Dear Dave,
I unhooked my RV and lights from the tow vehicle and the left taillight on the trailer continued to shine. I reconnected the RV and the left light continued to shine but no other trailer lights work. The left light shines regardless if the trailer is hooked up or not. My tow vehicle flashed left taillight error on dash while towing RV. —Martin, 2010 Open Range Journeyer JT287RLS
Dear Martin,
An RV taillight is typically powered by 12-volt DC from the 7-pin pigtail plugged into your tow vehicle. Here is a common wiring diagram from CURT that shows the wires.
The brown wire comes off the #3 pin of your truck and supplies 12-volt power to the side marker lights, taillights, and front/back marker lights. There should only be power coming from the tow vehicle if the running lights of the truck are on. So when you unhook the 7-pin connector and do not turn off the running lights on your truck, they should still stay on. However, if the taillight is staying on, it is getting power from another 12-volt source which can only be the house battery of your trailer. I would imagine that your truck is sensing stray voltage from the brown wire and that is why you are getting the left taillight error.
First thing to do
The first thing I would do is disconnect the house battery or batteries and disconnect the tow vehicle. The light should go out, which would indicate there is a wire from the battery touching the brown wire going to the light and energizing it. Most lights are grounded to the framework as the ground wire from the 7-pin, which is the #1 pin, is connected to the front trailer frame. If none of the other lights are coming on, then it isolates the issue at the brown wire going to the light fixture.
I would remove the fixture and see if there are any other wires that are visible. It is unlikely, as most trailer manufacturers just drill a small hole during production and stick the wires out, then screw the fixture to the back wall.
Look for the brown wire
Look underneath and see if the brown wire is visible along the trailer frame. If you can locate it, use a simple 12-volt test light to see if there is power at the wire. If not, the issue is closer to the fixture. But if there is, trace it back as far as possible and you should be able to find where the voltage is coming from. You might just need to run a new power wire and connect it to the marker light, either at the top or side. This might be a little difficult since the wire is most likely sandwiched in the sidewall or back wall.
Once you identify the issue with the taillight, you will need to check the 7-pin connection at the truck, since none of the other lights are working. I would imagine the continuous live wire in the taillight blew a fuse in the tow truck. You can use a CURT Tester, available here, or the 12-volt test light to verify power at the connection.
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Dear Dave,
Our issue is when the back lights on our RV go on, the left one is brighter than the right. But the bigger issue is that the brake lights don’t go on at all. Thank you in advance. — Kathy, Fleetwood Expedition 38K
Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”
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I had an issue once when I went to my campsite and noticed my TT that wasn’t hooked up to the truck had one tail light lite. I noticed the 7 pin connector had fallen to the ground. The moisture from the previous rain had seeped between the battery pin and tail light pin causing the tail light to light up. Cleaned and dried the connector and problem went away.
The only place this cross connection can happen is the cord that connects the trailer to the tow vehicle. The cord has been pinched, pulled, or otherwise damaged. Why send them on a wild goose chase checking fixtures and harness?
I would add that on most trailers using a 7-pin tow vehicle connector, there is some kind of junction box attached to the trailer frame just behind the tongue or king pin. Getting inside that junction box is in my experience where most wiring issues can be resolved. Also know that 7-pin and 4 or 5-pin color coding is different and there is often a ‘translation’ that happens inside that box. Sometimes the junction box will use terminal posts, but more often than not it’s just a bunch of wire nuts and something popped loose.
Thank you, Dave! 🙂
Sounds to me the left brake/turn signal wire is touching the charge line to the trailer at the wiring plug at the connector plug.
The brake light has separate wires from tow vehicle to trailer lights for brake/turn signal use.