Why Are Trailer Running Lights Coming on When Brakes are Applied

Question

Question:

Trailer 5th wheel clearance lights come on when brakes applied. As title suggests my clearance lights come on when braking, this all happened after installing a 4 point plug at rear ,now all lights work correctly on the secondary trailer . I also tried the trailer on another truck and same problem so that rules out my truck. Where did I go wrong ,all 5th wheel trailer lights work correctly.

asked by: Herman M

1

Helpful Expert Reply:

You are correct that the issue is on your trailer and based on what you are describing the circuits are somehow swapped when you installed your connector; unfortunately, I was unable to find your order number to see what you purchased from us for the install that may have caused this to happen. It is also possible that the brake and running light circuits shorted out and surged together. Either way, you will need to trace your wiring back, particularly at the running lights to see where any wires are crimped, broken or otherwise damaged. I have also added a link to a help article on wiring troubleshooting which I highly recommend reading.

expert reply by: Kyle S

1

Answer 1

Your problem is in the wiring in one of two places. It is either on the vehicle wiring harness or somewhere in the wiring on the trailer. The first thing to do would be to narrow it down to one of these two places, then we can pin-point the problem…

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Answer 2

If you check out the picture I attached the middle set of 7-ways is the wiring diagram/colors you'll need to use for the Ford Replacement OEM Tow Package Wiring Harness #118243 that you referenced. The harness is already wired up correctly though…

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Answer 3

What you are describing is normally the result one 1 of 3 issues: First, there could be some corrosion in or on the connectors on the vehicle or trailer side or both. Second there could be a bad ground. Bad grounds can cause all sorts of bizarre…

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Answer 4

I know you said you checked the fuses, but on your 1999 Ford F-250, the Power Distribution Box and Trailer Tow Relay Blocks are located in the engine compartment near the brake master cylinder, not to be confused with the Fuse Panel located below and…

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Answer 5

Since wire color codes are NOT universal, we always recommend wiring by function, rather than wire color. If you look at the Trailer Wiring FAQ article I have linked you to, you can see some diagrams showing the proper way to wire a vehicle and…

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Answer 6

It sounds like you have a ground issue. When trailer lighting does not have a sufficient ground many different scenarios can show up, but the most prevalent is when the most amperage is applied, like tail lights on and brake pedal pressed, the lights…

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Answer 7

I checked and Curt hitch #C13117 is listed for all models of the 2013 Mercedes ML350 as long as it does NOT have the Active Curve System which is a hydraulically activated anti-sway/roll bar. Installation is rank a 5 out of 10. I have linked the…

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Answer 8

We know from Tekonsha training on their universal-fit ZCI Circuit Protected Vehicle Wiring Harness #119250KIT that this item is very sensitive to the initial setup process, which must be followed exactly for correct operation. My technical contact at…

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Answer 9

The difference between the Class II and Class III hitches has to do with their Gross Trailer Weight (GTW), their Tongue Weight (TW), and their Hitch Receiver Size. Class II trailer hitch receivers have a 1-1/4 inch receiver and can be rated up to…

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Answer 10

In order to determine the torque rating for the lug nuts on your trailer you will need to know check out the chart I attached that shows all of the torque specs for all the trailer lugs available based on stud size, wheel material and wheel size.

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Answer 11

A 4-Way trailer connector has the basic lighting functions only; running lights, left turn signal and brake lights, right turn signal and brake lights, and ground. A 7-Way has these functions and a 12 volt circuit, a circuit for electric trailer…

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Answer 12

Hey Doug! The Firestone Ride-Rite Air Helper Springs #F35FV are the right kit for your 2026 Ford F-350 FX4 dually. However, you will need to measure to be sure it fits. The distance from the top of your rear axle to the top of the jounce bump stop…

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Why Are Trailer Running Lights Coming on When Brakes are Applied

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