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Trailer wiring basics for towing

Wiring up a truck for trailer towing isn’t as hard as it looks; many new trucks come prewired from the factory, and there are kits that make the operation plug and play. The one problem with most of these kits and prewired installations is that they leave the truck wiring and electrical components vulnerable to problems caused by short circuits, and in many cases the factory truck wiring is barely adequate for running the truck taillights, let alone additional trailer lights.

Duster towing

My father engineered this system of running trailer lights; I've made a few modifications that helped it along. By doing this you not only have the trailer lights protected separately from the truck lighting, meaning if a trailer lighting problem arises it doesn’t burn out the truck lights, but you are feeding the trailer lights with larger wires, meaning they will be that much brighter, necessary for safety.

Among the family pool of trailers, almost all of the trailers are wired the same; all my tow rigs are wired with 6 or 7 pin round connectors as that’s what most of the trailers are wired with. For the ones that aren’t, I have pigtail adaptors made up, that is they have the 6 or 7 pin on one end and whatever the trailer has on the other.

One trick I've found is to put a round style female plug on both the towing vehicle and the trailer, and use a pigtail that has the male end on both ends. This has several advantages:

Doing a proper job with trailer and truck wiring will ensure that the system will be trouble free for the life of the vehicles involved, and if there ever is a short it won’t burn up the truck wiring, switches, alternator, etc.

Wiring for the plugs is more or less standardized. Following the guidelines below makes it easier to tow a borrowed trailer or hook up to a new trailer with your old truck or vice versa, as they should all be similar. 18-wheeler pin positions were set by ICC regulations many years ago to avoid problems with trailer interchange; the RV industry more or less followed suit to make delivery of new travel trailers easier.Suburban towing

When running the relays, hook them to the truck wiring in the cab and run it through a grommet in the firewall. This puts all the connections from the truck wiring inside, where they are less apt to corrode, and avoids feeding the hot wires from the battery through the firewall, so they are less apt to short out if the worst happens. If you are using a trailer brake controller that has a line off the master cylinder, be sure to use a grommet on that line also, and don’t forget to bleed the air out of the brake controller.

When running the wires from the relays to the trailer plug, I recommend using 4/10, 6/10, or 7/10 multi strand cable, available from most heavy truck accessory shops. You can make your own cable but I wouldn’t use any less than 12 gauge wire — 10 is recommended — as you are carrying current over double the length of the truck in some cases. You don’t want dim trailer lights from line loss. Another advantage to 10 gauge wire on the truck is that if the worst happens and wiring gets toasted, it probably won’t be the truck wires that get cooked.

Route the wire as close as you can to the factory truck wiring, avoiding hot exhaust pipes and other things that make the wiring go “ouch.” I recommend soldering all connections and using di-electric grease in the plugs, so corrosion can’t get in there and make life miserable. Tinning the ends of the wires where they go into the connectors for the relays and trailer plugs will also help eliminate corrosion.

boat towing

I have found that the round type connectors seem to work better than the flat connectors, mostly I think because they have a cap over them to keep the crud out when not in use. A couple of the small utility trailers I tow have flat connectors on them, and it seems like if the trailer hasn’t been used in a while, it takes a bit of fiddling with the plug to get the lights to work.

Doing your trailer wiring this way will cost a bit more than the ready made kits, and takes a bunch more time to install, but in the long run it will save you time and trouble. This system has been in use on one of my dad’s trucks for over 30 years, with no problems. If done in a modular fashion, it isn’t too hard to remove it from the truck if you’re trading it in and install it on the new truck. One of my brothers has used a trailer wiring set like this on four different trucks as he’s traded up over the years. It may seem to be a bit overkill, but in several decades of trailer towing, I've discovered that Murphy was an optimist, and anything that you can do to make things trouble free helps keep his law from applying.

four-pin trailer connector

four pin round hole towing plug
6 pin RV (picture from Low Cost Trailer Supplies; wire color codes modified)

6-pin RV

Code Function Color   Code Function Color
S* Brakes Black RT Right Turn Green
GD Ground White A* Auxiliary Yellow or Blue
TM Taillights Brown LT Left Turn Red

7-pin RV wiring

Code Function Color   Code Function Color
1 Ground White 5 LH Stop & Turn Red
2 Electric Brakes Blue 6 RH Stop & Turn Green
3 Tail Running Lights Brown 7 Auxiliary


4 Battery Black      

7 pin tractor trailer style:

7 pin tractor trailer style




1 Ground White
2 Marker and Clearance Black
3 Left Turn Yellow
4 Stop Red
5 Right Turn Green
6 3 Bar Marker and Tail Brown
7 Auxiliary Blue

Pin 7 is always hot on ABS equipped trailers with air brakes; I use it for the electric brake controller on non air brake trailers. This requires a switch to go from one to the other on my semi tractor.

If so inclined you can use separate circuit breakers on all the relays. If you do, I recommend using 15 amps on all the lighting circuits. You can also use fuses instead of circuit breakers (my dad does on his truck), but a trailer short can eat up a lot of fuses before it’s located and corrected.
One final note, if your towing vehicle has separate brake and rear turn signals, you can buy the funky adaptor that makes a two taillight trailer work right, and use the 4 pin setup if you want, but good luck, that’s one of Murphy’s favorite items. Better is to add brake lights to the trailer, and use the 6 or 7 pin connector and run the brake light wire through the auxiliary circuit.

Thanks to and for inspiration for these images

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