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Trailer Brake Lights Not Working With 2003 Ford F-250


Hi Im working on a 2003 Ford F250. All light functions on the truck work correctly. The problem is that im getting turn signals on the trailer plug but not brake lights. Of course they are the same two wires in the RV7 plug. Turn signals function but brake lights do not. The RV7 plug on the truck is factory wiring. Whats causing this?


Expert Reply:

Since the relay for the trailer turn and stop is the same on a 2003 Ford F-250 we can rule that out but there are a number of other troubleshooting steps you are going to need to take in order find the specific issue.

you will need to use a circuit tester like the very handy Cordless Circuit Tester # PTW2979 to test the connector on your 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.

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.

expert reply by:
Kyle S

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