Brake light/ turn signal

Broken wire somewhere. Easiest just to run a new wire. Butt connector (weatherproof) it at the light and at the 4 pin connector.

You can try to track down the break, but why bother. Running the new wire is easier.

Another possibility is the socket is bad, but I am betting on the wire.

Start with a multi-meter or test light on the vehicle side of the 4 pin connector. If all is good there try cleaning the 4 pin connector. After that, a quick visual inspection of any wiring you can see for cuts or breaks. Then do as stated above, run new wires.

Get a cheap 12 volt test light. Less than $10 and more forgiving when chasing tail light.

You need to make a good ground somewhere. Use a scrap piece of wire if necessary. Once you have the alligator clip connected to a good ground you can probe the socket first since you changed the bulb. You could easily have purchased the wrong bulb or not have it inserted properly. A simple test is to remove the same bulb from the left light and put it in the right side. Test your old bulb in the left side as well.

If you still have the issue then you should really start at the trailer plug side of the tow vehicle. Could be a blown fuse in the towing vehicle.

You need to be careful to not introduce a new problem while trying to diagnose this one. Take pictures as you go and print some out. Document what you did.

What is the tow vehicle?
If it is the F150, check for blown fuse in the fuses under the hood.

START WITH THE BLUB.Turn and break are the same filament of an 1157 bulb. Unplug from TV and check TV plug. Disconnect trailer wiring from light. Plug in TV and check trailer wiring. Yellow and green are turn signal. Red is brake, white is ground.

Agree check the bulb. however you said the running light is working. But that doesn't tell you the brake light is working. The brake light, and the turn signal light is the same bulb. has 2 filaments in one bulb.

I'll bet the cheap light socket is not touching the base of the bulb. Try putting the turn signal on and then go push the bulb tighter into the socket. you can also remove the bulb and look down into the socket. It might be possible to bend the two electrical tabs up a bit so they make better contact when the bulb goes in.

I had a similar issue in on my 1986 and I could temporarily fix it by bending the tabs. In the end I had to pull the whole back panel down and replace the lights with some LED taillights.

As far as simple tests, it kind of depends on your skill level and available resources.

Do you have a couple of alligator clip test leads? If so unplug your 4 flat plug and use one test lead to connect the ground pins to each other. Use the second test lead to see if the tow vehicle yellow (left turn) wire can blink the right turn by connecting from yellow (TV) to green (PU) If the light blinks the problem is in the TV, if the light still does not blink the problem is in the trailer.

Use a meter to determine if the signal is actually reaching the socket and report back.

The most common running/brake light problems on a trailer are bad grounds.

The brake/turn bulb filament is higher amperage and requires a solid ground connection.

Terryallan wrote:
Agree check the bulb. however you said the running light is working. But that doesn't tell you the brake light is working. The brake light, and the turn signal light is the same bulb. has 2 filaments in one bulb.

tail light is one filament. Brake and turn is the other.