What to Test When There is no Power to a Vehicles 7 Way or 4 Flat

Updated 04/27/2021 | Published 04/24/2021

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Question:

Those fuse numbers you posted in the reply to Marks question dont match my 08 F150. None of them. I spent $280 on a new dealer bought central control module fuse box every fuse and relay is brand new. Still no power to my trailer lights plug. Any other suggestions?

asked by: Michael F

Expert Reply:

The fuses are typically the place I start when troubleshooting a plug with no power, but as you have completely replaced your fuse box, that is a bit of a moot point. What is most likely causing the lack of power is damage to one or more wires.

Using a tester, like the Circuit Tester # PTW2993, you will need to test the pins on the plug. If you have a 7 Way connection, the 2 o'clock position will be your 12-volt power, the 3 o'clock position will be the right turn signal and right brake signal, the 5 o'clock position will be your brake controller output, the 7 o'clock position will be your ground, the 9 o'clock position will be the left turn signal and right brake, the 11 o'clock position will be the taillight and running lights, and the center position will be the auxiliary circuit. If one or more of these are not working, you will need to then check the wires on the back of the plug to see if there is power going into the ports. If there is power at the back of the plug, you will need to replace the 7 Way with something like the Hopkins # HM40975. This would be a great replacement as long as the vehicle came pre-wired with the 7 Way plug. If it did not come pre-wired or if there was no power at the back of the plug, you will need to trace the wires back from the 7 Way connection as there is most likely either a break in one or more of the wires or the ground has become corroded or not secure. If this is the case, you can simply repair the wire and get back to towing, or you could rewire everything and use the Hopkins # HM40975-11998 at the end of the vehicle to give you a clean look.

If you have a 4 Flat connection, you will want to check the green wire for the right turn signal, the yellow for the left turn signal, and the brown wire for the brake signal. If one or more of these isn't working, you will need to replace the harness. This will again depend on if the vehicle was wired for a 7 way from the manufacturer. If it was pre-wired, you will need the Hopkins # HM40975. This is technically a 7-way but does have a 4 Flat connection as well. If it was not pre-wired for a 7 Way, you will need the Hopkins # 40185.

If none of this gets you up and running, let me know and we can try something else.