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Troubleshooting Trailer Wiring on 2004 Ford F-250 Super Duty


No brake lite or right or left turn just marker lites when hooked up to a trailer


Expert Reply:

Because you are losing functions when you connect your trailer, it sounds like you either have problems at the connectors with dirt/corrosion or you have a bad ground wire. I recommend testing each function individually using a circuit tester like # PTW2993 to test your trailer connection on your vehicle when your trailer is not connected. If you have a brake controller you will need to use the manual override to test that function.

If there is an issue, check your ground wire connection to make sure it is bare metal to metal and then check the owners manual for fuse and relays. After that, test each wire that isn't working properly probing the insulation for function every few feet until you find the short and replace wiring as needed.

If all functions on the vehicle check out, then you will test the trailer. First check the trailer main ground connection on the frame for a bare metal to metal connection. Next check the tail lights which are often grounded to your trailer frame through the mounting locations. These need to be bare metal to metal as well. If there is still issue, then you will trace from the trailer connector back probing every few feet to find the wire short and replace as needed usually with # 16-1-1 wire sold by the foot. For brake wires and 12V wires use # 10-1-1.

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