Idaho CDL Combination Practice Test (ID) 2

5 out of 5 (43 votes)
80% Passing score
20 Questions
4 Mistakes allowed
Prepare for your Idaho Commercial Driver's License (CDL) with our comprehensive Idaho CDL combination practice test. This test includes 20 multiple-choice questions covering crucial topics like inspection procedures, safe driving practices, air brake systems, and more. To pass, you'll need to correctly answer at least 16 questions, ensuring you're well-prepared for the real permit test. One of the key areas our combination vehicle test focuses on is the risk of jackknifing. This dangerous situation occurs when the trailer swings out of alignment with the tractor, often due to sudden braking or sharp turns. Our test helps you understand how to avoid jackknifing through smooth braking and careful handling, crucial for safe driving on Idaho's diverse roadways. From the rugged terrain of the Sawtooth National Forest to the snowy conditions in cities like Pocatello and Coeur d'Alene, Idaho offers unique driving challenges. The Idaho combination CDL test integrates these real-world scenarios to provide a truly authentic practice experience. While this DMV practice test doesn't replace a state-certified drivers ed or ELDT course, it serves as a valuable complement, helping you grasp essential concepts and build confidence. Upon completing the practice driving test, you can review your answers and explore hints and explanations for any questions you missed. This review process is crucial for reinforcing your understanding and ensuring you're fully prepared for the actual permit test. Although the DMV restricts the publication of actual permit test questions and answers, our Idaho practice drivers test closely mirrors the real DMV permit test experience. The questions are designed with similar phrasing and cover identical topics, providing an effective way to practice and improve your knowledge. Whether you're navigating the bustling streets of Boise or the winding mountain roads, our combination CDL practice test will help you feel prepared and confident.
FREE CDL Practice Test App
Pass your Idaho CDL Combination Vehicles test with the NextDoorDriving app. Real questions and answers—100% free and ad-free.
Download on the App Store

Sample Idaho Combination Test Questions

Question 1

What are the two main functions of the trailer emergency air line?

  1. A Operate the trailer lights and parking-brake warning
  2. B Control only the tractor’s emergency brakes
  3. C Supply the trailer air tanks and control the trailer emergency brakes
  4. D Cool the tractor cab and trailer brake drums

Correct answer: C – Supply the trailer air tanks and control the trailer emergency brakes

The emergency or supply line carries air used to charge the trailer air tanks. It also controls the trailer’s emergency brakes. If pressure is lost because of a breakaway, damaged line, or other major leak, the trailer emergency brakes apply and the tractor protection valve closes.

Source: Idaho Commercial Driver’s License Manual — § 6.2.4, “Trailer Air Lines,” “Emergency Air Line”

Question 2

What is the earliest reliable indication that a trailer has begun to skid sideways?

  1. A Tire noise heard through the cab
  2. B A vibration in the driver’s seat
  3. C Sideways trailer movement visible in the mirrors
  4. D A strong pull through the steering wheel

Correct answer: C – Sideways trailer movement visible in the mirrors

The Idaho CDL manual identifies the mirrors as the earliest and best way to recognize a trailer skid. Locked trailer wheels can cause the trailer to swing toward a jackknife. During hard braking, check the mirrors to confirm that the trailer remains properly aligned behind the tractor.

Source: Idaho Commercial Driver’s License Manual — § 6.1.5, “Prevent Trailer Skids”

Question 3

What adjustment should be made if the trailer is too high for proper fifth-wheel coupling?

  1. A Raise the trailer farther
  2. B Increase backing speed
  3. C Raise the tractor suspension fully
  4. D Lower the trailer

Correct answer: D – Lower the trailer

Before coupling, the trailer should be low enough that the tractor raises it slightly while backing under it. A trailer that is too high may not couple correctly because the kingpin may fail to enter and lock within the fifth-wheel jaws. Lower the trailer to the proper height before continuing.

Source: Idaho Commercial Driver’s License Manual — § 6.4.1, “Coupling Tractor-Semitrailers,” Step 6, “Check Trailer Height”

Question 4

Which trailer air line carries pressure controlled by the foot brake?

  1. A The emergency or supply line
  2. B The service or control line
  3. C The shut-off valve drain line
  4. D The tractor-protection exhaust line

Correct answer: B – The service or control line

The service line, also called the control or signal line, carries air pressure controlled by the foot brake or trailer hand valve. That pressure signals relay valves to apply the trailer brakes. The emergency or supply line has different functions, including charging the trailer tanks.

Source: Idaho Commercial Driver’s License Manual — § 6.2.4, “Trailer Air Lines,” “Service Air Line”

Question 5

How should you initially verify that a trailer is locked securely to the tractor?

  1. A Accelerate rapidly from a stop
  2. B Turn the tractor sharply in both directions
  3. C Gently pull forward while the trailer brakes remain locked
  4. D Check only the trailer position in the mirrors

Correct answer: C – Gently pull forward while the trailer brakes remain locked

Raise the landing gear slightly and gently pull the tractor forward while the trailer brakes remain locked. This checks whether the trailer is secured to the fifth wheel. The connection must then be visually inspected to confirm that the jaws surround the kingpin shank, the locking lever is locked, and the safety latch is in place.

Source: Idaho Commercial Driver’s License Manual — § 6.4.1, “Coupling Tractor-Semitrailers,” Steps 11 and 13

Question 6

How should you verify that air reaches both lines at the rear of the last trailer?

  1. A Open the rear emergency shut-off valve, then the service shut-off valve with service pressure applied, and listen for air from each
  2. B Open both air lines at the rear of the tractor and leave them open
  3. C Drive at road speed and make a hard brake application
  4. D Shut off the engine and drain only the tractor reservoirs

Correct answer: A – Open the rear emergency shut-off valve, then the service shut-off valve with service pressure applied, and listen for air from each

With normal pressure and the trailer air supply on, open the emergency-line valve at the rear of the last trailer and listen for escaping air, then close it. Apply service pressure and repeat with the service-line valve. If air is not heard from both lines, check that upstream trailer and dolly shut-off valves are open.

Source: Idaho Commercial Driver’s License Manual — § 6.5.2, “Combination Vehicle Brake Check,” “Check That Air Flows to All Trailers”

More Idaho CDL Practice Tests
Other CDL Endorsements