Power to Towing Harness

Just broke out my trailer for some springtime work and... no lights. I have a 2006 Doublecab that came with the towing package; my multimeter is getting me 12v on the battery terminal and the tail light terminal - zero volts on all others. I replaced the 30a fuse under the hood just for kicks... same issue.

I backed up my son's 98 4Runner to the trailer, hooked it up and all lights work.

Is it likely there is something internal and/or with the harness adapter? I looked underneath, not sure how easy it is to install a new one as it looked like multiple wire bundles using the same connector.

 

You said you replaced a 30A fuse, was it the towing fuse?
The turn signal terminals will only have power then the signals are on, did you check the ground terminal?
Here's the wiring diagram for the trailer plug.

 

Thank you for the diagram. Zero volts, and continuity, on the ground.

 

The Stop/Turn all go through the Trailer Tow Converter.
If you are not getting Power to the rear 7 Pin on those pins, you need to check the Trailer Tow Converter.
It’s under the dash on the drivers side above the left kick panel.

I believe this is it. None of it is “easy” to get too. Even trying to hold the spring loaded door open to check the 7-pin on the bumper can be difficult doing it by yourself.

 

You could get one of those LED trailer plug testers and plug it in to easily see what you have working, they are like $10-20.

 

Lol, I have one on my trailer harness.
I put them on all my trailers.

Buy yes, they do make one that is simply just for checking the connector.

 

My adapter has built in LEDs. The “TM” light is illuminated and the RT one lights up when I turn on my right blinker.

 

Sounds like you have power to your plug, what about the left turn?
I know some trailers are set up to ground through the ball hitch instead of the plug, this is a poor design IMO as all it takes is a bit of rust on the ball or trailer hitch and the lights won't work.

My fix is to hook a ground wire from the plug directly to the trailer frame.

You can check your trailer ground by hooking your test light ground to a good ground on the truck with the trailer connected and the lights on and touching the test light probe to the trailer frame, if it lights up at all your ground is bad to the trailer.

 

That is exactly how I do it too.
I use an eyelet and self tapping screw on the trailer frame. Be sure to paint the eyelet or coat it in liquid electrical tape. Other wise it will rust pretty quick. Or at least mine did anyways.

 

Ok. I unplugged the towing converter, sprayed contact cleaner on both ends. I also found my backup trailer harness adapter. After wiggling it a bit I have all lights except the two on the front of the wheel fenders. Should be easier to troubleshoot those at this point.

 

This actually had me curious. I forgot my connector has it built in too.
That way I know the truck is OK.
And then the one on the trailer too.

I must be getting old. Lol
I forget what I do and don’t have half the time.

 

As a former trailer tech, cheap ass adapters cause more problems than bad grounds. Buy a decent adapter without the led lights cause that's another connection in the wire that can break.

 

I have a similar problem, but only the tail light terminal at the rear trailer 7 pin connector has no power. Turn signals and brake lights work. I'll try doing the same on the towing converter. Hopefully, I'll have the same results.

 

How does one know a decent from a crappy adapter? Any brands to look for or avoid?

 

Do the tail lights go through this converter also? I have power to turn signals and brake lights to the 7 pin connector, but nothing at any terminal when tail lights are on. The lights on truck all work as normal.

 

@thmx you need to check the 30A fuse and the Relay for power.

 

What's the location for the relay and the 30a fuse?

 

Under hood

 

Never mind. I found it. This is my Brother in laws truck. He had replaced that fuse with one that plugged in, but was taller and didn't work. I replaced it with the correct fuse and that fixed it. Thanks for your help.