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I have a '40 White hopper trailer and the tail lights are the only thing not working. The truck is a w900 KW with 7 pin connector going to the trailer. I inspected the pins and holes and it appears that on both ends that it corrosion free. The trucks tail lights are working fine and turning signals and brake lights work on the trailer. I wouldn't necessarily think it was a ground issue since the other function work. What pin controls the tail lights and what else can I look for?
 

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Big pin is ground, pins on either side are tail and marker lights, pins below them on left and right are turn signals and the bottom pin is brake lights. Center pin is auxiliary pin, now used for ABS. The trailer should be wired so about half the lights go on with each circuit so if you have a short only half the lights go out. Allot of trucks and trailers have had jumpers installed between those top two pins. This is a stupid idea as the actual problem was never fixed and all the lights go out if there is a short in the trailer.

Use a tester to check those pins either side of the big pin in the truck plug, both should be on with headlight and marker lights turned on. If that's good then check the pins on the trailer plug, some you can spread if your real careful, the solid ones you can bend sideways a bit. The best way to check is to have someone watch the tail lights as you move the plug in and out and wiggle it to see if the lights come on or at least flicker.

If one of the pins is not lit up check to see if all the switches are on, also the other end of your trailer cord should be plugged in at the back of the cab of the truck (if it's built after 1985 or so), check if you have power on both pins there. Some trucks use manually resetable breakers that you have to push a button in, make sure they are set. Automatic breakers will get real hot after awhile which means you have a short somewhere in the system. Chances are the short will be in the rear light box of your trailer, had that last night at ten oclock, I'm going to strangle the ******* that wired that extension when I get to the farm tomorrow (read my signature...). Last week I also discovered somebody peeled the cord too far back when they installed a new trailer plug and crammed it in there, movement of the cable caused the wires to chafe and short together.
 

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I'm running 3 older tandem trailers and have light problems a couple times a year. On the older stuff, always start with checking the ground at the lights. 90% of the time, it's the ground on older stuff. If they're plastic or rubber mount, the taillights should have a ground wire to the frame. Some older ones ground through the mounting screws and you need a new screw or washer. If they are good, check the plug ins from tractor to trailer. A couple times I've had a bad connection on the screws holding the wire in the connecter plug.
 

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Brown wire is tail lights. Black wire is clearance lights. White wire is ground. If plate isn't grounding thru then it could be a ground on tail lights.
I wish... Most I find are mix of what you describe some being as bad as brown on the right and black on the left :(. There are more then a few manufacturers out there that don't take trailer light wiring seriously. When I wire trailers I always try to tie half the lights on each circuit, especially on the back. I'd have two outer tail lights and three center lights on black then four inner tail lights and corner lights on brown, this way I always have tail lights regardless of which circuit goes down.

I always ground through the white wire, nothing is tied to the trailer frame except for a jumper from the plug or a junction box. Those cheap junction box are a problem too if they are not placed and sealed up properly.
 
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