such as this under-$25 model is easier to work with:

X

If you want the best, you can't beat a Fluke, such as this $83 Fluke 106 model.

X

Yes, it's more expensive, but it's guaranteed accurate, it'll last a lifetime... and it's even simpler to operate than the previous one.

As an Amazon Associate Lazy Daze Owners' Group earns from qualifying purchases.
Andy Baird
2021 Ford Ranger towing 2019 Airstream 19CB
Previously: 1985 LD Twin/King "Gertie"; 2003 LD Midbath "Skylark"
Re: Power for Brake Controller
Reply #4 –
Thanks, Steve and Andy!  So no problem with tapping into that circuit then?  Would I need to add a fuse for the Demco wiring? If so, how do I determine what size fuse?
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Brake Controller
Reply #5 –
"Would I need to add a fuse for the Demco wiring?"

Maybe. Simplified answer: if the wires powering the Demco remote warning system are a smaller gauge than the wires leading to the “check entry step” light, then you should add a fuse. You should always use a fuse when going from larger to smaller wire, in order to protect the smaller wire.

But there's a good chance you won't be able to see the wire gauge printed on the “check entry step” light's wires, since the accessible section is so short. In that case you'll have to look at the physical sizes of the wires you're comparing. That's not precise, since insulation thickness can vary, but it'll get you close.

"If so, how do I determine what size fuse?"

I could refer you to ampacity tables, but it can be complicated to figure out what applies to your situation. So again I'll give a simplified answer: assuming you need a fuse, you'd probably be OK with one that is one size smaller than whatever is protecting the “check entry step” light. For example, if that's 20 amps, you'd go down to 15 A for the Demco indicator.
Andy Baird
2021 Ford Ranger towing 2019 Airstream 19CB
Previously: 1985 LD Twin/King "Gertie"; 2003 LD Midbath "Skylark"
Re: Power for Brake Controller
Reply #6 –
Thanks, Andy.  I just went out and was able to read the size on the two insulations. The step light is 16 AWG and the Demco is 22 so I will add a 15 amp inline fuse if I proceed with it.  BTW, I did buy an autoranging DMM based on a previous discussion we had, and looking at your website.  I just haven’t used it yet!
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Brake Controller
Reply #7 –
Greg,

As Andy said, you size the fuse to protect the smallest wire.  The various ampacity tables I looked at put the maximum current a 22 gauge wire can carry at around 7 amps.  Nobody in there right mind would ever try to run 7 amps through a 22 gauge wire and is waaaaay more current then the Demco indicator light will ever draw.  You could easily protect the Demco 22 gauge wire and light with a 1 amp fuse.  After all… this is just an indicator light, not the brake controller you’re fusing.

- John
John & Cathy enjoying life with our 2021 Mid-Bath, 1996 Cherokee and “Bandit” the wonder dog 🐶
Re: Power for Brake Controller
Reply #8 –
John, earlier I had to disassemble the 12V cigarette plug to see what size the wires were. Your comment spurred my memory that there was a fuse inside the plug so I went back out to see the rating. Yes, it’s a 1 amp so you hit the nail on the head!  Thanks!
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Brake Controller
Reply #9 –
John is correct: you can use a much smaller fuse than 15 A. I should have said that I was giving general advice based on a worst-case scenario, and not knowing the smaller wire's capacity. But since your Demco's 12 V plug's fuse is 1 A, that tells you what you need to know. :-)
Andy Baird
2021 Ford Ranger towing 2019 Airstream 19CB
Previously: 1985 LD Twin/King "Gertie"; 2003 LD Midbath "Skylark"
Re: Power for Demco Wireless Coachlink
Reply #10 –
Thanks, Andy.  BTW, I changed the subject of this thread to better reflect what we're talking about, and added a link to the product to my original post.
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Demco Wireless Coachlink
Reply #11 –
Quote from: JohnR  - 
As Andy said, you size the fuse to protect the smallest wire.  The various ampacity tables I looked at put the maximum current a 22 gauge wire can carry at around 7 amps.  Nobody in there right mind would ever try to run 7 amps through a 22 gauge wire and is waaaaay more current then the Demco indicator light will ever draw.  You could easily protect the Demco 22 gauge wire and light with a 1 amp fuse.  After all… this is just an indicator light, not the brake controller you’re fusing.

Thanks, John.  So I just noted in a photo on another thread that the fuse for the step light, which is behind the easily removable dash section under the steering wheel, is rated for 3 amps. Can I use that same fuse to also protect the 22 AWG wires for the Demco?  Or do you advise installing a separate 1 amp inline fuse.  Am I correct in assuming that only the “hot” wire be fuse protected?
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Demco Wireless Coachlink
Reply #12 –
Quote from: HiLola  - 
Thanks, John.  So I just noted in a photo on another thread that the fuse for the step light, which is behind the easily removable dash section under the steering wheel, is rated for 3 amps. Can I use that same fuse to also protect the 22 AWG wires for the Demco?  Or do you advise installing a separate 1 amp inline fuse.  Am I correct in assuming that only the “hot” wire be fuse protected?

Hi Greg,
Yes you only need to fuse the hot (+) wire.  Tapping into the  step light circuit is what I would do and not worry about putting in a separate fuse.  The 3 amp step light fuse is more than adequate to protect both the step light wire and the 22 gauge Demco wire.  Pull the 3 amp fuse out of its holder and use your multimeter to figure which side of the fuse holder is hot.  Tap into the non-hot side of the fuse holder with your 22 gauge Demco wire.  That way you are protecting both the step light wire and the Demco wire.  Also, by pulling the 3 amp fuse first, you don’t need to turn any other power source off.

- John
John & Cathy enjoying life with our 2021 Mid-Bath, 1996 Cherokee and “Bandit” the wonder dog 🐶
Re: Power for Demco Wireless Coachlink
Reply #13 –
Thanks for everyone's input! Finally finished this up today.  I tapped into the hot side of the "Step Light" circuit.  I thought that wiring was 16 gauge but closer examination revealed it's 12 gauge.  The Demco unit is 22 gauge.  This solves one of the issues I had on our last trip of the 12V cigarette plug providing "flakey" power.
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Demco Wireless Coachlink
Reply #14 –
Whoops, I just reread JohnR's reply #12.  He said to tap into the non-hot side of the step light wiring; I mis-read it and tapped into the hot side (before the fuse).  What are the possible consequences of that?
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Demco Wireless Coachlink
Reply #15 –
Hi Greg; Virtually none. Should work fine. Any problem with 22 gauge wire (receiver for Demco) will go unnoticed by the 12 ga. wire.  RonB
RonB (Bostick) living in San Diego
Original owner of "Bluebelle" a '99 TKB
Re: Power for Demco Wireless Coachlink
Reply #16 –
Quote from: RonB  - 
Hi Greg; Virtually none. Should work fine. Any problem with 22 gauge wire (receiver for Demco) will go unnoticed by the 12 ga. wire.  RonB

Thanks for your input, Ron.  I decided to re-do the power.  I removed the splice from the hot side and put in a new splice on the non-hot side of the 3-amp fuse.  I was a little worried about the Demco unit getting fried if something glitched.  Everything seems to be working fine.
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Power for Demco Wireless Coachlink
Reply #17 –
Who labeled the wires? That's a kind thing to do when someone else is looking at it. 
And next year when you have forgotten,  you are that 'someone else'  ;)
Joel & Terry Wiley
2013  31 IB   ORWAN   / 2011 CRV Tow'd LWEROVE
Re: Power for Demco Wireless Coachlink
Reply #18 –
Joel, those are factory labels.
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264