CDL General Knowledge TestDriving SafelyMedium
A commercial driver is driving a vehicle equipped with a manual transmission. To prevent engine over-speeding and maintain control on a long, steep downgrade, what is the most effective technique?
- AShift to neutral and coast down the grade.
- BApply the service brakes continuously to control speed.
- CShift to a lower gear before starting the downgrade.
- DPump the service brakes frequently to dissipate heat.
Show answer & explanationAnswer & explanation
Correct answer: C. Shift to a lower gear before starting the downgrade.
Shifting to a lower gear before starting a long, steep downgrade allows the engine's compression to provide significant braking power, known as engine braking. This reduces the reliance on the service brakes, preventing them from overheating and fading. Continuous braking (B) and pumping brakes (D) still rely on service brakes, and coasting in neutral (C) is extremely dangerous as it eliminates engine braking and control.
Why the other options are wrong
- A. Shifting to neutral and coasting down a grade is extremely dangerous as it removes engine braking and vehicle control, and is illegal.
- B. Applying service brakes continuously can cause them to overheat and fade, leading to a loss of braking power.
- D. Pumping the service brakes frequently still uses the service brakes and can lead to overheating if not done correctly with proper gear selection.
Engine Braking on Downgrades
Using the vehicle's engine and transmission to help slow down and maintain control on long, steep downgrades, reducing the need for excessive service brake use.
- Achieved by downshifting to a lower gear.
- Prevents service brake overheating and fade.
- Crucial for safe mountain driving.
Memory trick: Downhill, downshift, save your brakes, save your life.