CDL · Combination Vehicles · 6 min read
Checking the Complete Connected Rig
- Identify the critical coupling, landing gear, and connection components required during a combination vehicle walk-around inspection.
- Explain the step-by-step procedures to verify airflow through the trailer lines and test the tractor protection valve.
- Demonstrate how to safely test both the trailer emergency brakes and trailer service brakes before entering traffic.
The Combination Walk-Around Inspection
Inspecting a combination vehicle uses the seven-step inspection procedure described in Section 2 of the commercial driver manual. While the foundational process is the same as inspecting a straight truck, a combination vehicle requires inspecting more items than a single vehicle. Drivers must inspect additional tires, wheels, lights, and reflectors spread across the tractor and all connected trailers. These checks ensure that every vehicle in the combination is roadworthy before hitting the highway.
Walk-around combination vehicle checks are performed in addition to general walk-around inspection checks. You cannot simply inspect the tractor and assume the trailer is safe, or glance at the coupling without looking closely at critical connection points. Taking the time to check the complete rig prevents breakdowns and hazardous disconnects on the road.
Look closely at the connection lines between the tractor and trailer. The electrical cord to the trailer must be firmly plugged in and secured. Air lines must be properly connected to the trailer glad hands with no air leaks. The electrical cord must be firmly plugged in and secured; the air lines must be properly connected, secured, and have enough slack for turns. Check that all air and electric lines connected to the trailer are completely free from damage.
Common mistakes
- Failing to close the rear shut-off valves after listening for escaping air during the airflow check, which leaves an open air leak at the back of the train.
- Assuming the fifth wheel is properly locked without verifying that the locking jaws are closed around the shank of the kingpin rather than around the head.
- Using the trailer hand control (trolley valve) for regular stopping on the road instead of using the foot pedal, which operates the brakes on all wheels.
- Forgetting to check for tractor frame and cab clearance after adjusting a sliding fifth wheel, risking vehicle damage during tight turns.
Key points
- Combination vehicle walk-around inspections require checking extra wheels, tires, lights, reflectors, and coupling connections beyond standard straight-truck checks.
- The fifth wheel must have no space between plates, locking jaws closed around the kingpin shank, release arm seated, safety latch engaged, and slide pins locked.
- Airflow must be verified at the rear of the last trailer by listening for escaping air from both the emergency and service line shut-off valves.
- If air does not reach the back of the rig, verify that all intermediate shut-off valves on trailers and dollies are in the open position.
- With the engine off, the trailer air supply control should pop out as pressure is reduced to the manufacturer-specified range, usually 20 to 45 psi.
- After charging the system and checking that the trailer rolls freely, stop, pull out the trailer air supply control, and pull gently to check the emergency brakes; check the service brakes at normal air pressure while moving forward slowly, using the hand control if equipped.
- Always operate brakes with the foot pedal during normal driving so that service brakes apply to all wheels across the combination.
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