Washington DC CDL Combination Test (DC) 2026

5 out of 5 (42 votes)
80% Passing score
20 Questions
4 Mistakes allowed
Prepare yourself for the DC CDL combination practice test, designed to help you ace the combination vehicles endorsement for your Commercial Driver's License. This DMV practice test features an array of multiple-choice questions that cover essential topics such as coupling and uncoupling, pre-trip inspections, driving techniques, backing and maneuvering, and air brake systems. Each DC CDL combination test comprises 20 questions, and you'll need to answer at least 16 correctly to pass. The Washington DC learners permit practice test is designed with several study aids to make your preparation more efficient. If you encounter a challenging question, you can request a hint that will guide you toward the correct answer. Should you answer incorrectly, the DC CDL combination test provides a detailed explanation to help you understand the correct response. These aids ensure that you learn the material thoroughly and quickly. One of the standout features of our DMV permit practice test is the inclusion of real-life driving scenarios. These questions not only prepare you for the written test but also equip you with the knowledge to handle real-world driving situations. By practicing with these scenarios, you'll become a safer, more competent driver, ready to face the challenges of the road with confidence. The questions on the DC CDL combination practice test have been meticulously written and reviewed by professional driving instructors. These experts, with decades of combined teaching experience, have helped thousands of drivers obtain their licenses. Their expertise ensures that the questions are accurate, relevant, and reflective of what you'll encounter on the actual test. Understanding the increased stopping distances required for combination vehicles due to their greater weight is crucial for maintaining safety on the road. Our combination vehicle test covers this and other vital topics to ensure you're well-prepared for the unique challenges presented by these larger, heavier vehicles. Take advantage of the Washington DC DMV knowledge practice test to boost your confidence and knowledge. With comprehensive coverage of all key areas, detailed explanations, and professional insights, you'll be well-prepared to pass your combination CDL test and become a safer driver on the road.
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Sample Washington DC Combination Test Questions

Question 1

Before coupling to a trailer, which fifth-wheel conditions should be verified?

  1. A There are no damaged or missing parts
  2. B The locking jaws are open
  3. C The plate is greased as required
  4. D All of the above

Correct answer: D – All of the above

The fifth wheel must be inspected before the tractor is backed under the trailer. Check for damaged or missing parts and confirm that the mounting is secure. The plate should be properly greased, the jaws should be open, and the safety unlocking handle should be in its automatic lock position.

Source: DC DMV Commercial Driver License Manual — § 6.4.1, “Coupling Tractor-Semitrailers,” Step 1, “Inspect Fifth Wheel”

Question 2

For a standard combination vehicle with the engine stopped and service brakes released, the air-pressure loss must not exceed how many psi per minute?

  1. A 1 psi
  2. B 2 psi
  3. C 3 psi
  4. D 4 psi

Correct answer: C – 3 psi

Federal in-use inspection criteria limit air loss to 3 psi in one minute for a combination vehicle when the engine is stopped and the service brakes are released. This is the released-brake leakage test, not the fully applied-brake test. One additional psi per minute is allowed for each additional towed vehicle.

Source: 49 CFR § 570.57(a)(4) — Air brake system and air-over-hydraulic brake subsystem

Question 3

Before continuing to back a tractor under a trailer, the air system should be:

  1. A Between 60 and 80 psi
  2. B Reduced to half of its maximum pressure
  3. C At normal operating pressure
  4. D Completely empty

Correct answer: C – At normal operating pressure

The District of Columbia manual's coupling sequence requires the driver to connect the air lines, supply air to the trailer, and wait for normal pressure. The driver then checks for crossed lines and verifies that the trailer brakes work. After the trailer brakes are locked, the tractor may be backed under the trailer.

Source: DC DMV Commercial Driver License Manual — § 6.4.1, “Coupling Tractor-Semitrailers,” Steps 8–10

Question 4

If the service air line separates while driving but the emergency line remains connected, what may happen first?

  1. A The tractor's emergency brakes apply immediately
  2. B The combination stops immediately
  3. C The trailer tank immediately empties through the service line
  4. D The problem may not become apparent until the brakes are applied

Correct answer: D – The problem may not become apparent until the brakes are applied

The service line carries air controlled by the foot brake or trailer hand valve. If it develops a major leak while the emergency line stays intact, the trailer emergency brakes may not apply immediately. The leak may first become apparent when the driver applies the brakes, causing air pressure to fall quickly.

Source: DC DMV Commercial Driver License Manual — §§ 6.2.4 and 6.2.8, “Trailer Air Lines”; “Trailer Service, Parking and Emergency Brakes”

Question 5

Why must the fifth-wheel plate be lubricated as required?

  1. A To prevent steering problems caused by friction
  2. B To reduce tread wear on the trailer tires
  3. C To increase the tractor's fuel economy
  4. D To eliminate the need for a coupling inspection

Correct answer: A – To prevent steering problems caused by friction

The tractor and trailer pivot against the fifth-wheel plate when the combination turns. Without proper lubrication, excessive friction can develop between them and cause steering problems. Greasing the plate is part of the fifth-wheel inspection performed before coupling.

Source: DC DMV Commercial Driver License Manual — § 6.4.1, “Coupling Tractor-Semitrailers,” Step 1, “Inspect Fifth Wheel”

Question 6

During the coupling inspection, the fifth-wheel jaws are around the kingpin but the locking lever is not in the locked position. What must you do?

  1. A Release the parking brake and drive away
  2. B Replace the entire fifth wheel immediately
  3. C Correct the coupling before driving
  4. D Adjust the sliding fifth wheel for weight distribution

Correct answer: C – Correct the coupling before driving

A locking lever that is not in the lock position indicates that the coupling is not secure. The District of Columbia manual directs the driver not to operate the coupled unit until the problem has been corrected. The coupling inspection must also confirm that the jaws surround the kingpin shank and that the safety latch is properly positioned.

Source: DC DMV Commercial Driver License Manual — § 6.4.1, “Coupling Tractor-Semitrailers,” Step 13, “Inspect Coupling”

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