So, I am changing vehicle reverse lights to a switch I can control, and another switch to control the trailer reverse lights. Works great.
I then wanted to have a trailer switch inside trailer to turn on the same trailer reverse lights. (trailer is not attached to vehicle for this mod).
Blew the fuse.

Switch has 3 terminals, has a led indicator, and is just on/off switch.
Lights are hooked to positive wiring coming from trailer plug (which is not hooked up so is nothing right now/dead end), so I hooked positive from trailer battery to switch, then out to 12+ wiring to lights. Grounded the switch from the ground terminal (which is so led will light up on switch).

Turn on switch, fuse blew.

I wired 6 other of these same switches in the vehicle and they all work fine. Could switch be bad, or what kind of strange thing would blow the fuse?

 

Need a circuit diagram, for you and for us!
Max.

 

Here is crude diagram.
Simple, what am I missing, or what could be going on to blow a fuse? Sometimes I cant see whats right in front of me…
This is wiring of a light on a trailer. The vehicle is not plugged into the trailer. Ground is to the negative wiring to the battery then to the frame.
There are two lights, I should have drawn in, the positive from first light connects to positive to next, then gnd. LED, 18w each, so not enough to blow a 20amp fuse?
Lights will work off of vehicle when connected.


Ive done all the wiring on trailer last year, and just did relays and switches on the vehicle, so Im not a beginner, but not an expert either…
This is the switch:

 

Sounds like you have a short somewhere in your wiring. Only other few things it could be. 1 the load of the lights are to much for the size of fuse your trying to use. 2 the connection on the switch for power in and out and ground isn't connected right.

For tracking down the problem, I would start with Checking Fuse size, Switch is wired right then the wiring from the lights going to the the truck that you have marked un hooked. and take it off the lights post to make sure the short isn't past that going to the truck When not connected it could be in the run somewhere resting on ground.

Next I would remove the bulbs and test for ground *Short* in your wiring with a DVOM/DMM. With the Bulbs in you can still find ground in your wiring when probing the 12v side of the wiring because it will pass the ground through the bulb so that is why I would remove the bulbs. Now with only the 12v wires left if there is a ground showing then that is the line that has the short. Easy to get a staple or screw through the wiring

 

That drawing is awesome. Made my day.

Why do you have the switch connected to ground?
The way you have it (making some assumptions about the switch configuration), when you switch it one way it sends +12V to the light. When you have it the other way, it sends +12V to ground. That will blow a fuse real quick.

EDIT:
Just now saw that it's a lighted switch and the GND is probably for the LED in the switch. Still, I would check that connection and make sure you have it right. Does the LED in the switch work properly?

 

Thought about something while I was out shopping. Also a fast way to test this without all the testing lol. Would be remove the Ground from the switch for the LED and if by chance the Switch was bad *With short in switch* it wouldn't be grounded so it shouldn't blow a fuse if that was bad.

If still blows fuse with ground off switch then short in your wiring somewhere...

James

 

That's a good idea to remove the ground. The switch tested out fine w a multi meter. I re-thunk it, lol. I changed the switch out to an A-B (on-off-on) selector toggle switch, so I have to choose trailer power, off or vehicle power (when it's switch is on). This only benefit is keep trailer battery from feeding into vehicle, except that vehicle uses a relay which should isolate everything unless its wired backwards....
Here is new diagram just for fun since u enjoyed it so much. It all works fine now, at least trailer side (vehicle side worked before so should still be fine.)
So, can I rule out anything except a bad switch?

 

Sadly with removing the ground on the switch or even changing out the switch all together and everything works as expected with new switch then sounds like bad switch.

Now only thing to remember is that you need to change the position every time you want to use the light from the truck side.

Glad you got it working