What are the three lines on a tractor-trailer?

In Figure 42, the trailer has been coupled to the tractor and the service and supply lines of the units have been coupled by using gladhands.

The trailer has a reservoir installed. This tank provides a volume of air near the trailer chambers for normal or emergency braking. The tank is equipped with a draincock.

A relay emergency valve is mounted on the trailer reservoir. This valve can also be mounted directly on the trailer frame near the brake chambers. The relay emergency valve serves three main functions in the system:

1. The relay part of the valve relays air from the trailer reservoir to the trailer-brake chambers during a brake application. This part of the valve operates like the relay valve previously discussed. It also provides a quick release of the trailer brakes.
2. The emergency part of the valve directs trailer reservoir pressure to the trailer brakes causing an emergency application sometimes referred to as dynamiting. This action occurs automatically in the event of a ruptured or parted supply line between tractor and trailer, or loss of air from the main reservoir system. You may operate the cab-mounted trailer-supply valve to cause an emergency application of the trailer brakes.
3. The relay emergency valve has a one-way check valve that stops air in the reservoir from going back to the source of the supply. The driver has opened the trailer-supply valve to allow main-reservoir air pressure to be directed through the tractor protection valve to the trailer. Air pressure passes through the relay emergency valve to the trailer reservoir. Pressure builds up in the trailer reservoir to the same pressure as the main reser voirs on the tractor. This is known as charging the trailer system. The trailer-supply valve remains in the open position when pressure has built up to between 20 and 60 psi (138 and 413 kPa), depending on the make.
   
   Drivers can check the operation of the relay emergency valve by closing the supply valve on the tractor or by disconnecting the supply line between the tractor and trailer with the supply valve in the open position.

Figure 42. Typical tractor and trailer charged with air
Click for larger image.

Estimated reading time: 5 minutes

Safely and efficiently coupling and uncoupling semi-trailers is a crucial element of working as a truck driver.

Whether you’re a new driver or have been driving for years, it’s always good to review the essential steps in the process. Keep reading for our all-encompassing guide on how to couple and uncouple a tractor-trailer.

How to couple a semi-trailer

The key to coupling a semi-trailer is being safe at all times and following the steps of the process in the right order. The easiest way to remember the process is to A.I.M.:

• Align.
• Inspect.
• Make a secure couple.

Step 1: Align.

1. Back up until the tractor’s rear drive wheels are just under the nose of the trailer and the fifth wheel is not touching the trailer.
2. Set the brakes, put the truck in neutral, turn off the engine, remove the key from the ignition and safely exit the truck.
3. Check the height of the trailer. Make sure the front edge of the trailer will contact the lower half of the tilted fifth wheel’s ramp area. Adjust the trailer’s height as needed.
4. Make sure the trailer’s kingpin is in line with the center of the fifth-wheel opening. It must be less than two inches to the left or right of the center. Make sure there are no cracks or damage to the kingpin or trailer apron.
5. Ensure the fifth-wheel opening is free of obstructions and the jaw is completely open.

Step 2: Inspect.

1. Starting on the driver’s side of the truck, make sure:

• The fifth wheel is securely mounted with no cracks or broken welds.
• The pivot pin and cotter key are secure.
• The slider pins are locked in place.
• The slider stop blocks are secure.

2. Pull the handle all the way out and ensure it is:

• Not bent.
• The rear of the fifth wheel is fully tilted down.
• The plate is well-greased.

3. Check the landing gear for:

• Cracks.
• Bends.
• Broken parts.

4. Walk around the entire trailer and ensure:

• The tandem release handle is pushed all the way in and down.
• The slider locks are engaged.
• The brake shoes are tight against the drums.
• There are no obstacles above, below or around the trailer that would prevent it from coupling.

5. Inspect the fifth wheel from the passenger side and:

• Ensure it is securely mounted with no cracks or broken welds.
• Check the pivot pin and cotter key.
• Look at the slider pins and slider stop blocks.
• Make sure everything is locked in place and damage-free.

Step 3: Make a secure couple.

1. Slowly back up until the fifth wheel picks up the nose of the trailer. Stop before the fifth wheel locks. For heavier loads, wait at least 60 seconds for tractor suspension to adjust.
2. Once the trailer suspension adjusts, back up gently until the fifth wheel locks. Listen for a click or wait until you feel that the tractor will not back any further.
3. After it locks, gently pull the tractor forward until you feel it tug on the kingpin without moving the trailer.
4. Do this twice to ensure the kingpin is locked into the fifth wheel.
5. Put the truck in neutral, set the brake, turn off the engine, take the key and exit the truck.
6. Use a flashlight to check your connection. Make sure there is no gap between the trailer and the fifth wheel.
7. Push the release handle and ensure it is properly positioned. Check to make sure the locking bar of the fifth-wheel jaw is fully across the kingpin and the lower flange of the kingpin is visible under the locking bar.
8. Connect the air and electrical lines. There are three lines on the tractor – red, green and blue:

• Grab the blue line by the glad hand and pull it down to release it from the coupler on the tractor.
• Place the blue line’s glad hand at a 90° angle with the trailer’s blue coupler.
• Slide the glad hand’s groove into the coupler to lock the line into place.
• Grab the green line by the black plug and pull up to release it from the tractor’s socket.
• Find the round, silver, flap-covered socket next to the blue-line connection on the trailer, lift the flap and push the green line’s plug into the socket.
• Connect the red line’s glad hand to the trailer’s red coupler using the same steps as the blue line.

9. Crank up the landing gear and secure the handle.