Trailers lights are dim

[Rippinlips14]

So my trailer lights are dim. Its not the trailer because Everytime I plug them into a different truck they work but when i plug them into mine they do not. I ran a new ground and replaced the plug and they are still dim. Then i ran the negative ends of the jumper cables to act as a direct ground they worked flawlessly i ran the jumpercables from where I ground on the truck to where I grounded on the trailer any thoughts on why they are still dim?

[Molay1292]

Because vehicles and boat trailers are both frame grounded, if one or the other ground wires at the plug are loose or not making a good connection the lights might be dim.   What typically happens is because the primary ground through the plug is bad or broken the trailer will try to ground itself anyway possible, usually trough the ball and hitch, which is not  good ground. 

I know this does not help with the fix, but it might help with the diagnosis.

 

[Snakehead Whisperer]

Just go over everything thoroughly. You say you ran a new ground and replaced the plug, but you didn't mention whether that was on the trailer or the tow vehicle? You should check both. If you have a multimeter, you can actually check voltages and verify ground at both plugs. It's probably something really simple; just make sure to go over the wiring on the tow vehicle too, especially the ground wire.

I'm assuming that your tail lights on the tow vehicle are bright as they should be, and that this is isolated to the trailer. 

[BassnChris]

If lights on tow vehicle are correct.

And trailer lights are correct when connected to another tow vehicle.

i would start my search at the trailer light connector on my tow vehicle.

[Rippinlips14]

5 minutes ago, BassnChris said:

If the lights on tow vehicle are correct.

And trailer lights are correct when connected to another tow vehicle.

i would start my search at the trailer light connector on my tow vehicle. 

This is exactly where I started by putting a new plug on and connecting the ground wire in two different places which didnt work 

 

[Fishing Rhino]

The problem is with your vehicle.  I agree that the most likely culprit is a poor ground.  The trailer frame provides the ground for the trailer lights.  But you have a ground wire in the plug on your truck which goes to the truck frame.  From the plug on your truck, the ground connects to the plug on the trailer.  A wire from this goes to the frame of the trailer, making the frame the ground for all your lights.

Try connecting your jumper to another part of the truck where you  can make good contact, and connect the place where the ground from the trailer plug connects to the frame.  That has to be a good contact since it works on other vehicles.

I'm guessing the ground contact on your truck is poor since that is where you connected your jumper.

A side bit of information.  Most newer vehicles have a dedicated circuit for trailer wiring so the trailer lighting is isolated from the vehicles light system.  This prevents a short on the trailer from blowing a fuse for the tow vehicle's lights.

You could have corrosion in the truck plug which is preventing a good ground.  For future reference, apply some dielectric grease into the female receptacles on the plug.  It will prevent a multitude of problems caused by corrosion in the plug.

[Fishing Rhino]

Been thinking about this.  Are the brake and turn signal lights okay, or are they dim too?

If they are good, then it has to do with the circuit on your truck that feeds the running lights (tail and side marker lights).  The brake and turn signal lights connect are part of the same circuits.  The only difference is that when you turn on the signal light, it runs the brake light through a flasher which causes your signal light to blink.

Search for the fuse panel layout for you vehicle.  For example google "fuse panel layout 2007 Ford F150.

You can then see which circuits are for the trailer lights and then for the brake turn signal you'll find something like left, lft, L brk/trn, or right, rt, R brk/trn.  They abbreviate them differently.  You will also see something like trl to denote trailer.  Here's an example.

See numbers 5, 13, 17

[Molay1292]

I had one more thought on this and then I will step out.  It is possible that you still have a poor/bad trailer ground.  Remember that if the primary ground is bad the trailer will still try to ground itself through the ball and hitch.  Because your connections make for a dim light and another vehicle makes for brighter lights can be explained by just having a better grounding connection through the ball and hitch on the other vehicle. 

You stated that when you grounded using jumper cables everything was good, which would explain and support my theory.  I believe that you could test for a poor/bad trailer ground by not hooking the trailer to the hitch and just connecting the plug, this would eliminate possible grounding through the hitch.  If you have no lights then you know you have bad ground on the trailer.

 

[WIGuide]

7 hours ago, Molay1292 said:

I believe that you could test for a poor/bad trailer ground by not hooking the trailer to the hitch and just connecting the plug, this would eliminate possible grounding through the hitch.  If you have no lights then you know you have bad ground on the trailer.

This is what I was going to suggest. The fact it worked when he grounded it with a jumper seems that it's not grounded correctly, but this would help solve that by isolating the the trailer by itself. It's also worth a shot to make sure the ground has a good connection on both the trailer and the truck by sanding down the ground area to bare metal and use dielectric grease as well. 

[Way2slow]

The white wire has to have a good connection to the frame of the truck on that end and a good connection to the frame of the boat trailer on that end.  If you have a volt meter and familiar with how to use one, it's easy enough to check.  Take a voltage reading between the white wire and the frame of both, the truck and the trailer.  If either has a bad connection, there will be voltage on the white wire.  Since the white wire is supposed to be at battery negative (ground) there should be zero DC volts on it.   It can be very small, to several volts DC, depending on how bad the connection is.  If there is zero volts, then you have a bad connection else where.