trailer wiring

Follow the advice above to start pulling light fixtures, starting with the one that works. You've lost the 12VDC somewhere beyond the one that is working.

SweetLou wrote:

drsteve wrote:

wnjj wrote:
It can't be the ground wire. The brake lights work but several tail/marker lights don't. That's a problem with the hot wire.

It can always be the ground. Never say never.

That's correct! I had his exact same problem in my little utility trailer and it was a bad ground at the socket.

A bad ground at the socket of a tail/brake light could allow the brake light to work fine because it grounds through the tail light filament then back through the other tail light and marker lights to ground (lighting both filaments in the bulb with the missing ground) BUT only if the tail lights are turned off. Turn them on and there is no ground path anymore and the brake light quits.

While bad grounds may seem like it, they aren't voodoo. The symptoms match the failure. Unless the OP hasn't properly described how his is failing, it cannot be a ground problem. Now if his brake lights aren't actually working all the time, it could be.

SweetLou wrote:

drsteve wrote:

wnjj wrote:
It can't be the ground wire. The brake lights work but several tail/marker lights don't. That's a problem with the hot wire.

It can always be the ground. Never say never.

That's correct! I had his exact same problem in my little utility trailer and it was a bad ground at the socket.

But, did one filament of the dual filament bulb light up while the other filament did not? That is the situation with the OP.

drsteve wrote:

wnjj wrote:
It can't be the ground wire. The brake lights work but several tail/marker lights don't. That's a problem with the hot wire.

It can always be the ground. Never say never.

That's correct! I had his exact same problem in my little utility trailer and it was a bad ground at the socket.

muddyriver wrote:
1st off, sorry about the 2 post in different topics. I'll try not to let that happen again. As far as the ground. I don't believe it is the ground. It is a double filament and the brakes / turn signals works. Also I checked the wiring with it disconnect to the truck ball. I'm really not looking forward to start tearing into the walls to pull wire.

I agree about the ground and tried more than a few times to explain to others why I thought it wasn't.

You shouldn't have to tear in to walls. The bad connection is likely at a splice, and those will be behind the lights. Get some proflex and start pulling the light fixtures, but before you do, follow the wiring from the hitch first since any external connections are more likely to have failed.

Can you confirm the voltage you saw and that the socket with the voltage won't light a bulb?

1st off, sorry about the 2 post in different topics. I'll try not to let that happen again. As far as the ground. I don't believe it is the ground. It is a double filament and the brakes / turn signals works. Also I checked the wiring with it disconnect to the truck ball. I'm really not looking forward to start tearing into the walls to pull wire.

Ground.
Try the umbilical without the trailer on the ball.

BenK wrote:
Body grounding methods abound with El Cheapo...

The mounting nut might not be tight...or touching with a high resistance

The tab that is supposed to touch body ground may no longer have proper spring tension...therefore poor connection with high resistance

Dirt/water/etc gets into the contact area to corrode it and make a high resistance connection

That list goes on and on

Proper grounding is a dedicated 'wire' with proper UL/CSA/etc approve wire and connectors...along with approved methods on installation. That all costs more than an El Cheapo pushed on light with a spring clip to hold it in the hole...that also serves as the ground point...some times that spring clip is supposed to knife through the El Cheapo paint on that mounting hole edge...some times it does, some times is doesn't...some times it only kinda sorta does...

Most intermittent connections comes from body grouding schemes...

Again, agree 100%. In this particular case, the OP has a working brake light within the same fixture as a non-working tail light. They usually not only share the same ground, they are usually the exact same bulb with 2 filaments. That means the ground (in this case) is not suspect.

Including my response to the OP's duplicate post in the TT subforum since it seems to be fading away.

wnjj wrote:
The tail lights and all marker lights are from one pin on the trailer plug. Since you have at least one working, the truck, plug and trailer cord are fine.

Start by following the trailer wire cord from the plug to where it breaks out into the various circuits. There may be a splice block where wires from the front and rear marker lights join together with the trailer cord. If not, then there is something wrong inside the trailer wiring. You could pull each marker light, looking for a failed splice. I'd start with the left tail light since you're seeing "voltage" there (how much?). Worst case you could run a new wire from the trailer cord to one of the non-working lights and they should all start working.

Also, if you see 12V at the socket with no bulb installed but it doesn't light when you replace the bulb, that likely means a really poor connection to that socket, either right there or where the wire gets its power.

Body grounding methods abound with El Cheapo...

The mounting nut might not be tight...or touching with a high resistance

The tab that is supposed to touch body ground may no longer have proper spring tension...therefore poor connection with high resistance

Dirt/water/etc gets into the contact area to corrode it and make a high resistance connection

That list goes on and on

Proper grounding is a dedicated 'wire' with proper UL/CSA/etc approve wire and connectors...along with approved methods on installation. That all costs more than an El Cheapo pushed on light with a spring clip to hold it in the hole...that also serves as the ground point...some times that spring clip is supposed to knife through the El Cheapo paint on that mounting hole edge...some times it does, some times is doesn't...some times it only kinda sorta does...

Most intermittent connections comes from body grouding schemes...

BenK wrote:
Depends on how the OEM designed the electrical system...especially their grounding system

If they ran a separate ground wire (costs more) then wnjj is right, but if they used the metal of the vehicle, then yes parts can still work, while the parts not grounded or grounded well will not work

IMHO, a major El Cheapo fault of most trailer OEMs...am sure there are some fine ones out there...but on these forums...seems most are NOT...

drsteve wrote:

wnjj wrote:
It can't be the ground wire. The brake lights work but several tail/marker lights don't. That's a problem with the hot wire.

It can always be the ground. Never say never.

Agreed, but with a frame ground (likely on trailers), one local ground fault will not make multiple lights fail. That can only happen with a shared ground. Furthermore, how can the higher wattage brake light filament work when the tail light doesn't? The ground is the same exact connection on the bulb.