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Heavy Duty Equipment Technician Red Seal Exam Prep · Question

A variable displacement piston pump is operating, but the hydraulic actuator it powers is not moving. Upon inspection, the technician finds that the pump is not stroking to deliver flow. What is a common cause for this specific symptom?

A variable displacement pump relies on a compensator or control system to stroke the pump and deliver flow. If the compensator spool is broken or stuck in a de-

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Question: A variable displacement piston pump is operating, but the hydraulic actuator it powers is not moving. Upon inspection, the technician finds that the pump is not stroking to deliver flow. What is a common cause for this specific symptom?

Answer options: ✅ Broken or stuck compensator spool

  • Clogged return line filter
  • Excessive system pressure from a relief valve
  • Air in the hydraulic reservoir

Correct answer: Broken or stuck compensator spool

Explanation: A variable displacement pump relies on a compensator or control system to stroke the pump and deliver flow. If the compensator spool is broken or stuck in a de-stroked position, the pump will not produce flow even if the engine is running and the pump is technically spinning.

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