Trailer taillight wiring, is there a standard method?

[DMartin9N-2N]

Good evening: I bought a trailer (probably 20 years ago) to haul my 9N. Standard 2-axle trailer with brakes that activate when I press brake pedal in tow vehicle. The lighting is my question area: With plain, cheap tail lights from parts store, do most folks wire the rear bulb (running light) and the small side bulb so they both get power from the same wire? Any suggestions appreciated! EDIT: I am assuming the trailer lights get their ground from a wire that comes from the tow vehicle's harness.

 

[Jim.ME]

Good evening: I bought a trailer (probably 20 years ago) to haul my 9N. Standard 2-axle trailer with brakes that activate when I press brake pedal in tow vehicle. The lighting is my question area: With plain, cheap tail lights from parts store, do most folks wire the rear bulb (running light) and the small side bulb so they both get power from the same wire? Any suggestions appreciated! EDIT: I am assuming the trailer lights get their ground from a wire that comes from the tow vehicle's harness.

Trailer with brakes should use a 7 flat blade plug. There is supposed to be a dedicated 10 or 12 gauge wire for a ground back to the tow vehicle. There is a standard of where the different function wires connect to the terminals of the plug. The side market lights get connected to the taillight wire(s). Search wire diagram for 7 flat blade trailer plug and you should find a number of diagrams of how to wire them. There is a utility trailer standard and an RV trailer standard. The location of which blade a function goes to in the plug are the same. The difference in the two standards are the wire colors used for the functions.

 

[DoubleO7]

Good evening: do most folks wire the rear bulb (running light) and the small side bulb so they both get power from the same wire? Yes Any suggestions appreciated! EDIT: I am assuming the trailer lights get their ground from a wire that comes from the tow vehicle's harness.Yes

 

[504]

The best thing the state of Missouri did for trailer pullers is now we can mount the license plates sideways on the fender. And permanent plates are $58.00 for life

 

[DMartin9N-2N]

Good afternoon, Doubl07 and others. I have tried powering the trailer lights with temporary set-up, hitch ball not connected. I get some lights, but not all ok. Maybe I am missing a separate ground wire? BTW, I seem to have a round connector with small round pins, I think 4 pins. Maybe will get out there this afternoon, if rain does not prevent me. My truck is a 2005 Silverado, supposedly has factory supplied trailer stuff. Automatic trans has a special setting on column selector lever, keeps it from "hunting" in and out of 4th gear on rolling hill country. I bought truck used, near end of 2005, the dealer (who had used the truck for a rental unit), handed me a "glove box harness" which I installed, along with a couple wires underneath vehicle. I did get a trailer brake controller installed under dash, it has worked ok, but I don't know why I don't have a 7-flat-pin connector. I hope to have more info to post this evening....

 

[bmaverick]

Good afternoon, Doubl07 and others. I have tried powering the trailer lights with temporary set-up, hitch ball not connected. I get some lights, but not all ok. Maybe I am missing a separate ground wire? BTW, I seem to have a round connector with small round pins, I think 4 pins. Maybe will get out there this afternoon, if rain does not prevent me. My truck is a 2005 Silverado, supposedly has factory supplied trailer stuff. Automatic trans has a special setting on column selector lever, keeps it from "hunting" in and out of 4th gear on rolling hill country. I bought truck used, near end of 2005, the dealer (who had used the truck for a rental unit), handed me a "glove box harness" which I installed, along with a couple wires underneath vehicle. I did get a trailer brake controller installed under dash, it has worked ok, but I don't know why I don't have a 7-flat-pin connector. I hope to have more info to post this evening....

never seen a 7-way flat connect for trailer hookup.

This is the SAE standard,

 

[Jim.ME]

Good afternoon, Doubl07 and others. I have tried powering the trailer lights with temporary set-up, hitch ball not connected. I get some lights, but not all ok. Maybe I am missing a separate ground wire? BTW, I seem to have a round connector with small round pins, I think 4 pins. Maybe will get out there this afternoon, if rain does not prevent me. My truck is a 2005 Silverado, supposedly has factory supplied trailer stuff. Automatic trans has a special setting on column selector lever, keeps it from "hunting" in and out of 4th gear on rolling hill country. I bought truck used, near end of 2005, the dealer (who had used the truck for a rental unit), handed me a "glove box harness" which I installed, along with a couple wires underneath vehicle. I did get a trailer brake controller installed under dash, it has worked ok, but I don't know why I don't have a 7-flat-pin connector. I hope to have more info to post this evening....

You said you have brakes on the trailer. That means you need a minimum of 5 wires.

Ground (10 or 12 gauge)
Brakes (10 or 12 gauge)
Tail/marker lights (16 or 14 gauge)
Left Turn (16 or 14 gauge)
Right Turn (16 or 14 gauge)

The other two wires in a 7 pole plug are:

Auxiliary power (10 or 12 gauge) often used to keep the brake battery charged.
Reverse (backup) lights (16 or 14 gauge) Center pole of the 7 flat blade plugs. Could be used to power something else as not all trailers have backup lights

 

[Jim.ME]

never seen a 7-way flat connect for trailer hookup.

This is the SAE standard,

View attachment 3782

Agreed I have not seen a flat 7 connector plug used on a trailer, I have seen 3 to 5 pin flat plugs used. Your picture is a 7 flat blade plug vs a 7 round pin plug as is used on semi-trailers.

The function positions are SAE standard. The Utility trailer and RV trailer color standards vary, so one has to be careful and check plug blade to required trailer function, not just wire by color when replacing a cord on a trailer with a pre-made cord. Many pre-made cords are wired RV and things will be mixed up if wired color to color on a utility trailer for example.

 

[Brown David]

never seen a 7-way flat connect for trailer hookup.

This is the SAE standard,

View attachment 3782

That is how I took it first as well.

Then I re-read his post and realised he meant "7 flat blade connections", not a "Flat 7 blade plug".

He's right but could have worded it better.

 

[Jim.ME]

Good afternoon, Doubl07 and others. I have tried powering the trailer lights with temporary set-up, hitch ball not connected. I get some lights, but not all ok. Maybe I am missing a separate ground wire? BTW, I seem to have a round connector with small round pins, I think 4 pins. Maybe will get out there this afternoon, if rain does not prevent me. My truck is a 2005 Silverado, supposedly has factory supplied trailer stuff. Automatic trans has a special setting on column selector lever, keeps it from "hunting" in and out of 4th gear on rolling hill country. I bought truck used, near end of 2005, the dealer (who had used the truck for a rental unit), handed me a "glove box harness" which I installed, along with a couple wires underneath vehicle. I did get a trailer brake controller installed under dash, it has worked ok, but I don't know why I don't have a 7-flat-pin connector. I hope to have more info to post this evening....

Some of the manufacturer supplied harnesses were just a jumper harness to get from the terminal block on the cross member under the rear of the truck bed to the actual 7 blade socket. The socket you need to plug the trailer cord into, was not supplied, you have to buy the correct socket to match the connector on the socket end of that jumper harness. If you can't find the right socket, or the connector was removed from the jumper harness, you will need a universal socket and connect the individual wires to the correct socket terminals, I have encountered both scenarios.

 

[DoubleO7]

I have tried powering the trailer lights with temporary set-up, hitch ball not connected. You would definitely need a ground wire if the trailer coupler is not on the ball. A ground wire going thru the connector is highly recommended.

I get some lights, but not all ok. Maybe I am missing a separate ground wire? Another highly recommendation is to connect each light into a ground wire circuit. Not relying on the ground via the trailer frame.

BTW, I seem to have a round connector with small round pins, I think 4 pins. Four pins in a round plug would be odd but not unheard of, the small five pin round connectors are more common than the four pin. Both of those are considered old school nowadays.

As mentioned by Jim.ME, you will need at least five connections/wires. Unless your trailer has a breakaway battery to activate the brakes.

I would use the seven way connector. Your receiver hitch likely has the mounting bracket for one incorporated into the hitch already.

 

[Jimever11]

Good evening: I bought a trailer (probably 20 years ago) to haul my 9N. Standard 2-axle trailer with brakes that activate when I press brake pedal in tow vehicle. The lighting is my question area: With plain, cheap tail lights from parts store, do most folks wire the rear bulb (running light) and the small side bulb so they both get power from the same wire? Any suggestions appreciated! EDIT: I am assuming the trailer lights get their ground from a wire that comes from the tow vehicle's harness.

Yes, on my trailer the brown wires are all connected together

 

[caterpillar guy]

If you don't want to loan your trailer out use the semi plugs, They will not fit the plugs most will use and thus not be loaned out. IF you don't mind that then by all means use the flat blade style. For the semi plug the top by the nub on top should be the WHT/GRD then clockwise looking at the plug should be BRN/markr, GRN/ RTN, RED/Brk,Yel/LFTTN, BLK/tail lgts, Center BL for other accessories like your brake controller power. I use mine for a blinky light on oversized loads back when I was in the business, The positions will have small letters by the holes for the color of wire for the position and then just use the cord from an semi to work easily for the connection from trailer to truck. Most have a plug at both ends now so the cord can be removed and not drug when moving the trailer with a tractor to mow under it or so it is not stolen.

 

[mechanicfred]

Depends on the plug. Green is right, yellow is left and brown is tail lights. White is ground, green and yellow double as stop lights. As stated above 7 wires are as described.

 

[T in NE]

That is how I took it first as well.

Then I re-read his post and realised he meant "7 flat blade connections", not a "Flat 7 blade plug".

He's right but could have worded it better.

Better known as 7-way RV.

 

[Brown David]

Better known as 7-way RV.

Right.