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Old 05-16-2016, 12:25 PM   #1
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4-way TV to 7-way RV, do I need the Aux power?

Hello. Hope this isn't too noob a question, but I am one so it probably is one.

I'm towing a new-to-me 2003 17' Hi-Lo Towlite. I've got 4-way trailer wiring installed on my TV, but the Hi-Lo of course requires 7-way from the TV.

I bought and have (mostly) installed this guy: a 4-way to 7-way adapter from CURT.

The existing 4-way output from the TV goes right into the adapter. Same for my blue brake power wire. And I've grounded the adapter to the TV frame. But the adapter also has a black wire to take aux 12v power from the TV, apart from the 12v that's already a part of the 4-way.

Here's my question: what am I missing if I don't connect this black aux 12v? Will my travel trailer's batteries still charge during towing, or is that part of what I'll be missing?

I'm still trying to digest the electrical systems in the RV. How and when the batteries charge, and from where, etc. All help is appreciated.

Thank you!!
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Old 05-16-2016, 12:41 PM   #2
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You will not have the charge line from the TV.


While plugged in to shore power or a generator, your batteries should charge from a built in converter.


If you don't overnight more then 1 night between campground, you should be fine.


Without the charge line you may find yourself with run down batteries the second night.


A 12 or 10 gauge wire and a auto reset, 20 amp breaker is all you need. Find a power point in the TV, rated for 20 or more amps that works with the key on.


If you can't find a key controlled power point, just remember to unplug the trailer, if overnighting, while running on batteries. Otherwise you could run down the TV battery and not be able to start it in the AM
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Old 05-16-2016, 01:35 PM   #3
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Thanks, Twinboat!
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Old 05-16-2016, 02:48 PM   #4
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My truck allegedly has the 12V charge cable (voltage is there on both the TV and trailer), but my battery never sees any charge. I assume the wire gauge is too small to handle any meaningful current. I finally gave up and ran a heavier gauge dedicated wire from the truck's alternator to the camper's battery (fused on both ends).
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Old 05-16-2016, 05:22 PM   #5
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Quote:
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Here's my question: what am I missing if I don't connect this black aux 12v? Will my travel trailer's batteries still charge during towing, or is that part of what I'll be missing?
If you are looking at the inside of the trailer connector where the wires mount to the terminals starting at the notch at the top and working clockwise:

1:00 is Black and 12 volt power - normally used for RV battery charge. If the black wire is not "hot" when the engine is running, then your RV battery is not charging.
3:00 is Green and Right Turn and trailer Brake
5:00 is Blue and electric Brake Controller
7:00 is White and Ground
9:00 is Yellow and Left Turn and trailer Brake
11:00 is Brown and Tail and Running Lights
Center is Purple and is called the auxiliary 12-volt power wire, and is rarely used for anything. But when used, it is most often used for back-up lights.

Black (12-volt hot wire) is almost always used as the RV battery charge wire.
On most tow vehicles, that black wire will not be hot unless you (or your TV dealer) added a fuse and/or a relay to your power distribution box. On both my '99 F-250 and my 2012 F-150 with factory towing package, the fuse and relay were in a plastic bag in the glove box, so the dealer's new-vehicle prep team had to install them.

If your trailer has a 12-volt RV battery, then you want the black wire to be hot when the engine is running. If your trailer has backup lights, then you want the purple wire to be hot when the tranny is in reverse.
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Old 05-16-2016, 05:49 PM   #6
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Nobody has yet mentioned anything about activating your trailer brakes. Does your Curt adapter do that? Not being familiar with that setup would leave me worried that without a brake controller you might not have trailer brakes, which could be a huge safety issue. I really believe your best course of action is to install the proper 7 way RV plug on your TV, or a 6 round 'stock trailer' plug and a brake controller.
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Old 05-16-2016, 06:36 PM   #7
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The OP mentioned the blue brake wire going into the adaptor.
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Old 05-16-2016, 07:15 PM   #8
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Quote:
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My truck allegedly has the 12V charge cable (voltage is there on both the TV and trailer), but my battery never sees any charge. I assume the wire gauge is too small to handle any meaningful current. I finally gave up and ran a heavier gauge dedicated wire from the truck's alternator to the camper's battery (fused on both ends).
This is my plan for battery charging.
Why fused at both ends?
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Old 05-16-2016, 07:55 PM   #9
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You fuse to protect the wire from burning and causing a fire.

When plugged into the trailer it can short anywhere in the middle and get power from both batteries.
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Old 05-16-2016, 10:58 PM   #10
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I am a bit confused? You see the trailer is set up for 7 pin get your tow vehicle is only hooked up for 4 pin? This is where I am a little confused if you're set up for 7 pin you are probably set up with electric brakes. 4 pin could not control your brakes. I really think you need to properly set up your tow vehicle so that you will have electric brakes and the 7 pin connector. Just my observation. I think I got what you were saying right. Be safe... Dusty
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Old 05-16-2016, 11:18 PM   #11
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I have 7 pin on my tiny tent trailer.
Only extra wire I had to run is to charge battery.
If you have trailer brakes, you need to wire them also.
My trailer is only 950 lbs.
My understanding is under 1500, brakes aren't required by law in most states.
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Old 05-17-2016, 07:39 AM   #12
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The OP mentioned the blue brake wire going into the adaptor.
The blue wire will not activate the trailer's electric magnetic brakes unless there is also an electric brake controller installed in the tow vehicle.

The 4-pin plug has 4 wires:

1] Right Turn and trailer brake lights
2] Ground
3] Left Turn and trailer brake lights
4] Tail and Running Lights

A 5-pin plug usually adds a wire for electric brake controller. 5-pin plugs are not common, because most jurisdictions require that trailers with electric brakes also have an emergency disconnect circuit, and that circuit requires a charged-up battery.

A 6-pin plug usually adds both the wire for electric brake controller as well as the hot 12-volt wire for trailer battery charging. 6-pin round plugs are common on horse and livestock trailers. Even smaller livestock and cargo trailers with electric brakes will have at least a small 12-volt battery on the tongue to activate the trailer brakes in case the trailer comes loose from the tow vehicle. And that small battery requires the charge in the battery to be maintained. Most RV trailers don't have that small battery on the tongue because they use the larger 12-volt RV battery to power the disconnect circuit.

A 7-pin plug usually adds another 12-volt hot line used for trailer backup lights. In that case the "hot" line is hot only when the TV's engine is running and the tranny is in reverse gear.
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Old 05-17-2016, 07:59 AM   #13
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Yes, I do have a brake controller. I had the 4-way trailer wiring installed at the same time as my hitch, and then added my own brake controller shortly after. So the blue wire I mentioned is indeed functional. And the new camper has trailer brakes (though I have not yet tested them), but does not have reverse lights, so I'm hoping my only remaining wiring issue is the 12v hot lead.

Thanks everyone for the replies. My first couple of campsites will have 120v hookups, so I'm not in danger of discharging batteries just yet. But I definitely will get that 12v black lead hooked up and fused before I try and boondock anywhere.
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Old 05-17-2016, 11:08 AM   #14
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What is your tow vehicle? It may already be wired for a full 7 way connector and all you need is the info on how to utilize it.
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