Trailer wiring

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Turbo4whl

Turbo4whl

Supporting Member
Supporting Member
Joined
Oct 30, 2019
Posts
3,155
Reaction score
7,324
Location
Downingtown, PA
First Name
Wayne
Truck Year
1974
Truck Model
Jimmy
Engine Size
350
Some info that may help with all wiring upfits. I had a trailer with electric brakes for a safety inspection. Trailer brakes would work with the brakeaway cable pulled, but not through the truck/trailer plug. I found the builder used sealacrimp conectors.
You must be registered for see images attach


Notice the connector upper left with blue wire. The seal over part of the loop prevented the nut from tightening the nut to the metal loop. You can also see how a new connector, upper right also has sealing plastic on the loop, just not as much.

This trailer, two axle 12x4" brakes has a GVRW of 19,300 pounds. It can carry 3 large cable reels. Here is the factory wired connection box. (I use the word factory loosely)

You must be registered for see images attach
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
Hey I've got a somewhat related question that you may know the answer to. We got 4 new International medium duty trucks and we ordered the "in-dash" trailer brake controllers. Well, it turns out, that option simply means that the dealer installs an aftermarket brake controller to the underside of the dash. It's a joke really. But anyway, all 4 trucks will apply braking power to a trailer, but they do not show connection on the controller so it makes it a real guessing game to adjust brake bias. I haven't had much time to investigate... do you have an idea of what they might have done wrong? It's only 4 wires, I don't know what they could have possibly done.
 
Turbo4whl

Turbo4whl

Supporting Member
Supporting Member
Joined
Oct 30, 2019
Posts
3,155
Reaction score
7,324
Location
Downingtown, PA
First Name
Wayne
Truck Year
1974
Truck Model
Jimmy
Engine Size
350
@bucket

What brand is the aftermarket controller?
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
Also, something else that puzzles me is the wiring at the back of the truck. We ordered the optional rear body harness as well as a trailer plug harness. As delivered, the wires are not labeled for function, some of them are odd colors and many of them are just orange. The dealer we purchased the trucks through could not provide us with a schematic of any kind. I went through and figured them out, but each truck has a wire that is a constant 5 volts. It's part of the body harness which are all heavy gauge wires (trailer harness plugs are all smaller gauge wires). Pretty dumb, I think.

Long story short, I have no idea what use there would be for a constant 5v supply. Even the 12v Aux power wires are ignition switched.

I forgot I have a picture of the body harness:
You must be registered for see images attach
 
Last edited:
75gmck25

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
2,304
Reaction score
2,272
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
A guess at two possibilities/guesses, considering that maybe these vehicles were wired for to serve an “international” market. However, very odd that the dealer would not have a wiring diagram.

- wiring of plugs for European trailer power/lighting is different, so I assume color coding of wiring is different. For example, on older Land Rovers you need to use an adapter to plug in to a standard US trailer.

- 5 volts is used for many modern electronic devices and adapters. Could it be 5 volt power for some other type of controller or adapter that is needed for certain conversion bodies?
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
I'm sure someone at the dealership could provide a diagram, but nobody we talked to could find one that was accurate.

The trailer harness is actually two separate weather pack connectors. The guys at the dealership couldn't find us any kind of pigtails to plug directly into them either. They are odd too, as one connector has two wires for LT and two wires for RT.
 
Tonimus

Tonimus

Full Access Member
Joined
Sep 17, 2024
Posts
97
Reaction score
168
Location
Phoenix
First Name
Tony
Truck Year
1978
Truck Model
C10 Suburban
Engine Size
454
The +5V should be the actual brake control circuit. The red brake wire should be for the lights. The +5V should be pulsed and looking for a load when nothing is connected.
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
Tonimus said:
The +5V should be the actual brake control circuit. The red brake wire should be for the lights. The +5V should be pulsed and looking for a load when nothing is connected.
Click to expand...

The one I have labeled as "brake 12+" is for the trailer brakes. It only gets voltage when the controller sends voltage. The brake lights are shared with the turn signal function (single lights). The 5v wire is just a constant. It's always 5v, no matter what the conditions are. The 5v wire is part of the "body" harness only. The brake 12v supply is in the "body" harness as well as the "trailer" harness.
 
Last edited:
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
Oh yeah, and with the "brake 12+" wired to the trailer brakes, it does operate the trailer brakes properly. The indicator just does not light up on the controller and setting the gain is a guessing game because of it. The controller also can't be properly leveled because the indicator is needed to do so.
 
Tonimus

Tonimus

Full Access Member
Joined
Sep 17, 2024
Posts
97
Reaction score
168
Location
Phoenix
First Name
Tony
Truck Year
1978
Truck Model
C10 Suburban
Engine Size
454
Maybe this is the answer?

You must be registered for see images attach
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
Brake controllers are very simple to wire. They require a 12V+, a ground, a signal from the switched side of the brake light switch, and then an output wire to actuate the trailer brakes.
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
Tonimus said:
Maybe this is the answer?

You must be registered for see images attach
Click to expand...

There's no standard color for the wires really. Function is the most important part.

You must be registered for see images attach


I've wired many many trailers and plugs. There's never been a 5v wire before.
 
bucket

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
30,620
Reaction score
28,869
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
One other thing, the "7-way heavy duty" plug with the red brake wire is used on stuff like road tractors and farm tractors that don't use a trailer brake controller. Road tractors have separate brake lights that function independently from the turn signals.
 
Frankenchevy

Frankenchevy

Proverbs 16:18
Joined
Jan 3, 2018
Posts
6,098
Reaction score
7,806
Location
USA
First Name
Jeremy
Truck Year
Square
Truck Model
CUCV
Engine Size
Small
5v is almost always sensor or diagnostic related. I wonder if it’s for abs or stability. I’ve seen some trailers with ABS related features, but no idea how they work.
 
You must log in or register to reply here.

Similar threads

K
Trailer Wiring Adapter
Replies
9
Views
2K
Broken85
B
RanchWelder
Electrical dash wiring / wire color issues.
Replies
6
Views
2K
RanchWelder
RanchWelder
T
1977 C30 Fuel Gauge wiring
Replies
8
Views
4K
Mark80K15
Mark80K15
DutchSuburban79
Indicator issues '79 C20 Suburban.
Replies
4
Views
489
DutchSuburban79
DutchSuburban79
TommyGuns
Headlights, Radio, and Wipers work without ACC power
Replies
3
Views
546
TommyGuns
TommyGuns

Latest posts

Forum statistics

Threads
44,416
Posts
957,115
Members
36,752
Latest member
78 squarebody
Top