The excavator main control valve (distribution valve) is responsible for distributing hydraulic flow and pressure to each actuator (boom, arm, bucket, travel, swing). Any fault in the valve directly affects machine performance, causing weak, slow, or abnormal motions.

I. Common Symptoms of Main Control Valve Problems

1. Single Function Weak or Not Working

• Boom cannot lift

• Bucket has weak breakout force

• Arm moves slowly

• One-side travel weak or does not move

2. Multiple Functions Weak

• Entire machine moves slowly

• Works normally when cold but weak after warming up

3. Interference Between Two Operations

• Combined operations cause machine to stall

• One function becomes fast while another slows down significantly

4. Abnormal Noise or Vibration from the Valve

• Metal rubbing noise

• Buzzing sound

• Valve block local overheating

II. Common Causes of Main Control Valve Faults (By Likelihood)

1. Spool Sticking / Spool Wear (≈ 40%)

Causes:

• Contaminated hydraulic oil

• High operating temperature causing clearance change

• Spool scoring or rust

Symptoms:

• Jerky motion, intermittent movement

• Good when cold, poor when hot

• Overheating on one valve section

Solutions:

• Disassemble and clean the spool

• Polish/repair the spool and bore

• Replace worn spool or valve section

2. Pressure Compensator Valve (PC Valve) Failure (≈ 25%)

Causes:

• PC valve spring broken or weakened

• Compensator spool wear

• Unstable system pressure

Symptoms:

• Weak during combined operations

• One function normal, another extremely slow

• System pressure cannot reach specification

Solutions:

• Clean or replace the PC valve

• Adjust pilot pressure and main system pressure

3. Internal Leakage of the Valve (≈ 20%)

Causes:

• Increased clearance between spool and valve body

• Seal rings damaged

• Valve body surface wear or scoring

Symptoms:

• Pressure test normal but functions still weak

• Boom drifting down when lifted

• Travel weak, no pushing power

Solutions:

• Perform internal leakage test

• Replace spool, valve body, or seal kit

4. Pilot Circuit Problems (≈ 10%)

Causes:

• Low pilot pump pressure

• Pilot filter clogged

• Control lever (joystick) pilot valve malfunction

Symptoms:

• Large lever stroke required

• Dual operations cause machine to stall

• One direction significantly slower

Solutions:

• Adjust pilot pressure (standard 3–4 MPa)

• Replace pilot filter

• Repair or replace the pilot valve

5. Main Relief Valve / Safety Valve Failure (≈ 5%)

Causes:

• Relief valve stuck

• Spring fatigue or spool wear

Symptoms:

• System pressure cannot build up

• Weak functions, worse after warming up

Solutions:

• Recalibrate the relief valve

• Replace damaged relief valve parts

III. Quick Diagnostic Procedure

1. Pressure Test

• Main pump pressure: 18–35 MPa (varies by model)

• Pilot pressure: 3–4 MPa

• LS pressure (load-sensing)

2. Temperature Check

• A valve block section is significantly hotter → spool sticking or internal leakage

3. Sound Inspection

• Buzzing → relief valve open

• Metal rubbing → spool scoring

4. Swap Test

• Swap the suspected valve section with a normal one
  → Quick identification of valve-related faults

IV. Repair Guidelines and Precautions

• Ensure spool roundness & alignment during lapping/polishing

• Do not randomly adjust pressure screws

• Keep all valve components perfectly clean during reassembly

• Avoid unnecessary deep disassembly to prevent assembly errors

• Recommend using a professional valve test bench for verification

V. Summary Table