Cat 304 Won’t Swing or Walk

[MikePhua]

Introduction
The Caterpillar 304 is a modern mini-excavator in the 45 horsepower class, manufactured by CAT with reduced-radius / tail-swing configurations to work in confined job sites. Key specs include:

• Net power: about 45 hp (33.6 kW) at 2,400 RPM.
  
• Operating weight: roughly 9,867 lb (4,475 kg) in its heaviest configuration.
  
• Travel speeds: low speed ~2.2 mph, high speed ~3.2 mph.
  

Despite being compact, the hydraulic system is sophisticated: multiple circuits (swing, travel, boom/stick/bucket), relief valves, pilot or servo controls, gear and/or piston pumps. Failure in any part can render functions inoperative.

Symptoms of the Problem

• Boom, stick, and bucket functions worked (or intermittently), but no swing (cab rotation) movement.
  
• No track movement (“walk”) in either direction.
  
• Blade raise is extremely slow; not enough power to lift.
  
• Hydraulic fluid leak from a valve or manifold block under the cab, likely related to auxiliary circuits or the swing/travel block.
  
• Frayed or broken wiring harness/connector under cab.
  
• After repairs (o-ring replacements, fixing fluid leaks, correcting fluid condition), some swing function returned (though weak), but travel still non-functional.
  

Diagnostic Findings
From the repair narrative:

• Two hoses on the travel block had been swapped. This misrouting meant some functions got fluid flow, others got none or reduced.
  
• The small auxiliary pump (a gear pump, connected off the main pump) that supplies the swing, slew (swing/offset boom), blade circuits was operating extremely weakly: measured output ~180-200 psi, whereas spec is ~400 psi.
  
• After swapping hoses back to correct positions and replacing that auxiliary pump, machine regained all movement (swing, track walk, blade).
  

Terminology / Key Components

• Gear Pump / Auxiliary Pump: A smaller pump that supplies hydraulic fluid to a subset of functions (swing, blade, boom offset, etc.). If this is weak or fails, those functions suffer.
  
• Travel Block: The manifold or assembly that routes hydraulic lines to the track drive motors. Incorrect connections or block failures here will disable “walk.”
  
• Relief Valve: Protects circuits by limiting pressure; if stuck or damaged, may prevent pressure build-up needed for swing/travel motors.
  
• Pilot / Servo Lines: Control pressure signals; damage or air in pilot lines can disable functions.
  
• Hydraulic Fluid Viscosity & Condition: Thick or contaminated fluid can severely limit pump output, especially for smaller pumps.
  

Steps to Fix
Based on the case, these are the effective steps:

1. Locate and Repair Hydraulic Leaks First
   A significant leak from the valve/block under cab was one of the symptoms. Replace the faulty O-rings or seals; ensure all connections are tight and proper.
   
2. Check Hose Routing and Connections
   Verify hoses on the travel block (and swing / auxiliary circuits) are connected to the correct ports. Swapped hoses can redirect flow improperly, causing no movement or weak functions.
   
3. Test Hydraulic Pressures
   Use pressure gauges to test swing circuit output, travel motor (walk) circuit pressure, and auxiliary circuits (blade, boom offset). Compare to spec: e.g. swing pump ~400 psi expected; observed ~180-200 psi in the failure case.
   
4. Replace Auxiliary Pump If Required
   When the gear pump off the main pump is weak or damaged, replacing that auxiliary pump restored swing and blade function in the reported case.
   
5. Inspect Electrical System
   Frayed or broken wiring, blown fuses, damaged switches (e.g. 2-speed travel switch) can also disable swing or travel functions. Replace any damaged connectors; ensure continuity.
   
6. Bleed / Purge Air from Hydraulic System
   After repair, remove air from lines—particularly after filling, replacing hoses/pumps—by operating circuits slowly, opening bleed ports if present, etc. Air can compress and prevent pressure build-up.
   

Preventive Measures

• Use correct hydraulic fluid weight & change it per schedule; fluid that’s too heavy or degraded can overburden smaller pumps.
  
• Regular inspection of hoses, fittings and seals to catch leaks early.
  
• Keep hydraulic circuits clean; contaminants can damage pumps/motors.
  
• Label hoses/connectors during any disassembly to avoid swapping.
  
• Maintain electrical harnesses; moisture/abrasion leads to damage.
  
• Regular pressure checks on critical circuits to ensure capacity.
  

Conclusion
What looked like a major failure (“won’t swing or walk”) turned out to be a combination of:

• Swapped hoses in hydraulic block restricting flow,
  
• A weak or failed auxiliary gear pump serving swing/travel/slew/blade circuits,
  
• Leaks and damaged seals allowing fluid loss, pressure drop, and contamination,
  
• Some electrical issues (harness / fuses) compounding mechanical/hydraulic troubles.
  

After replacing O-rings, correcting hose routing, installing a new auxiliary pump, and addressing wiring/fuses, the 304 was made functional again — swing, walk, blade movement restored. The total cost was under USD 1,000 including parts for the pump, hoses, and minor electricals. For a machine bought for ~$3,000 in its condition, the repair represented good value.