CEM Technical Skills & Application 3 — Questions and Answers
Question 1: A motor grader blade drifts down during transport even though the hydraulic system pressure is within specification. What is the MOST probable cause?
- Relief valve set too high
- Internal leakage past the blade lift cylinder piston seals (Correct answer)
- Excessive pump flow
- Clogged case drain filter
Correct answer: Internal leakage past the blade lift cylinder piston seals
Internal bypass past worn piston seals allows fluid to transfer between cylinder chambers, causing slow drift under load even when system pressure is normal.
Question 2: Which diesel engine component is responsible for controlling injection timing in a common-rail fuel system?
- Mechanical governor flyweights
- Injection control pressure sensor
- Engine control module (ECM) commanding injector solenoids or piezo actuators (Correct answer)
- Fuel transfer pump speed
Correct answer: Engine control module (ECM) commanding injector solenoids or piezo actuators
In common-rail systems, the ECM precisely controls injection timing by energizing individual injector solenoids or piezo actuators at the calculated crank angle.
Question 3: During a hydraulic pump performance test, measured flow is 15% below the manufacturer's specification at rated pressure and speed. This result MOST likely indicates:
- Relief valve set too low
- Excessive internal pump leakage (volumetric inefficiency) (Correct answer)
- Air entrainment in the reservoir
- A clogged return filter
Correct answer: Excessive internal pump leakage (volumetric inefficiency)
A 15% flow deficit at rated conditions is the classic signature of worn pump internals causing volumetric inefficiency through internal slippage.
Question 4: When inspecting a boom cylinder on an excavator for structural integrity, a CEM notices a hairline crack at the base of a trunnion mount weld. The CORRECT action is:
- Monitor the crack and re-inspect at the next PM interval
- Immediately remove the machine from service for weld repair evaluation (Correct answer)
- Apply epoxy filler and return the machine to service
- Increase greasing frequency at the trunnion pin
Correct answer: Immediately remove the machine from service for weld repair evaluation
Cracks at structural weld locations are a critical safety defect requiring immediate removal from service and professional structural assessment.
Question 5: A technician measures 0.5 volts on the signal wire of a 0–5V pressure sensor while the system is at high pressure. This reading MOST likely indicates:
- Normal sensor output at high pressure
- An open circuit or failed sensor producing a low-clamp signal (Correct answer)
- The sensor is over-ranged
- The ECM analog input is shorted to ground
Correct answer: An open circuit or failed sensor producing a low-clamp signal
A 0–5V pressure sensor should produce 4–4.5V at high pressure; 0.5V is near the zero-pressure value and suggests an open-circuit or failed sensor.
Question 6: Which type of gear is MOST commonly used in the final drive of a track-type machine to achieve the high torque multiplication needed at low speed?
- Helical gear set
- Planetary gear set (Correct answer)
- Bevel gear set
- Worm gear set
Correct answer: Planetary gear set
Planetary gear sets provide very high torque multiplication in a compact, concentric package and are the standard choice for track machine final drives.
Question 7: What distinguishes a two-speed axle from a standard single-speed axle in vocational trucks used in construction fleets?
- It uses a torque converter instead of a differential
- It contains an additional gear set in the axle housing to provide a lower ratio for heavy haul (Correct answer)
- It uses an electronic limited-slip differential
- It omits the ring and pinion to reduce weight
Correct answer: It contains an additional gear set in the axle housing to provide a lower ratio for heavy haul
A two-speed axle incorporates an auxiliary planetary gear set that engages to provide a numerically higher ratio for improved gradeability under heavy load.
CEM Technical Skills & Application 3
A motor grader blade drifts down during transport even though the hydraulic system pressure is within specification.
What is the MOST probable cause?