DTC DTC Hydraulic Systems and Accessories 2 — Questions and Answers
Question 1: What is a Power Take-Off (PTO) unit on a diesel truck used for?
- To increase engine horsepower for highway driving
- To transfer engine power to auxiliary equipment such as hydraulic pumps or winches (Correct answer)
- To engage the four-wheel drive transfer case
- To power the air conditioning compressor
Correct answer: To transfer engine power to auxiliary equipment such as hydraulic pumps or winches
A PTO transfers rotational power from the truck's transmission or engine to drive auxiliary equipment like hydraulic pumps, vacuum pumps, or winches.
Question 2: A diesel technician finds contaminated hydraulic fluid with a milky, white appearance. What is the most likely cause?
- Overheating of the hydraulic oil
- Water intrusion from a failed heat exchanger or reservoir seal (Correct answer)
- Excessive additive depletion
- Normal oxidation over time
Correct answer: Water intrusion from a failed heat exchanger or reservoir seal
Milky or whitish hydraulic fluid indicates water contamination, often from a leaking hydraulic oil cooler (heat exchanger) or a compromised reservoir vent/seal.
Question 3: What does a hydraulic flow control valve do in a diesel truck's hydraulic circuit?
- Sets the maximum system pressure
- Regulates the speed of hydraulic actuators by controlling fluid flow rate (Correct answer)
- Divides flow equally between multiple circuits
- Prevents fluid from flowing backward through the circuit
Correct answer: Regulates the speed of hydraulic actuators by controlling fluid flow rate
A flow control valve meters the volume of hydraulic fluid passing to an actuator, which controls the speed of a hydraulic cylinder or motor regardless of load pressure.
Question 4: Which component in a hydraulic system prevents fluid from flowing backward when the pump stops?
- Relief valve
- Flow control valve
- Check valve (Correct answer)
- Directional control valve
Correct answer: Check valve
A check valve allows hydraulic fluid to flow in one direction only and closes to prevent reverse flow when pump pressure drops or the system is de-energized.
Question 5: What is the correct method for bleeding air from a truck-mounted hydraulic cylinder?
- Increase system pressure above relief valve setting
- Cycle the actuator slowly through its full stroke multiple times with the system running to purge air to the reservoir (Correct answer)
- Drain the entire hydraulic reservoir and refill
- Install an air separator upstream of the cylinder
Correct answer: Cycle the actuator slowly through its full stroke multiple times with the system running to purge air to the reservoir
Slowly cycling the cylinder through full extension and retraction several times allows entrained air to migrate back through the return line to the reservoir where it can escape.
Question 6: What hydraulic system test uses a flow meter and pressure gauge to evaluate pump output performance?
- Residual pressure test
- Pump flow test (pump efficiency test) (Correct answer)
- Continuity test
- Thermal imaging test
Correct answer: Pump flow test (pump efficiency test)
A pump flow test measures actual pump output (GPM) versus rated flow at a specific pressure, determining whether the pump is worn and bypassing internally.
DTC DTC Hydraulic Systems and Accessories 2
What is a Power Take-Off (PTO) unit on a diesel truck used for?