Only the headlights will turn on the trailer lights

I bought the

$55 Curt 55379 Vehicle-Side Custom 4-Pin Trailer Wiring Harness,Fits Select Toyota Pickup,Tacoma,Black

Curt 55379 harness for my 95 taco and after test installing it, the trailer lights will only light up when I turn on the headlights. The turn signals and brake lights on the trailer don't work.

I know for a fact that the trailer lights are functional and I'm fairly certain that the connectors are fine since all the truck lights work when plugged into the piggyback T-connectors. I've even tried running a direct ground from the battery to the harness. Using a multimeter between the different pins, the brown pin gets a full 12.6 volts while the yellow and green only get around 5 volts.

Has anyone else encountered this problem? Did I just get unlucky and get a defective unit?

 

Welcome aboard new member

 

It’s a four pin. One should be ground, one brake light, one blinker, one running lights. At the curt connector check each location for power while using turn sig, brake and running lights, if all Of them work at the connector the issue is on the trailer side. I’m assuming you haven’t already done this.

 

I'll check the voltages between the T-connector and the light tomorrow but when I checked the 4 pin trailer connector on the curt harness, the yellow and green pins were only at 5 volts while the brown wire was fine at 12.6 volts. The trailer lights should be fine because I've towed it with a Honda odyssey with all the lights working. I'll keep you updated

 

Encountered this type of problem every spring after Horse trailers sit all winter.In the fall after the first snow fall funny how the lights on the snow mobile trailers don`t work.
Be glad your not dealing with trailer brakes as well .

If your trailer has been sitting for sometime welcome to fixing and replacing lights .

If you have battery voltage at the truck harness side but only 5VDC at the flat plug sounds like the box is bad already.

I hate those converter boxes much less frustrating for me to add a stand alone Turn signal on the trailers.

You do have a good clean and tight ground for the converter box ??

Sometimes the only way to get a good ground at the rear of the vehicle is an extra main ground to the frame from the battery I now do this to every vehicle .

Checking one thing at a time. Happy Hunting

 

If you just bought it, and you are having problems already, is it worth it to try to diagnose and fix the issue? Or would it be easier to ask for an exchange and see if the problem persists before diving further?

 

It should be easy enough to diagnose pretty quickly with a volt meter. I'd at least confirm if you're getting proper voltage to and from the harness before returning it.

I have a similar harness, and recently the tail lights on my trailer stopped working. I was able to confirm I was getting voltage IN the harness but not out, so obviously a bad harness. It's 10+ years old, so only *slightly* out of warranty, lol, so I just bypassed the harness with a piece of wire.

 

Alright now I'm digging myself into a deeper rabbit hole. I grabbed a few random wires to test the converter box directly with a battery and I think I ruled out the box as a problem. All the pins work and have full power when poking wires at a battery. The reason the converter box is only putting out about 5 volts on the yellow and green pins is because its only getting 5 VOLTS when all hooked up. I even tried a direct ground from the truck's negative terminal!

As it turns out, if I start the truck, the first turn signal I do will actually light up the trailer light and flash but once I cancel the signal and signal again (in any direction), it completely goes dead again except for the running lights. Same goes for hitting the brakes after starting and then letting go. I checked the voltage at the truck's turn signal using one of those battery/alternator testers with 6 LEDs because they respond quicker than multimeters and with one turn signal plugged in, it's only around 5-6 volts. With both tail lights removed, instead of immediately blinking the 3 LEDs for 12.5 volts, it kinda slowly rolls up to a dimly lit 12.5 volt LED each time it blinks. Seeing that, I decided to start the truck up and whaddaya know, the three 12.5 volt LEDs flash immediately and very brightly until I cancel the signal and turn it back on.

I'm really confused because if the wiring harness takes its power from the same connector as the tail lights, and the turn signals only get 5-6 volts, how in the world are the truck's turn signals still able to light up?? All the stock lights light up just fine. One thing forgot to do was check the turn signal voltage through its own ground in the same connector so I'll have to do that when I get the chance.

If it turns out that the truck's circuit is the issue, where could the problem lie? The only things I can think of are the signal flasher relay and the turn signal stalk but both of those have always worked. Maybe I'll try messing with the flasher relay next.

 

Are your tail lights LED?
I believe on some of the 2nd Gen truck, you needed to run an LED Turn Flasher.
Could some one have done this with your truck?

 

Now it is such a habit I forget to even think about it I always have the battery on the charger while trouble shooting trailer lights

Some older vehicles have such line loss some lights will not work

Your tail lights and turn signals are different circuits different fuses .

Check the plug where the new converter harness plugs into the truck harness Some pictures of this would be great

I wish I was close to figure this out in person