Fuse Mystery

Toyota Tundra 2014 CrewMax, already took it to 2 mechanics and 2 trailer shops, and nobody knew how to fix this issue:
When I pull my trailer, the inside lights don’t work (they work when hitched to a different truck); all the other lights, stops, etc work fine. It must be the 12V fuse box but they can’t find it (it’s not in the engine fuse box and the manual doesn’t help). I read there’s a second fuse box somewhere else but how do you access it?
Any advice on how to fix this issue? Thanks

 

There's another set of fuses on the passenger side firewall I believe.

 

^ this.

You can find the fuse box locations and layout in the wiring diagrams.
You can find those here..

Ultimate TUNDRA Wiring Diagrams Collection | Toyota Tundra Forum (tundras.com)

 

Is 15V Stop the one am looking for? I consulted the 700-page manual with the complete wiring and am still confused if this fuse is in the engine or passenger fuse box

 

What exactly are you referring to, the INSIDE lights?
The lights in the truck?
The lights INSIDE the trailer?
What kind of trailer is it; hauling, camper, etc..?

Clarifying that would help a lot..

 

Check #38 under the hood. I believe that's a 40A trailer accessory fuse.

 

It’s a horse trailer, the lights inside the trailer aren’t working. Running lights, stop lights, blinkers are all working. Thanks!

 

They don’t work with the truck off or on? Did you check both?

The 7 pin hot wire/auxilary power is only powered when the truck is running. When turned off, it’s not powered and that’s what powers your interior light (most likely).

my enclosed trailer is the same way. The Chevy I have is powered all the time, so the light works even with the car is off.

 

Yes, the interior lights of the trailer do not work when the truck is running. I tried a different truck (running…), and all the interior lights of the trailer were running fine. All 4 mechanics I consulted told me to find the 15V fuse which they could not see marked in the engine fuse box…

 

That circuit is going to be your SUB BATT 40 amp fuse.
Black wire, Pin 5 on your trailer plug.
Maybe test it with a DMM.






I'm guessing the fuse is in the engine fuse box, I can't find the location.
Although the SUB BATT relay is in the engine fuse box.

 

SUB BATT 40A is #38 in the diagram i posted above.

 

thank you, going to try this!
Also they did test the 7-way pin and one wasn’t working