Interior Trailer Lights are Not Working but All Exterior Lights Do Work

Updated 08/18/2025 | Published 08/06/2013

Products Featured in This Question

Pollak Replacement 7-Pole, RV-Style Trailer Connector Socket - Vehicle End

(739)

$14.85

Hook Probe Circuit Tester

(65)

$13.35

Question:

Help on getting interior lights working.

asked by: Russ

Expert Reply:

You will need to determine what circuit the lights on your trailer are connected to. If the lights are hooked up to either the 12-volt circuit on the 7-Way or the auxiliary power circuit, then it could just be a matter of making that function active on the vehicle side. If the lights go first to a trailer mounted battery, make sure the battery has a charge and that the power wire and grounds are intact. They can also be tied into the running light circuit so you would check it as well.

To check the 7-Way functions on the vehicle side, use a circuit tester like # PTW2993. The pin in the 1:00 position if facing the connector is the 12-volt circuit. The center pin is the auxiliary circuit though it can also be reverse lights. If power is getting through to these pins (or whichever pin the interior trailer lights are connected to) then you know the issue is a wiring problem on the trailer side.

But if there is no power on the vehicle side then you will need to check any fuses or relays related to the tow package wiring. Use the vehicle owner's manual to find their locations. If you happen to have a Chevy or GMC truck, there may be some connections under the hood you have to make to make everything fully functional.

expert reply by: Michael H

Scott S.

8/16/2025

I just sold my 2003 Keystone Cougar 241FBS Travel Trailer, that I’ve owned since new. Excellent condition, everything works as new.To a nice elderly couple. Question is, when he plugged it in to his tow vehicle ( newer Ford 150 pickup) all the exterior lights on the TT worked but none of the interior lights or the slide out would work. I’ve never had this problem with my 2002 Chevy avalanche hooked up to the TT. Everything works great when hooked up to shore power. I haven’t used external 12 volt batteries on the TT for 20 years. We have always camped with shore power. I’m pretty positive that when hooked up to my vehicle all interior and exterior lights and the slide out would work. I think the most likely problem would be with his 7 pin plug on the Ford 150. What do you think? Thank you. Scott S

Jesse M.

8/18/2025

@ScottS I would definitely start by testing the 7-way on the Ford truck. It could be the 7-way does not have 12v aux power hooked up. It definitely sounds like a problem with that truck and not with the trailer itself since it works fine with your Chevy truck. That is typically the first thing we would try for troubleshooting this kind of issue. Plug the trailer into another truck and see how things work. If it works with your truck, but not with theirs, we can pinpoint their truck as the issue.

Scott S.

8/18/2025

@JesseM Thank you for the response, Jesse. I will pass on the information to the new owners. Scott S