Running Lights on trailer

[Pony]

My running lights are working intermittently on my trailer and I’m looking for some direction. They will work sometimes and then stop working once turn the truck off and come back and occasionally come back on while driving.

I started by checking grounds and looking for wiring issues. I have a 7-4pin adapter that has the the little LEDs on it to indicate that a circuit is working. The running light circuit is on until I plug in the harness.

I just finished running a new harness on the trailer and it’s doing the same thing which leads me to believe it could be on the truck side, but that circuit indicator shows it’s getting power to the plug so I’m a little lost on where to start.

What i need to do is hook up a different vehicle to my trailer and or hook mine up to a known working trailer....I just haven’t had a chance or opportunity yet.

 

[alldodge]

Is the white wire (ground) attached to the frame of the trailer?

 

[dingbat]

I started by checking grounds and looking for wiring issues. I have a 7-4pin adapter that has the the little LEDs on it to indicate that a circuit is working. The running light circuit is on until I plug in the harness.

How many wires do you have connected to your running lights?

If you don’t have two wires, you need to make sure your light is grounded through the trailer back to the connector.

Disconnect the trailer and check for continuity between the running light (brown) and ground (white) pins in the trailer connector.

Should be continuity. If not, you have a ground problem

 

[Silvertip]

For testing you don't need another vehicle. If you have a battery charger and nail you have all that's needed to verify the trailer. 1) Plug in the charger. 2) With the trailer plug disconnected, connect the charger ground lead to the trailer frame or coupler ensuring the area is not painted. 3) Clip a nail into the positive charger clip and then one by one, insert the nail into each terminal on the plug BUT DO NOT STICK IT IN THE GROUND TERMINAL. Each set of lights should light brightly using this technique. If so, the problem is the truck. If not, troubleshoot the trailer.

 

[Pony]

Is the white wire (ground) attached to the frame of the trailer?

On the new harness yes. I have a frame ground at the tongue and on the individual lights.

Im going to go back and recheck the grounds on the side markers though.

 

[Pony]

How many wires do you have connected to your running lights?

If you don’t have two wires, you need to make sure your light is grounded through the trailer back to the connector.

Disconnect the trailer and check for continuity between the running light (brown) and ground (white) pins in the trailer connector.

Should be continuity. If not, you have a ground problem

There are two brown commons coming off the 4 pin flat plug for the running lights. One going to each side and then only spliced in where there are markers.

The ground is at the tongue via an independent wire and at each fixture

 

[Pony]

For testing you don't need another vehicle. If you have a battery charger and nail you have all that's needed to verify the trailer. 1) Plug in the charger. 2) With the trailer plug disconnected, connect the charger ground lead to the trailer frame or coupler ensuring the area is not painted. 3) Clip a nail into the positive charger clip and then one by one, insert the nail into each terminal on the plug BUT DO NOT STICK IT IN THE GROUND TERMINAL. Each set of lights should light brightly using this technique. If so, the problem is the truck. If not, troubleshoot the trailer.

I did not know I could do that! Thanks!

Also nice seeing some names I recognize in here

 

[dingbat]

There are two brown commons coming off the 4 pin flat plug for the running lights. One going to each side and then only spliced in where there are markers.

The ground is at the tongue via an independent wire and at each fixture

The ground(s) need to be tied back into the connector (white wire) from the truck.

Sounds like you have a non-standard connection harness.

Four (4) pin connector:
Brown (tail lights and markers)
Green (right turn signal)
Yellow (left turn signal)
White (ground)

 

[Pony]

The ground(s) need to be tied back into the connector (white wire) from the truck.

Sounds like you have a non-standard connection harness.

Four (4) pin connector:
Brown (tail lights and markers)
Green (right turn signal)
Yellow (left turn signal)
White (ground)

From the 4 pin connector that is what I have. The ground in grounded to the frame. There are two brown wires coming off the same pole for the common circuit to run down each side of the trailer...and obviously green and yellow for the signals. It’s a normal harness. It is brand new.

Each marker light is connected to the common circuit and has an independent ground to the trailer frame. The grounds are all clean and tight.

I have used the battery charger method to confirm that all the circuits work correctly so I now believe the problem is the truck side.

What is confusing to me is that the 7 pin to 4 pin adapter that I use has the testing lights built into it. The common circuit is lit until I plug in the trailer. On my truck there is a double connection. The harness has a 7 pin and a separate 4 pin next to it so you don’t have to use the adapter. Both connections perform the same way.

 

[dingbat]

From the 4 pin connector that is what I have. The ground in grounded to the frame. There are two brown wires coming off the same pole for the common circuit to run down each side of the trailer...and obviously green and yellow for the signals. It’s a normal harness. It is brand new.

Each marker light is connected to the common circuit and has an independent ground to the trailer frame. The grounds are all clean and tight.

I have used the battery charger method to confirm that all the circuits work correctly so I now believe the problem is the truck side.

What is confusing to me is that the 7 pin to 4 pin adapter that I use has the testing lights built into it. The common circuit is lit until I plug in the trailer. On my truck there is a double connection. The harness has a 7 pin and a separate 4 pin next to it so you don’t have to use the adapter. Both connections perform the same way.

Adaptors suck....more problems than they are worth. First thing I do to new trailer is to add a breakout box on the tongue and convert over to a 7 pin connector.

I still think you have a ground problem.

Do do the other functions work?

What model and year of truck?

A lot of new trucks use current sensing technology. They don’t power a connection until they sense a load on the ground connection at the load center. Drove me nuts trying to wire the aux lighting in horse trailer to a new F150 until I figured it out. Worked no problem with the old truck.

if you put a meter between the brown and white pins on the trailer side connector, do you have continuity?

Using a battery charger proves nothing unless you connected the charger ground to the truck, not the trailer. Connecting it to the trailer adds a secondary ground path that doesn’t exist in normal operation.

 

[Pony]

Adaptors suck....more problems than they are worth. First thing I do to new trailer is to add a breakout box on the tongue and convert over to a 7 pin connector.

I still think you have a ground problem.

Do do the other functions work?

What model and year of truck?

A lot of new trucks use current sensing technology. They don’t power a connection until they sense a load on the ground connection at the load center. Drove me nuts trying to wire the aux lighting in horse trailer to a new F150 until I figured it out. Worked no problem with the old truck.

if you put a meter between the brown and white pins on the trailer side connector, do you have continuity?

Using a battery charger proves nothing unless you connected the charger ground to the truck, not the trailer. Connecting it to the trailer adds a secondary ground path that doesn’t exist in normal operation.

All other light functions work perfectly fine. It is only the running lights that are having the issue. Wouldn’t an improper ground on the truck harness side affect the other circuits?

its a 2004 Silverado.

I would have to check for that continuity when I’m off of work next....

 

[Batmanrjw]

I too have lighting problems on trailer.
Shocked when I saw you has 2004 chevy as well.
I upgraded to led lights then problem started. Also Bought new light connector to truck.
Spent hours on light problem.
sanded all grounds to bare metal, added new grounds battery to frame, .....etc.

One More thing I want to try is getting back to OEM light connector. lights were fine before so I’ll try that.

 

[Scott Danforth]

I too have lighting problems on trailer.
Shocked when I saw you has 2004 chevy as well.
I upgraded to led lights then problem started. Also Bought new light connector to truck.
Spent hours on light problem.
sanded all grounds to bare metal, added new grounds battery to frame, .....etc.

One More thing I want to try is getting back to OEM light connector. lights were fine before so I’ll try that.

please do not hyjack other people threads

 

[Grub54891]

Each marker light is connected to the common circuit and has an independent ground to the trailer frame. The grounds are all clean and tight.


I see the issue as this. Yes they are grounded to the frame, but they can lose continuity through the joints in the frame. I always run a continuous ground directly to the ground on the plug. Lighting issues seem to be a thing of the past ​