RV Education
Towing
Series
Tow Dolly
Towing 4 Wheels Down
Tow Lights and Wiring
Tail Light
Wiring
Braking
Units M&G
Braking Unit
Air-Brakes
RV Resources and Tech Advice
Mike Wilson is a RVDA Certified Master RV
Tech. with over 25 years experience and now he's sharing his
knowledge with you.
Mike is the president of Wilson RV
and developer for The RV Help
Desk.
Tow Lights and Wiring
There are several options for
rear lights while towing. Light bars and separate lights are one
method.
Using the towed vehicles existing lights is another method.
A light bar can be used if
towing for a limited number of times. There is little labor and time
involved in set up.
Since most RV’s have a wiring connector already in place, you can
simply run an extension along the tow bar and back to the lights.
A magnetic mount is generally used to attach the lights to the
vehicle. Storing a light bar, installing and removing a light bar
become tiresome if towing frequently.
Wires dangling down and difficulty securing them is another good
reason to hard wire the towed vehicle’s rear lights.
There are now wireless tow lights available. I have not tried a
wireless light set up, so I will not comment on them at this time.
Most often the rear lights on
the towed vehicle are hard wired to work off the RV. Wiring kits are
available from most tow bar manufacturers.
These kits include wiring, connectors, and diodes. The diodes are
necessary to avoid electrical feedback through the towed vehicles
electrical system.
Due to the design of the automotive electrical system, a turn signal
switch or a brake light can feedback through a vehicle possibly
damaging the system or running an unwanted circuit.
A diode simply isolates the current flow to the direction of the
light and not in reverse.
The diodes designed for towing should be used since they are
designed for exterior use and typically have a heat sink built on
the diode.
Hard wiring tow lights can be
very complicated and difficult. In some applications, separate light
bulbs may be needed.
Vehicles that use ground switching to activate lights will require
separate bulbs.
Separate bulbs can create problems since there is usually not enough
room for another socket and bulb.
Be sure to check your rear brake lights for voltage before deciding
on which towed light system to use.
Tow vehicles use positive switching to activate the towed vehicle
lights and cannot activate negative switching lights.
In other words, a vehicle that uses ground switching has positive
power at the rear light when the light is off and uses ground
continuity to activate the light.
You can test for ground switching with a simple voltage test at the
rear lights. I have found a couple of vehicles that use ground
switching for brake lights, but not running lights.
Remove the rear light fixture and test for constant voltage at the
positive feed for the brake light. If it has constant voltage with
the light off, it is ground switching.
If it gets voltage only when applying the brake, it uses positive
switching. It is important that you understand you will be using the
red brake lights.
I do not recommend using amber turn signals for towing. Some RV’s
have rear amber lights.
They do not typically use an amber circuit for towing, but rather a
converter for red light only use.
You can have an RV with amber turn signals and still use the red
brake lights only for both brakes lights and turn signal lights on
your towed vehicle.
I make my own wiring kits. I
use bulk wiring and have a bulk supply of diodes. Roadmaster sells
their diodes in bulk bags and they are very good quality.
Using a bulk roll of wire eliminates the problem of the wiring being
too short. I find that Remco supplies a harness that is too short
for almost all vehicles.
Most auto parts stores have 4 wire trailer wire in bulk. Generally I
would buy about 30 feet to be sure you have enough. You don’t want
to splice the harness.
I usually run my wiring outside the vehicle. It is easiest to run
the harness back while the front fascia or bumper is off during base
plate installation.
Find a good location for the light socket. One of the reasons I like
Blue Ox base plates is because they provide a mounting point for the
socket.
You can use any type trailer socket you’d like. I usually figure how
many wires in total I will be running before choosing a socket.
There will be 4 wires for lights. If using a lube pump, there will
be 2 more. If using a braking unit there may be more, and if using a
battery charging system, there will be more.
I will cover charging systems and braking systems in another
article.
Mount your socket, but do not connect any wires yet. Start to run
your 4 wire harness to the rear. Lead the harness into the engine
compartment.
I usually go to the driver’s side. Stop at an area at the front
where you can attach the ground wire to a good ground source.
I look for a factory harness ground point. Split the white wire off
from the 4 wire harness and cut to length. Install a terminal then
attach at ground point.
Continue to run the remaining 3 wires to the rear going down near
the firewall then to the under side of the vehicle.
There are usually brake lines and/or fuel lines to secure the
harness to. Be careful to avoid sharp or rotating objects and areas
that will get hot. Support harness with tie straps.
When getting near the rear decide how the harness will reach the
lights. Remove the rear light fixtures to see how the factory
harness runs to them.
If there is outside access to the factory harness, run the 4 wire
harness directly to that area.
If the factory harness is inside the vehicle, you will need to find
an easy access point to run the harness through.
Typically I find trucks and sport utility vehicles have outside
harnesses and cars have inside harnesses.
I look for a factory grommet or vent flap to run the wire to the
inside for cars. Be sure to poke a small hole and seal it with
silicone seal.
Split the green wire off from the 4 wire and run it to the right
side rear light. Run the other 2 wires to the left side rear light.
Using only the red brake light (not amber), find the factories wire
that feeds the left brake light. Be sure to test for voltage right
next to the light bulb while someone applies the brake.
Some vehicles use “bulb out” indicators. You will need to be on the
bulb side of the indicators.
Cutting the brake light wire very close to the bulb/socket will
ensure you are on the correct side of a bulb out indicator.
Cut the wire; install the terminal connector provided with the diode
to both ends of the cut wire.
Be sure to use dielectric terminal grease to all terminals and
connections. Poor or corroded connections are the number 1 problem I
see with tow lights.
Connect the wire coming from the vehicles harness (feeding the
light) to a diode input using the diagram on the diode for proper
electrical flow.
Run the yellow from the new harness to the other diode input.
Install a terminal on the cut wire (bulb side) and connect to output
diode terminal.
Follow the same procedure on the right rear red light using the
green wire from the 4 wire harness.