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Question

Difficulty: MediumMountain Driving and Steep Downgrades

You are operating a loaded combination vehicle approaching the crest of a steep mountain downgrade. Which of the following procedures must be followed regarding transmission gear selection before beginning the descent?

  1. A
    Place the transmission into neutral and maintain control of your road speed by applying steady, light pressure to the service brakes.
  2. Shift into a low gear that allows engine braking to hold the vehicle at a safe speed before starting down the grade.Answer
  3. C
    Maintain standard cruising gear during the crest and downshift only after vehicle speed exceeds the posted safe downgrade speed.
  4. D
    Keep the transmission in top gear while activating maximum auxiliary retarder power on wet or icy road surfaces.

Answer

Shift into a low gear that allows engine braking to hold the vehicle at a safe speed before starting down the grade.
Gravity causes a commercial motor vehicle to accelerate rapidly on steep downgrades. The primary braking force must come from engine compression braking, which requires choosing and shifting into the correct low gear before starting down the grade. Once downhill descent has begun, gravity forces make gear synchronization extremely difficult, often leaving the vehicle stuck in neutral without any engine braking.

Step-by-Step Solution

1
Evaluate the downgrade factors including vehicle gross weight, steepness of grade, road condition, and weather.
Determine the safe maximum speed and the appropriate gear needed to hold that speed using engine braking.
Engine braking provides the primary speed control on steep hills, reducing reliance on friction service brakes.
2
Complete the downshift into the selected low gear before the vehicle crests the summit and begins descending.
The transmission is locked into the proper gear with the engine providing retarding force.
Once momentum builds on a downgrade, matching road speed and engine RPM to complete a downshift becomes nearly impossible.
3
Descend utilizing engine braking supplemented only by intermittent snub braking as needed.
The vehicle descends under complete control without overheating the foundation brakes.
Proper gear selection prevents mechanical brake fade and loss of control.

Key Concept

Pre-descent gear selection and the critical role of engine compression braking during steep mountain driving.
Estimated Time:1m 0s
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Topics

Mountain Driving and Steep Downgrades

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