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Combination Vehicles Free CDL Practice Tests
Page 6

Prepare For The Combination Vehicles Portion Of Your CDL Written Exams

Combination Vehicles Questions

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When uncoupling, the tractor should:
  • Be pulled out at a 45 degree angle
  • Be at a 90 degree angle
  • Be lined up however it is convenient
  • Be lined up with the trailer
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From The CDL Manual

STEP 1: Position Rig

Make sure surface of parking area can support weight of trailer

Have tractor lined up with the trailer. (Pulling out at an angle can damage landing gear.)

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To help release the fifth wheel locking lever:
  • Back up quickly and hit the brakes
  • First, lower the landing gear
  • Back up gently to ease the pressure on the locking jaws
  • Set parking brake and pull forward firmly
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From The CDL Manual

STEP 2: Ease Pressure on Locking Jaws

  • Shut off trailer air supply to lock trailer brakes.
  • Ease pressure on fifth wheel locking jaws by backing up gently. (This will help you release the fifth wheel locking lever.)
  • Put on parking brakes while tractor is pushing against the kingpin. (This will hold rig with pressure off the locking jaws.)
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Before coupling or uncoupling, the trailer wheels should always be:
  • Flattened
  • Raised off the ground
  • Chocked
  • Checked
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From The CDL Manual

STEP 3: Chock Trailer Wheels

Chock the trailer wheels — especially those trailers manufactured before 1975 — if the trailer does not have spring brakes or if you are not sure. (The air could leak out of the trailer air tank, releasing its emergency brakes. Without chocks, the trailer could move.)

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To make coupling and uncoupling a loaded trailer easier:
  • Turn crank in low gear a few extra turns to lift trailer off of the fifth wheel
  • Turn crank in high gear a few extra turns
  • Just lower the landing gear and pull out
  • Turn crank in low gear a few extra turns to lift some weight off the tractor
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From The CDL Manual

STEP 4: Lower the Landing Gear

If trailer is empty, lower the landing gear until it makes firm contact with the ground.

If trailer is loaded, after the landing gear makes firm contact with the ground, turn crank in low gear a few extra turns. This will lift some weight off the tractor. (Do not lift trailer off the fifth wheel.) This will make it easier to unlatch the fifth wheel and easier to couple next time.

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To prevent moisture from entering the electrical cable when disconnected:
  • Leave it hanging on the ground
  • Tie it in a double knot
  • Hang it with the plug down
  • Wrap it in plastic wrap
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From The CDL Manual

STEP 5: Disconnect Air Lines and Electrical Cable

  • Disconnect air lines from trailer. Connect air line glad hands to dummy couplers at back of cab or couple them together.
  • Hang electrical cable with plug down to prevent moisture from entering it.
  • Make sure lines are supported so they will not be damaged while driving the tractor.
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In case of landing gear failure while pulling out from under it, you should:
  • First back up quickly
  • Pull completely free of the trailer
  • Stop with tractor frame under the trailer
  • Use a spotter to tell you if there are problems
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From The CDL Manual

STEP 7: Pull Tractor Partially Clear of Trailer

Pull tractor forward until fifth wheel comes out from under the trailer.

Stop with tractor frame under trailer (prevents trailer from falling to ground if landing gear should collapse or sink).

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All of these are part of the final 3 steps of uncoupling the trailer except:
  • Secure tractor
  • Inspect trailer supports
  • Chock trailer wheels
  • Pull tractor clear of trailer
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From The CDL Manual

STEP 3: Chock Trailer Wheels

STEP 8: Secure Tractor

STEP 9: Inspect Trailer Supports

STEP 10: Pull Tractor Clear of Trailer

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When unlocking the fifth wheel in Step 6, all of the following apply except:
  • Pull tractor forward until fifth wheel comes out from under the trailer
  • Pull the release handle to “open” position
  • All of these are part of that process
  • Keep legs and feet clear of the rear tractor wheels to avoid serious injury in case the vehicle moves
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From The CDL Manual

STEP 6: Unlock Fifth Wheel

  • Raise release handle lock.
  • Pull the release handle to “open” position.
  • Keep legs and feet clear of the rear tractor wheels to avoid serious injury in case the vehicle moves.
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The locking jaws should be around:
  • The kingpin and all the lines
  • The head of the kingpin
  • The kingpin and the electrical line
  • The shank of the kingpin
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From The CDL Manual

Securely mounted to frame

  • No missing, damaged parts
  • Enough grease
  • No visible space between upper and lower fifth wheel
  • Locking jaws around the shank, not the head of kingpin
  • Release arm properly seated and safety latch/lock engaged
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When inspecting the coupling area, all of the following are required except:
  • All lines free from damage
  • Air lines properly secured with enough slack for turns
  • Electrical cord unplugged and swinging freely
  • Electrical cord firmly plugged in and secured
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From The CDL Manual

Air and electric lines to trailer

  • Electrical cord firmly plugged in and secured
  • Air lines properly connected to glad hands, no air leaks, properly secured with enough slack for turns
  • All lines free from damage
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A specific item to check on a sliding fifth wheel is:
  • It is not too far forward
  • Check all of things on a sliding fifth wheel
  • It is properly gresaed
  • Locking pins are all in place
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From The CDL Manual

Sliding fifth wheel

  • Slide not damaged or parts missing
  • Properly greased
  • All locking pins present and locked in place
  • If air powered, no air leaks
  • Check that fifth wheel is not so far forward that tractor frame will hit landing gear or cab will hit the trailer during turns
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When you open the emergency shut-off valve at the back of the last trailer after supplying air to trailer(s):
  • You should hear air escaping, indicating that the system is charged
  • The trailer should jump forward 6 inches
  • The low air warning should immediately sound
  • You should not hear any air escaping
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From The CDL Manual

Check that air flows to all trailers –

Use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red “trailer air supply” knob. This will supply air to the emergency (supply) lines. Use the trailer hand brake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer.

You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer hand brake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the other trailer(s) and dolly(s) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

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When driving a combination vehicle with multiple trailers or dollies, all air shut-off valves except the last one at the back must be:
  • Open
  • Closed
  • Half-closed
  • Half-open
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From The CDL Manual

Check that air flows to all trailers –

Use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for air pressure to reach normal, then push in the red “trailer air supply” knob. This will supply air to the emergency (supply) lines. Use the trailer hand brake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer.

You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer hand brake or the service brake pedal is on), then close the valve. If you do NOT hear air escaping from both lines, check that the shut-off valves on the other trailer(s) and dolly(s) are in the OPEN position. You MUST have air all the way to the back for all the brakes to work.

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To test the tractor protection valve, pump the brakes with the engine off until the trailer air supply knob pops out, usually at:
  • Between 40 to 90 psi
  • 120 psi
  • Between 20 to 45 psi
  • Between 60 to 80 psi
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From The CDL Manual

Test tractor protection valve –

Charge the trailer air brake system. (i.e., build up normal air pressure and push the “air supply” knob in.) Shut off the engine. Step on and off the brake pedal several times to reduce the air pressure in the tanks.

The trailer air supply control (also called the tractor protection valve control) should pop out or go from “normal” to “emergency” position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi).

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The trailer service brakes should be tested with the hand control, and controlled during normal operation by:
  • The foot pedal
  • The trolley valve
  • The parking brake knob
  • The hand control
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From The CDL Manual

Test trailer service brakes –

Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped. You should feel the brakes come on.

This tells you the trailer brakes are connected and working. (The trailer brakes should be tested with the hand valve but controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels.)

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Trailers will have an ABS malfunction lamp:
  • Om the back
  • On the left side
  • On the right side
  • On the front
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From The CDL Manual

Trailers will have yellow ABS malfunction lamps on the left side, either on the front or rear corner. (See Figure 6.7.) Dollies manufactured on or after March 1, 1998, are required to have a lamp on the left side.

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