Trailers · Air and electric lines
Tractor-trailer air and electric lines: gladhands, hoses and coils
Behind every tractor cab hang three lines: two air lines for the trailer brakes and one electrical cord. Often sold together as a "tri-coil" or line kit, they are covered by different standards and fail in different ways.
The three lines
| Line | Color convention | What it does | Key standards |
|---|---|---|---|
| Service (control) air | Blue | Carries the brake application signal from the tractor to the trailer brakes | SAE J318 (gladhand); SAE J1402 / FMVSS No. 106 (air brake hose) |
| Emergency (supply) air | Red | Supplies air to the trailer and releases the trailer spring brakes; loss of pressure applies them | SAE J318; SAE J1402 / FMVSS No. 106 |
| Electrical | Green is common for ABS cords | Lighting and trailer ABS power through the 7-way | SAE J560 (connector); SAE J1067 (cable) |
In North America the blue service and red emergency coding for air brake couplers is set out in SAE International's Recommended Practice SAE J318, which covers gladhand design, interchangeable dimensions, performance and identification. The electrical side is covered in detail in the SAE J560 pinout guide and the 7-way jumper cable guide.
Gladhands
A gladhand is the quick-disconnect coupler at the end of each air line. Two gladhands interlock face to face and seal on a rubber gland seal, so a driver can couple and uncouple without tools. Points to check:
- Gland seals crack and harden with age and cold. A hissing coupling is usually a seal. They are a routine replacement item.
- Color coding and identification should be intact on both tractor and trailer. Crossed lines are a serious fault.
- Mounting angle and port position on the tractor side should suit how the hoses hang, so the hose is not kinked at the fitting.
- Dust covers or dummy couplings on the tractor keep dirt and water out of the lines when no trailer is attached.
Air hoses and coils
Tractor-to-trailer air lines are made either as rubber air brake hose, commonly built to SAE J1402 and certified by its maker to FMVSS No. 106, or as coiled nylon tubing. Federal Register material on FMVSS No. 106 notes that coiled nylon brake hose meeting the Federal Motor Carrier Safety Regulations at 49 CFR 393.45 is excluded from certain FMVSS No. 106 test requirements; 393.45 references SAE J844 for nylon air brake tubing. For a specific part, rely on the maker's marking and documentation rather than the description on a listing.
Rubber hose hangs more naturally and tolerates heat; coiled nylon retracts like a coiled cord. Either way, the lines need the same care as the electrical cord: support so they do not lie on the deck plate, clearance from exhaust and moving parts, and enough working length for full articulation.
Why the electrical cord is usually the weak link
The three lines live together, but the electrical cord has the most to lose from the same abuse:
- an air hose with a scuffed cover can still hold pressure; a cord with a scuffed jacket lets water in, and the corrosion spreads along the conductors;
- air couplers tolerate some dirt; electrical contacts do not;
- a cord's conductors fatigue at the plug with every articulation, while the outside may look fine.
Line kits that bundle all three with a common wrap are convenient, but make sure each line in the kit still meets its own standard, and that the wrap does not trap water against the cord.
Coupling and uncoupling habits that protect the lines
Most line damage happens during coupling, uncoupling and while the tractor runs without a trailer. These habits cost nothing and protect all three lines:
- Pull on the plug and the gladhand, never on the cord or hose. Pulling on the cable loads the strain relief and the conductors behind it.
- Stow the lines when bobtailing. Lines left hanging whip against the deck plate and fifth wheel. Use the dummy couplings and a stowage point for the electrical plug.
- Close the socket covers on tractor and trailer when nothing is plugged in.
- Check before pulling away. Confirm lines are connected, supported and clear of the fifth wheel and driveline, and that trailer lamps and brakes work.
- Report damage early. A scuffed cord is cheap to replace before it lets water in; a corroded nose box and trailer harness are not.
Line management
- Support. A hanger, spring or tender arm keeps lines off the deck plate and clear of the fifth wheel.
- Separation from heat. Keep all three lines clear of exhaust components.
- Consistent lengths. Lines that are much longer than the others sag and chafe.
- Protection at the tractor bulkhead. Fittings and sockets should be secure and sealed.
Common questions
Which gladhand is red and which is blue?
Under SAE J318 practice in North America, red is the emergency (supply) line and blue is the service (control) line.
What is a tri-coil?
A set of the two air lines and the electrical cord, usually coiled and often sold together. Each part is still a separate component with its own standard.
Can air and electrical lines be wrapped together?
Many kits do this. Make sure the wrap allows water to drain, does not hold the cord against a hot or sharp surface, and does not stop you inspecting each line.
How often should gladhand seals be changed?
Follow your maintenance program. Replace any seal that is cracked, hardened or leaking; they are inexpensive and a leaking coupling wastes air.
What to specify for the electrical line
- SAE J560 plugs and SAE J1067 cable, coiled or straight, with working length matched to the other lines.
- Jacket material and color suited to cold and oil exposure (see cable jacket materials).
- Strain relief, sealing and handle or spring guard requirements.
- Continuity and circuit verification on every cord.
Last reviewed 2026-09-17