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6.4 Inspecting a Combination Vehicle

Use the 7-Step Inspection Method described in Section 2 to inspect your combination vehicle. There are more things to inspect on a combination vehicle than on a single vehicle, which are discussed below.

Walkaround Inspection

Perform the following checks in addition to those listed in Section 2: Step 5: Do Walkaround Inspection.

Coupling System Areas

Fifth wheel (lower):
  • Securely mounted to frame.
  • No missing, damaged parts.
  • Enough grease.
  • No visible space between upper and lower fifth wheel.
  • Locking jaws around the shank, not the head of kingpin.
  • Release arm properly seated and safety latch/lock engage.
Fifth wheel (upper):
  • Glide plate securely mounted to trailer frame.
  • Kingpin not damage.
Air and electric lines to trailer:
  • Electrical cord firmly plugged in and secured.
  • Air lines properly connected to glad hands, no air leaks, properly secured with enough slack for turns.
  • All lines free from damage.
Sliding fifth wheel:
  • Slide not damaged or parts missing.
  • Properly greased.
  • All locking pins present and locked in place.
  • If air powered, no air leaks.
  • Check that fifth wheel is not so far forward that tractor frame will hit landing gear or cab will hit the trailer during turn.
Landing gear:
  • Fully raised, no missing parts, not bent or otherwise damaged.
  • Crank handle in place and secured.
  • If power operated, no air or hydraulic leaks.

Perform these checks in addition to those in Section 5.3: Inspecting Air Brake Systems.

Combination Vehicle Brake Check

The following section explains how to check air brakes on combination vehicles. Check the brakes on a double or triple trailer as you would any combination vehicle.

Check that air flows to all trailers:

Use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red “trailer air supply” knob. This will supply air to the emergency (supply) lines. Use the trailer hand brake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer hand brake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut off valves on the other trailer(s) and dolly(s) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

Test tractor protection valve:

Charge the trailer air brake system. (i.e., build up normal air pressure and push the “air supply” knob in.) Shut off the engine. Step on and off the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out or go from “normal” to “emergency” position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).

If the tractor protection valve does not work correctly, an air hose or trailer brake leak could drain all the air from the tractor. This would cause the emergency brakes to come on, with possible loss of control.

Test trailer emergency brakes:

Charge the trailer air brake system and check that the trailer rolls freely. Then stop and pull out the trailer air supply control (also called “tractor protection valve control” or “trailer emergency valve”) or place it in the “emergency” position. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on.

Test trailer service brakes:

Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped. You should feel the brakes come on. This tells you the trailer brakes are connected and working. (The trailer brakes should be tested with the hand valve but controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels.)

Test Your Knowledge

  • Which shut-off valves should be open and which ones should be closed?
  • How can you test that air flows to all trailers?
  • How can you test the tractor protection valve?
  • How can you test the trailer emergency brakes?
  • How can you test the trailer service brakes?
  • What is the minimum tread depth for front tires?

Study section 6.4 if you can't answer all of these questions.

The following info only applies to trucks carrying multiple trailers.
Remember: The tractor protection valve (red 8-sided knob in the truck) should pop out between about 20 and 45 psi.

The tractor protection valve (red 8-sided knob in the truck) also controls the trailer emergency brakes. It serves two main purposes:

  • Activates the trailer emergency brakes.
  • Keeps any remaining air in the tractor air tanks to prevent the tractor emergency brakes from engaging. Once air pressure drops to about 20psi, the tractor spring brakes (emergency brakes) will then engage and the yellow diamond shaped knob (also controls the parking brake) will pop out.

Combination Vehicle:

A vehicle with two separate parts - the power unit (tractor) and the trailer. Tractor-trailers are considered combination vehicles.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

OWI:

Operating While Intoxicated

Review Questions - Click On The Picture To Begin...

What is a correct way to test the trailer emergency brakes?
  • While stopped, pull out the yellow parking brake control and pull gently on the trailer with the tractor
  • While idling in the lowest gear possible, pull out the trailer air supply control to see if you come to a safe stop
  • While idling in the lowest gear possible, pull out the yellow parking brake control to see if you come to a safe stop
  • While stopped, pull out the trailer air supply control and pull gently on the trailer with the tractor

Quote From The CDL Manual:

Test trailer emergency brakes: Charge the trailer air brake system and check that the trailer rolls freely. Then stop and pull out the trailer air supply control (also called "tractor protection valve control" or "trailer emergency valve") or place it in the "emergency" position. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on.

Next
The air pressure range when the tractor protection valve will pop out occurs at:
  • Between 100 and 120 psi
  • Between 20 and 45 psi
  • Between 50 and 75 psi
  • Between 5 and 15 psi

Quote From The CDL Manual:

The trailer air supply control (also called the tractor protection valve control) should pop out or go from "normal" to "emergency" position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).

Prev
Next
Which statement about inspecting a combination vehicle is false?
  • Check the the air lines and electrical line to be certain they are properly secured and connected
  • Make sure the fifth wheel locking jaws are around the head of the kingpin and not the shank of the kingpin
  • Be sure the landing gear is fully raised, no missing parts, not bent or otherwise damaged
  • If you have a sliding fifth wheel, you must make sure the fifth wheel is securely locked in place

Quote From The CDL Manual:

Inspecting a Combination Vehicle

  • Locking jaws around the shank, not the head of kingpin.
  • Landing gear is fully raised, no missing parts, not bent or otherwise damaged.
  • Air lines properly connected to glad hands, no air leaks, properly secured with enough slack for turns.
  • All sliding fifth wheel locking pins present and locked in place
Prev
Next
What is the best way to test the Trailer Service Brakes?
  • Release all brakes except the trailer service brakes using the trailer hand control valve (trolley valve) then walk back to the rear of the trailer and visually check to be sure the brakes are engaged
  • With the engine off, step on and off the brake pedal several times to reduce the air pressure in the tanks and make sure the tractor protection valve pops out
  • Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped
  • Pull out the trailer air supply control and pull gently on the trailer with the tractor to make sure the brakes work

Quote From The CDL Manual:

The trailer service brakes: Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped. You should feel the brakes come on. This tells you the trailer brakes are connected and working. (The trailer brakes should be tested with the hand valve but controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels.)

TruckingTruth's Advice:

Many companies are no longer installing trailer hand control valves which is very unfortunate as this is really the only way to be certain the trailer service brakes are functioning properly. If your truck does not come equipped with a trailer hand control valve, test your brakes before you start driving on a major roadway. If you notice the truck struggling to stop, the problem most likely is with your trailer brakes and needs to be inspected.

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Next
How can you test the Tractor Protection Valve?
  • With the engine off, engage and release the parking brake several times until the trailer air supply control pops out
  • With the engine off, step on and off the brake pedal several times until the trailer air supply control pops out
  • With the engine on, engage and release the parking brake several times until the trailer air supply control pops out
  • With the engine on, step on and off the brake pedal several times until the trailer air supply control pops out

Quote From The CDL Manual:

Test tractor protection valve: Charge the trailer air brake system. (i.e., build up normal air pressure and push the "air supply" knob in.) Shut off the engine. Step on and off the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out or go from "normal" to "emergency" position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).

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