Inspection of Combination Vehicles
What this domain covers
The inspection domain tests a systematic pre-trip of the combination: coupling system, air lines, electrical, tires, wheels, lights, frame, cargo, and brake tests. The point is to catch defects that affect both tractor and trailer, not just the power unit.
Core rules
- Inspect the fifth wheel for cracks, missing jaws, and a secure mounting; check the kingpin and safety latch.
- Check air lines for chafing, cuts, and secure glad-hand connections. Red is emergency/supply; blue is service. Check the electrical cord and receptacle.
- Test the trailer brakes separately from the tractor brakes. With the engine off and system charged, a combination vehicle should lose no more than 4 psi in one minute with brakes released, and no more than 4 psi in one minute with the service brake applied.
- Confirm the low-air warning before 60 psi and that spring brakes apply in the 20–45 psi range.
- Check tires for tread depth, sidewall damage, and matching sizes; check wheels for cracks and loose lug nuts.
Worked example
A fully charged combination gauge reads 100 psi. With the engine off and brakes released, it falls to 95 psi in one minute. That is a 5 psi drop, above the 4 psi limit for a combination vehicle, so the vehicle fails the leak test. Find the leak before driving; do not just watch the gauge.
Traps that cost marks
Checking only tractor brakes; accepting a trailer ABS lamp that stays on; missing a cracked fifth wheel; forgetting to check landing gear and mud flaps; and doing the leak test with the engine running or the system not fully charged.