Interior Dome Light on Discovery Trailer Only Works with Brake Pedal on Ford Expedition

Updated 03/26/2019 | Published 03/25/2019

Question:

All of the lights and brakes work on my Discovery tandem axle cargo trailer except the interior dome lights. The trailer dealer says everything is wired correctly on the trailer and i can confirm this as I have plugged it into other make vehicles and all works fine. On the Nissan the interior dome light is operational with the parking lights. On my 2018 Ford Expedition LTD the only time the interior dome light of trailer comes on is then the vehicle is running and the brakes are applied. The Ford Dealer is claiming the truck is wired correctly. Not sure how that can be... I cant see the circumstance where the interior dome light should only be operational with the truck running and brakes applied. Can you help?

asked by: Derek R

Expert Reply:

I can certainly say that is a unique issue that I have not heard before. However, I suspect the wiring on the trailer is what is causing the odd lighting funcion. The trailer may be wired correctly according to the manufacturer but that does not necessarily mean it matches what is on the tow vehicle. Generally speaking, the interior dome light on a cargo trailer will be ran to a separate battery on the trailer. If the trailer does not have a seperate battery then it will be ran to the 12 volt circuit on the 7-way and will work when connected to the tow vehicle or when connected to the running tow vehicle.

I recommend you reach out to the trailer manufacturer to see where the interior dome light is ran on the trailer. If this cannot be done then you will need to trace the light yourself to see where it is ran. If you run the power wire for the interior dome light to either a separate battery on the trailer or the 12 volt circuit on the 7-way then you should have normal lighting functions.

expert reply by: Conner L