fix trailer lights
How To Fix Your Trailer Lights
We have written this to make it easy for anyone to understand and test trailer
lights. Feel free to print this document and wiring diagrams to take them out in
the shop or driveway to help you find and fix the problem. There are full page
plug and receptacle diagrams located at the end of this document. Step one is to
find out if the problem is in the tow vehicle electrical system, or if the
problem is in the trailer wiring. We will show you how to check both.
Grounding
Before you dive into testing and fixing
trailer lights, understand about how important
grounding is to trailer lighting. It doesn’t
matter whether your trailer has LED (Light
Emitting Diode) lighting, or filament bulbs,
they both need a good ground to illuminate. A
light fixture may use the metal part of the
frame to serve as the ground, or a more common
(and better) way is to run a separate ground
wire to each fixture on a trailer. This ground
originates at the towing vehicle battery negative terminal and must travel back
thru the truck to trailer plug, then to each trailer light fixture. A bad or
missing trailer ground is a VERY COMMON reason that lights don’t work on a
trailer, or they only work sometimes. If you have ever followed a truck/trailer
at night and you see the lights flickering, that is a bad ground between the
truck and trailer. Never rely on the metal to metal contact between the tow ball
on a truck and the hitch on a trailer to serve as the ground connection between
the two. A pin on the truck socket and a pin on the trailer plug are dedicated
for this ground connection, so test that connection FIRST when troubleshooting a
problem.
Wiring Colors – Be Careful!
We encourage you to ignore the wiring color codes that are shown on trailer
plugs. There is a wide variety of colors used by trailer manufacturers, truck
manufacturers, RV manufacturers, not to mention a variety of mechanics and
previous owners that may have worked on your truck or trailer before. We want
you to focus on the pin location and function on a trailer plug, rather than the
color of the wire connected to it. Basically, assume the wiring colors are wrong
when troubleshooting, and then you can be pleased if they turned out to be
right. Also be mindful of the orientation of the plug when matching each pin to
its function. The recessed rectangular area at the top of the receptacle is a
helpful way to assist in determining the orientation.
This photo shows the orientation of the receptacle rotated clockwise 90 degrees.
Note that the recessed rectangle is located on the right hand side.
Note that
the recessed rectangle is at the top of the diagram. Use this as a reference
point when determining which pin is for which function.
This is all
that you
will need
to test the
wiring on
your towing
vehicle and
trailer. We
will show
you two
different
ways to
test the
wiring on
your towing
vehicle.
Testing the Truck / Vehicle
Tools Required: Use the Circuit Light Tester and Alligator Clip Test Leads.
Helpful Documents: It would be helpful to print out the wiring diagram for the
vehicle end receptacle to look at while you are testing to assure that you are
testing the correct pin.
Assistant: You will need someone in the vehicle to help you with this portion of
the test.
Step by Step:
1. Wire, tape, or prop the spring loaded door of the receptacle open to keep it
out of your way.
2. Connect the test light to one end of a test lead.
3. Connect the opposite end of that test lead to the “Ground” pin on the
receptacle. Make sure that it is making good contact to the metal on the
pin.
4. Have an assistant turn the key of the vehicle on and leave it turned on for
the remaining tests.
5. Touch the test light to the “Running Lights”.
6. Have an assistant turn on the vehicle’s lights.
This should cause your test light to illuminate.
7. Touch the test light to the “Right Turn/Stop” pin.
8. Have an assistant turn on the right hand turn signal.
The test light should illuminate, blinking as the turn signal would.
9. Have an assistant also press the brake pedal.
This should cause your test light to illuminate.
10. Touch the test light to the “Left Turn/Stop” pin.
11. Have an assistant remove foot from brake and turn on the left hand turn
signal.
The test light should illuminate, blinking as the turn signal would.
12. Touch the test light to the “12V+” pin.
Your test light should illuminate, proving that you are receiving
voltage from the vehicle battery and that your ground pin is good.
13. Have an assistant also press the brake pedal again.
1. This should cause your test light to illuminate.
14. Touch the test light to the “Accessory or Reverse Lights” pin.
15. Have an assistant put the emergency brake on and firmly press the brake
pedal and put the vehicle into reverse. Note: Highly Dangerous, perform this
step with great care!
This should cause your test light to illuminate.
If you have an accessory wired to this pin, turn it on and your test
light should illuminate.
Optional: If Your Trailer has Electric Brakes
1. Touch the test light to the “Electric Brakes” pin. This only applies if your
vehicle is equipped with an electric brake controller.
2. Have your assistant slowly squeeze the hand operated slide for the electric
brake controller.
3. The test light should start out dim and get brighter as the outgoing power
from the brake controller increases.
4. Some brake controllers have a test feature, which will cause the test light
to blink.
Test Failures – Light Did Not Light
If your circuit tester did not illuminate for any of the functions, it is likely
that you have a bad ground.
You can test this by removing the test lead from
the ground pin on the plug and grounding it directly to the vehicle.
Make sure
that you are making contact with bare metal.
Retry all of the tests to see if the circuit tester now illuminates, this proves
a bad ground pin.
This could be cause by the adapter on the truck or the wiring
to the adapter, both should be thoroughly checked.
Testing the Truck
Tools Required: Using a Truck
Trailer Plug Tester with LED
Display.
This tester can be
purchased in our online
trailer parts store, it plugs
into the receptacle and has an
LED light indicator for each
of the pins.
Assistant: You will need
someone in the vehicle to help
you with this portion of the
test.
1. Plug the tester into the truck’s receptacle.
2. Have an assistant complete each of the functions: turn on vehicle (12v),
turn on vehicle’s lights (TL – trailer/running lights), turn on left turn
signal (LT – Left Turn), turn on right turn signal (RT – Right Turn), apply
brakes(LT & RT), squeeze electric brake controller slide (BK – Electric
Brakes), put vehicle into reverse/turn on accessory (BU – Backup/reverse or
accessory).
3. The corresponding LED light on the tester should illuminate for each of the
applicable functions. The LT and RT lights should blink when the left and
right turn signals are turned on. The BK light should brighten as you
increase the outgoing power from the electric brake controller. It may also
blink if your brake controller has a test function.
If the ground pin on the vehicle receptacle is not working, none of lights will
illuminate.
Testing the Trailer
Tools Required: Battery or Jump Box and Alligator Clip Test Leads.
Alligator
Clip Test Leads can be purchased in our online trailer parts store.
Assistant: You will need to view and assist the trailer while performing
portions of the test.
Electric Brakes: If your trailer is equipped with electric brakes you will need
a floor jack that enables you to jack up one wheel at a time.
It is important to remember that the pins for the trailer plug will not
correspond to the same functions as they do on the vehicle end receptacle
diagram. Since they are plugging into each other, the plug and receptacle
diagrams are a mirror image.
Remember to match the orientation of the plug to the diagram. When holding the
plug, the rectangular “bump” should be at the top.
Standard Factory Wiring for 7 Pin Plug
Note that the rectangular “bump” is located at the top of when looking directly
at the face of the plug.
Step by Step
1.
Using a battery or jump box, connect one end of
the black test lead to the negative terminal and the other end to the
“Ground” pin. It is helpful to use corresponding colors for the test leads,
as shown. Black for the negative terminal and red for the positive terminal
is typical.
2. Connect the second test lead to the positive terminal of the battery or jump
box. Now we will begin to the test the functions on the trailer by touching
the opposite end of the second (red) test lead to each of the pins.
3. Make contact with the alligator clip to the “Left Turn/Stop” pin.
The left turn signal/stop light should come on when you energize the
circuit.
4. Touch the alligator clip to the “Running Lights” pin.
When you energize the circuit, the trailer’s running lights should come
on.
5. Make contact with the test lead to the “12v+” pin.
If you have a trailer that is equipped with electric brakes and a
breakaway battery that has a “charging” indicator light, it should
illuminate when you energize the circuit.
6.
Touch the alligator clip to the “Right
Turn/Stop” pin.
The right turn signal/stop light should come on when the circuit is
energized.
7. Make contact with the “Accessory or Reverse Lights” pin.
If your trailer is equipped with reverse lights, the backup lights
should illuminate when you energize the circuit.
If you wired an accessory up to this pin, that accessory should turn on
when the circuit is energized.
Trailers with Electric Brakes
8.
For the “Electric Brakes” function,
you will need to jack the trailer up one wheel at a time.
9. Have an assistant spin the wheel in the direction it would spin when
traveling forward on the road.
10. Connect the alligator clip to the “Electric Brakes” pin.
When you energize the circuit, the
brakes should be applied and the wheels should lock up.
11. Repeat this step for each additional wheel.
If the “Ground” pin on the trailer is not working, none of the functions will
work. It is common to overlook the ground when problem solving. It is important
to test it first!
If you
pinpointed
an issue on
your towing
vehicle, be
sure to
check your
fuses first.
You can use
your circuit
tester light
to do this
as well (see
above). Look
in your
vehicle’s
owner manual
to see which
fuses are
for which functions.
Hopefully, this helped you to identify the problem in your truck or trailer
wiring system. Once you have narrowed down the issue, the next step is fixing
it!
Did you determine that your vehicle receptacle or trailer plug needs to be
replaced? Check out our large selection of trailer plugs and trailer
receptacles!
Back to our Trailer Repair How To section.
Based in Centreville, Maryland, Centreville Manufacturing builds stock and
custom trailers to fit a variety of needs for the residential, rental,
government, military and business market. Innovation and customer service drive
the company to provide top quality trailers for sale. With a desire help
customer on an ongoing basis, Centreville Manufacturing has local trailer parts
stores serving customers in Maryland & Virginia and online trailer parts store
serving customers nationwide.
Do you have a trailer troubleshooting, repair or maintenance question – submit
it here.