Troubleshooting Trailer Wiring

Updated 07/02/2018 | Published 06/28/2018

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Question:

Parking, brake, and both turn signals work.....until you turn on vehicle lights....then the left turn does not work but all the others do!

asked by: Darren L

Expert Reply:

In most cases a bad ground is to blame for this type of issue, but in order to determine the specific problem you'll need to use a circuit tester such as # PTW2993 to test the connector on your vehicle after disconnecting it from your trailer. Check the wiring going into your harness from your taillights to see if those wires are carrying the correct signals, if not then the issue is with the vehicle wiring. If the signals going to your harness are okay, you'll need to test the signals coming out of the harness, and if they are correct then your issue is likely going to be with the trailer side wiring.

As I mentioned above, an important issue you will want to take a look at are the ground connections, as they can be the cause of numerous lighting problems. You'll want to make sure your grounds are securely attached to bare metal surfaces free of any paint, dirt or corrosion of any kind. Also check your wiring thoroughly to ensure there are none that are crimped, broken or otherwise damaged. I've added links to a pair of help articles on trailer wiring that you may also find useful.

expert reply by: Kyle S