CEM Technical Skills & Application 4 — Questions and Answers
Question 1: A CEM reviewing oil analysis data notices a sharp increase in silicon content across multiple machines sharing the same job site. The MOST likely root cause is:
- Internal bearing failure generating metallic debris
- Dirt ingestion through compromised air filtration systems (Correct answer)
- Coolant contamination from a failed head gasket
- Additive package depletion
Correct answer: Dirt ingestion through compromised air filtration systems
Silicon is the primary elemental marker for dust and dirt ingestion, and a fleet-wide spike on one site strongly indicates a common air filtration failure or improper filter servicing.
Question 2: Which specification MOST directly limits the allowable worn clearance in a diesel engine main bearing?
- OEM maximum wear clearance published in the engine service manual (Correct answer)
- ISO 4406 cleanliness code for the lubrication system
- SAE viscosity grade of the crankcase oil
- API service classification of the oil in use
Correct answer: OEM maximum wear clearance published in the engine service manual
OEM service manuals specify maximum allowable main bearing clearance; exceeding this value causes oil pressure drop and accelerated crankshaft wear.
Question 3: When pressure-testing a hydraulic accumulator for pre-charge nitrogen pressure, the technician should:
- Use shop air compressed to system working pressure
- Use a dedicated nitrogen charging kit and check gas pressure only with the hydraulic side depressurized (Correct answer)
- Fill with hydraulic fluid to verify accumulator volume
- Test at operating temperature immediately after the machine has been working
Correct answer: Use a dedicated nitrogen charging kit and check gas pressure only with the hydraulic side depressurized
Accumulators must be hydraulically depressurized before checking nitrogen pre-charge to prevent dangerous measurement error and injury; only nitrogen (inert gas) is used.
Question 4: A scraper elevator chain stretches beyond tolerance after only half the expected service life. Which maintenance factor is MOST likely responsible?
- Excessive application load
- Under-lubrication of the chain pins and bushings (Correct answer)
- Over-lubrication causing seal wash-out
- Use of the wrong chain pitch
Correct answer: Under-lubrication of the chain pins and bushings
Inadequate lubrication of chain pins and bushings accelerates wear and elongation, dramatically shortening chain service life.
Question 5: On a wheel excavator, the stabilizer cylinders lock up and become inoperative after the machine sits overnight. No hydraulic fluid loss is observed. The MOST probable cause is:
- Relief valve failure
- Internal rust or corrosion causing piston seizure due to moisture intrusion (Correct answer)
- Overpressure from the boom circuit
- Worn hydraulic pump unable to build pressure
Correct answer: Internal rust or corrosion causing piston seizure due to moisture intrusion
Stabilizer cylinders exposed to temperature cycling can allow condensation, and if protective coatings are worn, moisture causes corrosion and seizing of the piston-to-barrel interface overnight.
Question 6: What is the function of a torque converter lock-up clutch in heavy equipment power trains?
- It multiplies torque during high-load acceleration
- It mechanically couples the engine to the transmission input at higher speeds to eliminate slippage and improve efficiency (Correct answer)
- It disconnects the transmission from the engine during shifting
- It provides dynamic braking when the operator releases the throttle
Correct answer: It mechanically couples the engine to the transmission input at higher speeds to eliminate slippage and improve efficiency
The lock-up clutch bypasses the fluid coupling in the torque converter at cruise speeds, creating a direct mechanical drive that eliminates torque converter heat and improves fuel efficiency.
Question 7: A pneumatic tire on a rigid-frame haul truck shows accelerated wear on the outer shoulder only. Which operational condition BEST explains this wear pattern?
- Chronic overloading of the haul truck
- Excessive cornering speed on haulage curves causing lateral scrub (Correct answer)
- Under-inflation below the recommended cold-inflation pressure
- Alignment error causing excessive positive camber
Correct answer: Excessive cornering speed on haulage curves causing lateral scrub
Outer shoulder wear on haul truck tires is the classic signature of excessive lateral forces during cornering, which scrubs material from the outer edge.
CEM Technical Skills & Application 4
A CEM reviewing oil analysis data notices a sharp increase in silicon content across multiple machines sharing the same job site.
The MOST likely root cause is: