Troubleshooting Left Trailer Turn Signal That Does Not Function  

Question:

Expert Reply:

To check your trailer wiring set up I suggest that first you unplug the trailer and confirm that the left turn signal on your vehicle itself is functioning correctly. Also turn on the vehicle headlights to make sure the left taillight works too. Trailer wiring combines these two signals onto one wire, usually a yellow wire for the left side of a vehicle/trailer.

If the vehicle left blinker light does not work then you may need a new flasher unit that you can get at any auto parts store.

If the vehicle blinker and tail lights work properly then you can move on to testing the trailer lights on their own by directly connecting a 12V battery to the pins on the trailer connector. Do this with the trailer disconnected from the vehicle. Connect the battery negative terminal to the ground pin on the trailer plug and connect the positive terminal to the pin for the yellow wire. If the left trailer light works when powered directly like this then you know the issue is on the vehicle.

The vehicle's power distribution box under the hood will usually have a guide or legend on the lid that tells you which fuse or relay controls each particular function, including trailer lighting. Check any fuse locations that are labelled trailer/tow and verify the fuse is good.

If you like you can tell me about your vehicle (year/make/model/version) and the type of wiring it has (4-pole or 7-way, etc) and whether this was factory-installed or aftermarket and I will be glad to assist further.

expert reply by: Adam R

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