I am in the process of rewiring my 1999 Shoreland'r boat trailer and when I began removing the old wiring harness I found it had a separate ground for the left side and the right side aside from the ground up by the hitch. Is there a reason for this even though it is grounded up by the hitch? I plan to run seperate yellow-brown to the left and green-brown to the right side and bypass the clearance lights on the side. Any opinions or recommendations? Thanks

One ground back is fine. Just cross it over back at the lights from one side to the other. Or run a ground for each side from the hitch/ ball area like it was. One way is not any better than the other.

4 wire plug standard is:

White Ground
Brown Tail lights
Yellow Left turn/brake
Green Right turn/brake

A dedicated conductor for ground is always better then relying on the hitch connection as ground.
Ever seen a small trailer behind a pickup at night, an the trailer lights flicker off/on every few seconds,
this could be a bad/loose ground termination point, no ground conductor to tow vehicle, just relying on the hitch connection.
There are better ways.
FWIW

Doug
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A bad ground is a real pain. Makes lights do all kinds of weird stuff. Run a single ground wire to each light fastening it to the light. Some lights have a ground wire and some ground to a mounting bolt. Run the ground wire to your plug. The cost of a few more feet of wire is worth it for not having constant problems. Dont use but connecters or wire nuts or any of that stuff to make connections. Solder and shrink tubing.

If your wiring the plug on the vehicle spend 3 more bucks to run a dedicated ground wire to the negative post on the battery. I have never had a ground problem with factory installed trailer plugs. If your doing it though a ground wire to the battery prevents a lot of headaches

Back as a teenager when I first started working with trailers, my buddy’s Dad told me the most important connections are the grounds. One main for the system and ground individual lights where possible.

I agree about never using blade connectors. They last about 10 minutes. Also agree on using solder and shrink tubing. The connectors with solder and shrink tubing are great. The kits are inexpensive on Amazon.

Use dielectric grease everywhere possible to prevent corrosion. Secure the wiring with clips or zip ties. Use wire loom to protect the wires. Tape or zip tie the loom in place. Put some dielectric grease in the couplers. When not in use, put a heavy plastic bag secured with a zip tie over the connector to prevent wasps from packing mud.

Especially in salt water, a dedicated ground to each light, and to the truck.

My advice is to run all your wiring through PVC pipe and use submersible LED lights. It's what I did on my shorelander of the same vintage. I haven't had to touch them in 5 plus years, that's unheard of for boat trailer wiring in my circle of friends and family. I just have the one ground by the hitch on the trailer.

Well today hooked all of my trailer wires and a ground to each side at the back of the trailer. I have power to the lights but no lights. I ran a jumper wire from the hitch on the P/U back to the tail light and had power but when I grounded to the bolt where the ground wires split at the Y of the trailer tongue and go to each side no power. I have polished the plate on the back with a angle grinder to make sure there is a good ground but doesn't seem to help. I'm wondering if it might be the lights as there is power to the bulb and the bulb checked out ok On mu utility trailer that I wired I just ran my green brown and yellow brown to the lights on the back and ground to the tongue of the trailer and it works fine. Makes me wonder.

Just for fun hook a wire to the negative post on the vehicle battery. Run it back to a bolt on back of a trailer light strip the insulation without cutting the wire, wrap the bare spot around the bolt and hook the end of the wire to the other light. Then see if they work

Do you have a tilt-trailer? I worked on a boat trailer once that lost the ground where the trailer tilted so I had to run a jumper wire to get ground back to the lights.

Thanks for the ideas on getting the lights on the trailer to work. I am going to try the single wire from the batt. when it get's light & warms up a bit this morning but I got to thinking when I went to pull my receiver hitch out the other day it wouldn't budge and I finally had to put a log chain around a tree and to the ball to get it to come out of the receiver. There was so much rust and crud on the receiver and inside of the hitch that I'm wondering if that may have something to do with the problem? I cleaned the hitch and receiver but am wondering if that might still affect the ground?

Sure could.

Instead of grounding to the frame through the ball hitch, run a ground wire to the plug on your vehicle. Run a ground wire from the trailer plug back to the lights. That extra 10 bucks worth of wire will prevent 99% of trailer light wiring problems if all your connections are SOLDERED AND SHRINK TUBED

If the vehicle plug is a factory installation just run a wire from the trailer plug to the lights

Are you saying all the light don't work or only the back 2 that have turn, tail and break? Some of the light have to be individually grounded to work. We wire trailers up all day of all kinds some light are better then others.

Update: I took the hitch off the P/U and shined up the tube with a grinder to make sure it was bare and no rust then replaced it but no lights so them I ran a ground wire from the batt. to the both lights on the trailer and I had left turn signal but no right signal but when I turned on the headlights both lit up so that could be the bulb. Looks like I might have to run a wire from batt. to trailer frame as there might be too much rust left inside the hitch that I can't get at to remove without some kind of a grinder or flap wheel.

It wouldn't have to be from the battery.......just a good solid ground on the frame at the back of the vehicle. Even drill a hole and put a bolt thru it just for the sake of a good ground.

That's what I was thinking of trying as it definitely looks like a ground problem.

Are the lights used / old? Incandescent or LED? Sealed?

Start with a new set of lights. Old trailer lights were very poor quality. The new LED sealed lights are good.

No need to go to the battery for a ground. Drill a hole on the vehicle somewhere that won’t damage. Same thing on the trailer.

Look at etrailer.com for a new set of lights if needed.

Your plugs should have a ground wire to each frame, clean those and the ground from light to trailer frame.