Lighted rocker switch puzzle (long)

Here's one for you electrical experts. On a trailer I have built I have installed a lighted rocker switch to light up the tail lights from a battery mounted on the trailer. This trailer will be pulled primarily with a tractor but it sometimes will be behind a pickup, so I thought it might be a good idea to be able to light up the tail lights if it's pulled by a tractor if he gets caught up in the dark before he gets home. The battery supplies power to some other things on the trailer too.

On the tongue I have a cord with a regular RV plug to plug into the pickup when needed and it powers the brake/tail lights.

The rocker switch on the trailer has three terminals - in/out/ground. The ground is for the light in the switch. When I plug the RV plug into the pickup and turn on the lights it nturally backfeeds to the rocker switch, but it pops the fuse because the rocker switch grounds it out somehow, even though the switch is turned off. If I unplug the ground wire from the switch the fuse in the pickup does not blow.
Are we confused yet? The switch is wired right, I've checked it multiple times. A check with the OHM meter shows resistance between the ground terminal and the 'power out' terminal when the switch is turned off, but the circuit between ground and out is open when the switch is on.

So now after reading this long confusing post, does anyone out there know anything about how these lighted rocker switches work internally? Why does this switch turn the power out terminal into a grounded termional when turned off?

Looks like I'm gonna have to isolate the switch with either a diode or relay. Jim

I think if it was my trailer I would not want a switch which can draw power. If I wanted a light to let me know when the taillights are on, it would be a small indicator light next to the switch. If I remember right, I think some of those lighted switches light when not turned on, others when turned on.

Paul, I've thought about a separate pilot light, but it will mess up the order of the switch panel. This 'trailer' is a trolley car replica, and it's as fancy as anything can be, with oak woodwork, stained glass windows and the works, so I'm being very fussy about every nit-picking thing on it. Maybe I should have explained exactly what I'm working with right away, but I've been holding off until it's completed and I can post a pic. Hopefully it will be done in a week or two, providing I don't make any more trips to the operating room.LOL.

What I'm curious about is why the terminal on this switch, that sends the power out to the lights, turns into a grounded terminal when the switch is turned off, instead of just being open and connected to nothing. I'm wondering if there's a lighted switch out there that doesn't do this. Jim

It sounds to me like your switch is single pole double throw (SPDT), with the "out" terminal being common. That is a common configuration for rocker and toggle switches: A SPDT switch can be used to replace a single pole single throw switch. so stores only have to stock one part instead of two.

It would be a good idea to put a diode in the circuit anyway. If your battery is dischared and you forget to turn the switch off, you're almost certain to blow your tail light fuse.

I went through just the same thing a while back

It is actually a SPDT switch and uses one side as a ground for the bulb.

I would GUESS they make them that way to be CHEAP, is it simply involves adding a light to an existing 3-terminal switch.

Coincidently, that throws a dead short to ground from the middle terminal when the switch is thrown to the "OFF" position.

When used as intended, that is not a problem, but when the switch is "backfed" so the output is powered in the "OFF" position you get a dead shot.

Just use a normal switch, and a separate indicator lamp, if you REALLY need the "idiot light" function.

Huh! So I'm stuck with working around this problem if I want to keep the lighted switch, and I do. If there's no indicator light of some sort I can see someone leaving the tail lights on, exiting the trolley, getting in the pickup and leaving. The switch panel is in the headboard in the front of the trolley.

One idea I had was to have the lighted switch turn on a relay. That would isolate the tail lights from the switch, but the diode idea sounds better. There are just two taillights and they are LED so the draw is very little.

There are 16 individual lights on or in this thing, all are LED, all are clear except for the taillights and if all lights are left on it takes three full days to kill a 850 amp deep cycle battery, I tried it out. Jim

A lighted double throw double pole switch should do it.

1 4
2 5
3 6

Power to switch from truck to #1
Power from switch to trailer lights #2
power from on board battery to #3 and #5
With switch connecting 1-2, and 4-5, The switch light should be off, and the trailer lights controlled by the truck lights.
With the switch connecting 2-3, and 5-6, the battery should be lighting the switch, and the trailer lights. (much depends on where this switch gets its internal light power, but that should be easy to determine. JimN

Why not simplify things, forget the switch. Make a pigtail to plug into the trailer connector to power the lights when not connected to the truck?