CDL Combination Vehicles Test: Everything Covered in 20 Questions

Every Class A CDL driver needs to pass the Combination Vehicles test. It's 20 questions covering coupling, uncoupling, inspecting combination rigs, and understanding why these vehicles behave differently from single-unit trucks. Here's what you need to know.

Why Class A Drivers Must Pass This Test

Combination vehicles — tractor-trailers, doubles, triples — have unique dynamics and systems that a straight-truck driver has never encountered. Incorrect coupling can cause a trailer to separate at highway speed; improper inspection can lead to brake failures. The Combination Vehicles knowledge test (and its associated CDL restrictions) exists because the consequences of mishandling these vehicles are severe.

If you pass the CDL skills test in a combination vehicle, you can drive combinations. If you test in a straight truck (Class B vehicle), your CDL will carry a combination restriction.

The Test: 20 Questions, 80% to Pass

The Combination Vehicles test is 20 questions requiring 16 correct (80%). It can be taken the same day as General Knowledge and Air Brakes at the DMV. Plan for it — don't show up having studied only for General Knowledge.

Coupling Procedure: What the Test Expects You to Know

The correct coupling sequence is a major test topic. Here's the standard procedure:

1. Inspect the fifth wheel — ensure it's greased, properly mounted, and jaws are open
2. Check trailer height — trailer should be slightly above the fifth wheel
3. Secure the trailer — trailer brakes should be applied (emergency line disconnected)
4. Back under the trailer slowly — stop before contact, then ease under
5. Connect the trailer — fifth wheel should lock around the kingpin with an audible "click"
6. Test the coupling — apply tractor brakes, gently pull forward to verify lock
7. Connect air lines — emergency line (red) first, then service line (blue)
8. Connect electrical plug (pigtail)
9. Raise landing gear — fully raise and secure crank
10. Inspect — perform complete pre-trip inspection of coupled combination

Uncoupling Procedure

1. Park on firm, level surface
2. Apply trailer emergency brakes
3. Chock trailer wheels
4. Lower landing gear until firmly on ground
5. Disconnect electrical plug
6. Disconnect air lines — service line (blue) first, then emergency (red)
7. Release fifth wheel locking lever
8. Pull tractor away slowly while watching for clearance

Inspecting the Fifth Wheel

The fifth wheel is the coupling plate on the tractor that accepts the kingpin from the trailer. Proper inspection includes:

• Greased with adequate lubrication (no dry metal-on-metal contact)
• Mounting bolts tight, no cracks in mounting
• No visible cracks in the fifth wheel plate itself
• Jaws fully closed around kingpin when coupled (test by attempting to pull tractor away)
• Safety latch/lock properly engaged
• Slide pins (if sliding fifth wheel) fully engaged and locked

Converter Dollies

A converter dolly connects the lead trailer to the rear trailer in doubles operations. The test may ask:

• How to inspect a converter dolly (check drawbar, pintle hook, safety chains, air connections)
• Where the dolly's air supply comes from (from the lead trailer)
• Proper coupling sequence when attaching converter dolly

Air Line Color Codes

Memorize these — they appear on both the Combination Vehicles and Air Brakes tests:

• Red glad hand = Emergency line (applies trailer brakes if disconnected)
• Blue glad hand = Service line (controls normal brake application)

When coupling, connect emergency (red) first. When uncoupling, disconnect service (blue) first. The logic: you want the trailer brakes applied (red line disconnected = brakes on) during coupling and after uncoupling.

Combination Vehicle Handling

The test also covers how combination vehicles handle differently:

• Off-tracking: The rear trailer wheels follow a tighter arc than the tractor in turns. Swing wide enough to prevent trailer wheels from jumping the curb.
• Jackknifing: The trailer pushes the tractor sideways, usually from hard braking or brake imbalance. Prevent by braking smoothly and maintaining proper brake adjustment.
• Rearward amplification (in doubles/triples): Each subsequent trailer amplifies movement — drive smoothly.

Ready to test your Combination Vehicles knowledge? Practice at greenlightcdl.com/test/ — dedicated Combination Vehicles question sets are included.