South Carolina Combination Vehicle By Erin Monahan · March 22, 2023 · 0 comments The Combination Vehicles CDL practice test contains 20 questions you can expect to find on the South Carolina CDL exam. You will need to know how to operate a combination vehicle which includes tractor-trailer, straight truck, doubles and triples. This includes coupling and uncoupling, connecting air lines, emergency braking and more. Take the test to prepare for the South Carolina Combination Vehicles portion of the CDL test. START 1. You are hooking a tractor to a semi-trailer and have backed up but are not under it. What should you do before backing under the trailer? Hook up the electrical service cable Hook up the emergency and service air lines Connect the ground cable Nothing. Back up and secure the fifth wheel to the trailer 2. After pushing in the trailer supply valve, you should not move the tractor until the whole air system is Charging At normal pressure Bled down to half the maximum pressure Between 50 and 60 psi 3. There are two things that a driver can do to prevent a rollover. They are (1) Keep the cargo as close to the ground as possible, and (2) Make sure that the brakes are adjusted properly Keep both hands firmly on the steering wheel Drive slowly around turns Keep the fifth wheel free-play loose 4. With the engine off and the brakes released, a combination vehicle air brake system should not leak more than how many psi in one minute? 1 2 3 4 5. The fifth wheel locking lever is not locked after the jaws close around the kingpin. This means that The trailer will not swing on the fifth wheel You cannot set the fifth wheel for proper weight distribution The hand valve is released and you may drive away The coupling is not correct and should be corrected before driving the coupled unit 6. A driver crosses the air lines when hooking up to an old trailer. What will happen? The hand valve will apply the tractor brakes instead of the trailer brakes The brake pedal will work the trailer spring brakes instead of the trailer brakes If the trailer has no spring brakes, you could drive away but you wouldn’t have trailer brakes The brake lights will not come on when the brake pedal is pressed 7. After coupling a semi-trailer, you should crank up the front trailer supports (dollies) how? Raised ½ way with the crank handle secured Raised ¾ way with the crank handle removed Fully raised with the crank handle secured Three turns off the top with the crank handle secured in its bracket 8. Air brake equipped trailers made before 1975 Are not required to have spring brakes Are easier to stop than newer trailers because they are heavier Usually need a glad hand converter Cannot be operated on interstate highways 9. The hand valve should be used to park a combination vehicle when? When you park at loading docks When you are parking for less than one hour When parking on a step grade Never 10. Air lines on a combination vehicle are often colored to keep from getting them mixed up. The emergency line is ______. Red Black Blue Orange 11. How do you supply air to the air tank on the trailer? You supply air to the trailer tank by Pushing in the trailer air supply knob/valve Pulling out the trailer air supply knob/valve Connecting the emergency line glad hand Applying the trolley valve 12. In normal driving, some drivers use the trolley (hand) valve before the brake pedal. Which of these statements is true? It should never be done It results in less skidding than using the brake pedal alone It lets the driver steer with both hands It is the best way to brake in a straight line 13. You are hooking up a tractor and semi-trailer. You have connected both air lines. Before backing under the trailer you should Pull forward to test glad hand connections Supply air to the trailer system, then pull out the air supply knob to lock the trailer brakes Make sure that the trailer brakes are off Blow the horn twice to alert others 14. When backing up under a trailer, you should line up About 15 degrees off the line of the trailer The right mirror along the right edge of the trailer Directly in front of the trailer To the far left of the trailer 15. _____ are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer. Clamps Shut-off valves Hand valves Glad hands 16. After hooking up, you should check the fifth wheel connection by Driving away at 20 mph and pulling down the trailer hand valve Backing up with the trailer brakes released Pulling the tractor ahead sharply to release the trailer brakes Pulling the tractor ahead gently with the trailer brakes locked 17. How often should you drain your vehicle’s trailer air tanks. Daily Weekly Monthly Yearly 18. The emergency air line supplies air to the trailer air tanks and Supplies air to the cab Controls the emergency brakes on combination vehicles Are coded with the color green to keep them from getting mixed up with other lines All of the above 19. How much space should be between the upper and the lower fifth wheel plates? About 1 inch About ½ inch About ¼ inch None 20. When not pulling a trailer, why is it a good idea to lock the glad hands together or to a dummy coupler? It keeps air from escaping The brake circuit becomes a secondary air tank It keeps dirt and water out of the lines All of the above Great job you’ve finished the Texas Combination Vehicle CDL Practice test! Please enter your name and email. Your results will be presented on the next page. Feel free to retake the test as many times as you need to. Worried about passing your CLP exam? 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