Testing for 12 Volt Power for Interior Lighting in an Enclosed Trailer

Updated 11/15/2011 | Published 11/14/2011

Question:

2004. F150 with 7 pin connector, 7x14 Atlas trailer with battery disconnect for brakes. I have lights inside the trailer but they do not function. Is it the truck or the trailer wiring that is the problem? Thanks in advance for any help.

asked by: Joe N

Expert Reply:

You will need to do some testing to figure out exactly where the problem is with the 12 volt power for your trailers interior lighting. The first thing I would do is check to see if the interior lights run off of the taillight circuit. Turn on the headlights on the vehicle then try the trailer interior lights, just to be sure this is not the problem.

If the interior lights are not attached to the tail light circuit, use a circuit tester, # 3808 if needed, and start out by checking the vehicle 7-Way connector for power. Attach the ground clip from the tester to the # 1 terminal inside the 7-Way connector and then touch the probe to the # 4 terminal inside the connector. Our wiring page, see link, shows the function for each terminal inside the 7-Way connector.

If you do not have power to the # 4 terminal in the 7-Way you will need to check the wiring on the truck for issues. You may find that a fuse is blown or missing in the power distribution box under the hood. There should be a legend under the cover for the power distribution box that tells you which fuse supplies power for the 7-Way trailer connector.

If you do have power at the vehicle side trailer connector you should turn your attention to the trailer. Make sure the ground wire is securely attached to the trailer frame and that the connections inside the trailer side connector are attached to the correct terminals.

If your trailer was equipped with a trailer mounted battery and it is no longer attached, you will need to find the wire that leads to the interior lights from that original connection and attach it to a new trailer mounted battery, or to the power wire coming from the 7-Way trailer side connector. If you add a new battery to the trailer the power wire from the 7-Way is attached to the positive post of the battery, and the negative post of the battery should be grounded to the trailer frame.

Since you have a breakaway kit mounted on the trailer, you may find that the power wire has been used to charge the trailer mounted breakaway battery and has been disconnected from the interior lighting.

expert reply by: Robert G