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Troubleshooting Trailer Wiring Running Lights


I’m having trouble with my hookup for a 5 flat trailer connector I seem to have a extra wire which is the running lights. Left and right and brake lights come on but running lights do not. Do I need to tie in a wire or I’m a miss wiring something?


Expert Reply:

A 5-way will have the following functions: left turn and stop, right turn and stop, running lights, auxiliary, and ground. If everythying is working except for one function there are a number of potential issues you will need to troubleshoot for. you will need to use a circuit tester such as # PTW2979 to test the connector on your tow vehicle after disconnecting it from your trailer. Check the wiring going into your harness from your tail lights 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. You will also want to check both your vehicle and trailer connectors for corrosion, which can be green or white in color. A build-up of corrosion can interfere with the signals getting through the connectors so if it is present you will need to either replace the plug or clean the corrosion and be sure to use Dielectric Grease # LT37534 to help stop any future corrosion.

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 have added links to a pair of help articles on trailer wiring, including a troubleshooting guide which I highly recommend taking a look at.

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expert reply by:
Kyle S

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