CDL Practice Test

CDL Vehicle Systems Practice Questions

Vehicle Systems makes up 4 of every 50 questions on a Tigertest CDL practice test. Here are 10 of the 50 questions on it, each with the answer and the reason.

Updated 2026-11-10 by the Tigertest team. Covers the CDL general knowledge exam. 80% to pass on the real test.

1. 1. What should you do if your engine temperature gauge shows the engine is overheating?

   • A.Continue driving but reduce speed to avoid further damage
   • B.Stop driving as soon as safely possible and shut off the engine
   • C.Turn the heater on full and keep driving to help cool the engine
   • D.Remove the radiator cap at once and add cold water

   Answer: B. An overheating engine can cause severe damage and should be shut off immediately when it's safe to stop.

2. 2. Which component regulates the flow of coolant through the engine?

   • A.Water pump
   • B.Radiator cap
   • C.Cooling fan
   • D.Thermostat

   Answer: D. The thermostat opens and closes to control coolant flow based on engine temperature, maintaining optimal operating temperature.

3. 3. What is the primary function of the alternator in a commercial vehicle?

   • A.Generate electricity to charge the battery and power electrical systems
   • B.Regulate voltage to prevent electrical damage to sensitive components
   • C.Start the engine and provide initial power during startup
   • D.Store electrical energy for later use during engine shutdown

   Answer: A. The alternator converts mechanical energy from the engine into electrical energy to charge the battery and power all electrical systems while running.

4. 4. If your battery warning light comes on while driving, what does this typically indicate?

   • A.The electrical system is using too much power
   • B.The starter motor is failing
   • C.The charging system is not working properly
   • D.The battery needs to be replaced immediately

   Answer: C. A battery warning light usually indicates the alternator or charging system is not functioning properly, meaning the battery is not being charged.

5. 5. Why should you never remove the radiator cap when the engine is hot?

   • A.It will damage the radiator cap threads
   • B.Cold air will shock the hot engine block
   • C.Hot coolant and steam can cause severe burns
   • D.The cooling system will lose all its pressure

   Answer: C. A hot cooling system is under pressure, and removing the cap can cause scalding hot coolant and steam to spray out violently.

6. 6. What should you check if your engine is using more fuel than usual?

   • A.Air filter, fuel filter, and engine tune-up items
   • B.Brake adjustment and the transmission fluid level
   • C.Tire pressure and wheel alignment only
   • D.Oil level and engine coolant temperature

   Answer: A. Clogged filters and poor engine tune can significantly increase fuel consumption by affecting engine efficiency.

7. 7. How often should you typically change the fuel filter on a diesel engine?

   • A.Every 100,000 miles regardless of manufacturer recommendations
   • B.Every oil change
   • C.Only when it becomes completely clogged
   • D.According to manufacturer's maintenance schedule

   Answer: D. Fuel filter replacement intervals vary by manufacturer and operating conditions, so following the maintenance schedule ensures optimal performance.

8. 8. What can happen if you use gasoline in a diesel engine?

   • A.Nothing significant will occur
   • B.Severe engine damage can result
   • C.The engine will produce less emissions
   • D.The engine will run more efficiently

   Answer: B. Gasoline lacks the lubricating properties of diesel fuel and can cause serious damage to fuel injection components and engine internals.

9. 9. If you notice black smoke coming from your exhaust, what does this typically indicate?

   • A.The cooling system is overheating and needs immediate attention
   • B.The engine is burning too much fuel or has poor combustion
   • C.The exhaust system has a minor leak in the manifold
   • D.The engine oil level is too high and needs draining

   Answer: B. Black smoke indicates incomplete fuel combustion, often caused by over-fueling, dirty air filters, or fuel injection problems.

10. 10. What is the purpose of a diesel particulate filter (DPF)?

    • A.To silence exhaust noise and reduce cabin sound levels
    • B.To increase engine power output and improve acceleration performance
    • C.To improve fuel economy by optimizing combustion efficiency
    • D.To reduce harmful particulate emissions from the exhaust

    Answer: D. The DPF captures and burns off soot particles from diesel exhaust to meet environmental emission standards.