Flat deck trailer turn signal not working


Trailer wiring can be alot of fun... heres acouple ways to figure out why a tail light isnt working.

Tools I used

-screw drivers

-digital multimeter

Possibly afew small sockets depending on junction box on the trailer

1. Remove light from socket gently prying on light to not crack the plastic, work around the light untill it pops free

Flat deck trailer turn signal not working-069d #1-2. check to make sure all connections and wiring to the led are clean of dirt/grit and tight, Ill put a new coat of dielectric grease in these connectors when the light works

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With alot of trailers having LED tail lights and side markers, i like to swap lights from side to side to confirm its not the light. Granted you only have 1 side not working

3. connections seem tight, confirmed the light isnt burnt out. now check the socket on the truck. is the truck supplying power? Turn the trucks lights on, vehicle doesnt have to be running but watch you dont kill the battery

Snapon sells a quick easy tester you plug in and small lights illuminate to tell you if all circuits are working

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If you dont have the easy tester a voltmeter will do, most newer trucks have a diagram on the cover of the 7 pin connector

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I turn my hazard lights on so i can check both left hand and right hand lights at the same time, using your volt meter (set to volts) touch the red positive lead to the corresponding terminal and the black lead to any ground. You should see your meter fluctuate from around 12v to 1v this confirms you have voltage coming from your truck to power the signal lights. Some models of trucks have fuses under the hood for trailer lights. If theres no power in the socket and you have had a short in your trailer theres a good chance its popped its corresponding fuse.

4. Turn your vehicles lights off.

5. The next step is to inspect your 7 pin connector on the trailer end

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Make sure the connector is free of dirt, debris and corrosion. If any is present try and clean out the terminals of the connector. Depending on how corroded this connector is I might just replace it.

6. Open up the 7 pin connector, a small set screw on one side and a larger set screw holding the wires from being pulled out of there connections. The body will slide down the wire and expose the connections.Flat deck trailer turn signal not working-069d #7-

Make sure all connections are tight nothing is bridging any of these terminals. In my case a small bit of the white wire was loose touching the yellow brake signal wire causing a short.

7. Reassemble the connector

8. At this point everything looks good. Have power coming from the truck, its time to dig abit deeper. Most trailers have a junction box on the inside of the frame rail. Locate box, Unbolt the cover and dig in.

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Junction boxes can be full of grease, dont clean it out unless its full of crud. That dielectric grease keeps unwanted water from corroding the terminals. Look for any broken wires or loose connections. The terminals should be labelled on the inside of the cover or under all the grease beside the terminals in the back of the box.

9. Turn vehicle hazard lights on, set voltmeter to volts and test the terminal corresponding to the signals. For my situation it was the drivers side light causing an issue so i tested the terminal with the yellow wire. Confirmed 12v to the junction box and 12v back at the faulty light

10. Tail light plugged back in, If the light still doesnt illuminate the only thing left is a break in the wire somewere from that juntion box to the light, peel back the wire loom and your issue will most likely reveal itself. If the wire is old and full of cracks it might be best to run new wires and get rid of future problems