RV wiring for trailers

[2400]

OK, the most popular question was about the 7 blade RV style plug for trailers.

I like this one and have it on all 4 of my trailers. I use all 7 on 3 of my trailers but just because theres 7 blades there you don't have to.

With 7 blades I can do a lot. Here's what I use them for and the wiring gauge I used on mine, if you see 2 choices I used the bigger one (smaller number).

10 gauge-------12 volt power
10 gauge-------Ground
12 gauge-------Trailer brakes
12/14 gauge---Left turn/brake lights
12/14 gauge---Right turn/brake lights
12/14 gauge---Tail or running lights
12/14 gauge---Back up lights


OK, I know someone is wondering what I need 12 volt power to the trailer for. I have a winch and battery on 2 of my trailers so I use it to keep the batteries charged, on my camper I use to to keep the 2 camper batteries charged. On the other trailer I don't use it at all.

I have a 10 gauge ground wire in addition to the trailer frame grounding through the hitch and ball on the truck. If you have a bad, corroded or weak ground you're going to have nothing but trouble with lights and brakes on your trailer.

One of the blades is for the trailer brakes and break away switch.

One is for the running or tail lights.

One is for the left turn and brake.

One is for the right turn and brake.

One is for back up lights. Everywhere I go I'm usually either starting, ending or both ends of the trip in the dark. I like seeing where the trailer is headed as I'm backing up. In addition to working off reverse in the tow vehicle I also have a switch in the tow vehicle that turns them off and on. That way I can leave the truck running with the A/C or heat on in park and have the lights on too.

Here's a diagram of the truck and trailer ends of the 7 blade RV style. I know that some of your trailer and trucks do not have the same colors in the same places, don't worry we'll get to that.

The truck side on on the left and the trailer is on the right.

 

[2400]

With 7 blades I can do a lot. Here's what I use them for and the wiring gauge I used on mine, if you see 2 choices I used the bigger one (smaller number).

10 gauge-------12 volt power
10 gauge-------Ground

I use at least 10 gauge wire for power from the tow vehicle to the trailer. That way I know I'll have at least 30-40 amps for charging. As I mentioned I have a battery on 2 trailers that I use to run winches, thats a high load use and I need to be able to recharge the batteries fairly fast. On the camper I have 2 BIG batteries that need to be recharged quickly when I'm towing or just sitting and using the truck to top them off. On my camper electrical system I use 2/0 welding cable for the positive cable to a 500 amp switch that lets me use battery 1, battery 2, both batteries or just shut them completely off.

I have a 10 gauge ground in addition to the coupler/hitch connection. The operation of ALL your lights, battery charging and brakes depend on a good ground. I have a dedicated ground for all my lights, winches, brakes and auxiliary batteries on my trailers.

On the 2 trailers that have winches I welded a 3/8" bolt to the trailer frame and use it to ground the winches, batteries and the ground wire from the trailer plug. That way as long as I have a bare spot on metal I have a good ground, I welded several 1/4" bolts for grounds for lights on it.

I did the same thing for my camper, the ground wire from the plug, the batteries and grounds from the camper electrical system all go to bolts welded to the frame. I use 2/0 welding cable from the batteries to 3/8" bolts welded to the frame so I have a good clean ground for everything.

 

[2400]

With 7 blades I can do a lot. Here's what I use them for and the wiring gauge I used on mine, if you see 2 choices I used the bigger one (smaller number).

12 gauge-------Trailer brakes

I have brakes on 3 out of 4 trailers. In AZ it's required that you have a breakaway switch and dedicated battery for the trailer brakes. So since the 3 trailers with brakes all have at least 1 big battery I was able to use it instead of the little battery that comes with the trailer.

The first thing I check when the brakes either don't work or just work once in a while is the ground.

Does the trailer have a good ground to the tow vehicle? If not fix it.

I take the set screw out of the barrel on the trailer plug and slide it back off the plug. That way I can use my circuit tester to check for voltage and make sure everything is working. The wiring diagram is correct the colors of the wires from your vehicle or trailer may not match this one.



If so the next step is with the trailer hooked up and plugged in, to have someone step on the brakes. Is there power to the brake wire in the plug? Is the brake wire is hooked up in the plug? If it's not or it looks iffy unscrew the set screw, pull the wire out, cut if off, strip it and put it back in and screw the set screw down.

Did that fix it?

Next I jack up one side, then put a jack stand under the trailer to keep from killing yourself. Look at the brake wires, there are 2 of them. One is a ground and the other is the power wire.

Usually the axle has a hole drilled in both ends and the wires are fished in one side through the inside of the axle and back out the hole in the other side. After a while the wires get chaffed where they go in the holes and start shorting out. So while you're looking at them pull them out a little on each end and look to see if the insulation is rubbed off.

If the wires have cuts, scrapes or are just worn out I pull new wires. I use a butt connector and hook my new wire up to the old wire and pull one at a time through the axle. When I hook up the new wires, I use a butt connector, electrical grease, and heat shrink. I strip the ends of the 2 wires I'm connecting, slip a piece of heat shrink tubing over one of them and push it out of the way, put a little electrical grease on each wire, slide them into the butt connector, crimp them, pull on the wires to make sure you have a solid crimp, then slide the heat shrink over the butt connector and heat it until it seals the wires and butt connector.

I run the power wire from the brakes through convoluted tubing to protect it up to the front of the trailer and into the plug.

Convoluted tubing




I have a 1/4" bolt welded to the trailer frame between the axles and ground the wire from the brakes to it and then run a wire from there to the ground in the plug. That way I KNOW I have a good ground for the brakes.

I mentioned a breakaway switch and a small dedicated battery for it. The breakaway switch wire is supposed to be hooked to the tow vehicle so that if the hitch or coupler breaks it will set the trailer brakes and stop the trailer. The battery is dedicated to the brake system and is charged by the tow vehicle.

Here is a diagram of the brake wiring, the colors may or may not match your trailer.



- - - Updated - - -

With 7 blades I can do a lot. Here's what I use them for and the wiring gauge I used on mine, if you see 2 choices I used the bigger one (smaller number).

12/14 gauge---Left turn/brake lights
12/14 gauge---Right turn/brake lights


So if your brake/stop light doesn't work on one or both sides.

With the trailer hooked up and plugged in and the 4 way flashers on.

Start with the trailer ground, is it good? If not fix it.

If you have incandescent lights check them. If one is bad go on and replace both sides. If one is burned out the other isn't far behind. If you have LEDs go on to the next step.

Then with the tow vehicles 4 way flashers on, using a test light check the wires at the plug and see if you have power.



If you have power at the plug, check the wire all the way back to the light looking for scrapes, holes and maybe missing wires. Fix the problems as you find them working your way to the back of the trailer.

If all is good check for power at the light. Once you find the problem fix it and then recheck everything. It's a whole lot easier to fix stuff at home in the daytime rather than on the side of the road in the rain with a pissed off cop looking over your shoulder.

I have LED stop/turn/tail and marker lights on all my trailers. They don't draw much power and are visible in the rain, snow, fog and daylight. I want people to notice my trailer and really notice my turn and brake lights.

On my camper and 18' flat bed have an extra light on the side that works with the brake or turn signal. I think that extra light on the side helps visibility.

I have a 1/4" bolt welded to the trailer on both sides in the rear for a good ground and also run a wire from it to the common ground at the plug.

 

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Excellent mini article 2400. I’m going to hit PRINT and file your posts in my shop for reference. Every time I go to trouble shoot something I have to run to the house and grab my I Pad and do a search of which blade is which. thanks.
All my stuff, with the exception of a couple small brakeless utility trailers, I’ve wired like you. I agree it’s important to have a dedicated ground line! Without a reliable ground, there’s nothing but frustration. Yep, lots use the ball hitch, but it’s marginal and unreliable at best.
Good advice on wire size, etc. Might mention to keep the plugs clean and even squirt a shot of oil in them once in a while. Is oil the best or something else?
Thanks for the post. I just hit the print button!