Cat 315 Excavator Half-Function Failure

[MikePhua]

Understanding the Situation
When only half of the hydraulic functions work on a Cat 315 excavator—such as boom and swing working, but stick and bucket not—this usually points to an issue with the auxiliary (i-pump) hydraulic circuit, a leaking pilot valve, or a load-sense malfunction. The Cat 315 (and similar 313/317 siblings) rely on tandem pumps—primary for travel/swing and a secondary (i-pump) for boom, stick, bucket, attachments—so partial failure often reflects a split between those circuits.
Terminology Clarified

• I-pump (Implement pump) – secondary hydraulic pump dedicated to boom, stick, bucket and attachment movements.
  
• Load-sense pressure – communication line that adjusts pump output to demand; low or absent load-sense causes sluggish or dead functions.
  
• Pilot pressure – low-pressure control line that actuates main control valves; a leak here disables spool movement.
  
• Spool valve – directional control component; opens or blocks flow to cylinder based on joystick input.
  

Common Causes of Half-Function Loss

• I-pump failure or belt slip – if the belt is worn or loose, the implement pump spins slowly and can’t produce pressure.
  
• Pilot leak from spool block – internal seepage from a worn or contaminated pilot section prevents spool activation.
  
• Load-sense line disconnection or leak – pump can’t sense demand and shuts down pressure to sections.
  
• Valve block contamination – grit or varnish in spools makes certain functions sticky or non-responsive.
  
• Electrical issues impacting hydraulic sensor or control solenoids – intermittent shutdown of a circuit can disable some functions while leaving others live.
  

Diagnostic Roadmap

1. Visual and safety check – secure the machine on flat ground, lock out power, then inspect belts, hoses, connections.
   
2. Engine RPM test – run engine and monitor i-pump drive belt tension and slippage under load.
   
3. Hydraulic pressure measurement – use gauges on the implement circuit to test i-pump output; target pressure ∼30 MPa (4,350 psi) bar.
   
4. Test load-sense line – cap the line and observe if pump pressure rises; if it remains low, the pump or load-sense system may be suspect.
   
5. Pilot pressure test – use gauges on pilot ports to confirm 140–280 psi (10–20 bar) on spool block.
   
6. Spool function check – with machine warm, operate levers while sniffing for leaks or sluggishness; if necessary, remove and clean valve block.
   
7. Electrical check – inspect wiring, fuses, and solenoid coils on the hydraulic control modules for continuity and proper activation.
   

Field Story
A Florida excavation crew had a Cat 315 where the boom raised and the bucket curled, but the stick movement failed. After confirming the implement pump spin and belt condition, technicians fitted a gauge to the pilot port and noticed no pilot pressure when moving the stick lever. Cleaning residue out of the valve block and replacing a small pilot seal restored full function—and saved the owner thousands by avoiding pump replacement.
Specifications Recap

• I-pump output: approximately 30 MPa / 4,350 psi max
  
• Pilot requirements: ~140–280 psi (10–20 bar) to command spool motion
  
• Belt condition check: less than 10 mm deflection at mid-span required
  
• Load-sense pressure range: 100–120 psi differential above pump relief setting
  
• Service interval for hydraulic filter: every 500 hours (or per manual)
  

Preventative Maintenance Advice

• Check and tension implement pump belt every 250 hours.
  
• Replace hydraulic filters and observe contamination levels; dark slick sludge indicates varnish or shear breakdown.
  
• Apply clean high-zinc hydraulic oil and maintain reservoir cleanliness.
  
• Warm up the implement circuit hydraulics before heavy use in cold weather to prevent pilot seal stiffening.
  
• Regularly lubricate and inspect spool block for early signs of wear.
  

Parts and Support
Cat 315 aftermarket and OEM parts remain accessible through dealer and parts retailers. Common replacements include i-pump seals, pilot valve spools, pressure relief cartridges, and load-sense pipes. Because Cat’s machines are highly popular, operator communities also offer service bulletins and rebuild guides specifically for the 315's hydraulic system.
Summary
Half-function hydraulic loss on a Cat 315 typically traces back to issues in the implement (i-pump) circuit—most often the pump belt, load sense, or pilot spool. By systematically measuring pressures and flow, flushing the valve block, and maintaining proper belts and fluid cleanliness, most of these faults can be quickly identified and fixed. A good preventive maintenance schedule keeps the Kubota 315—or any Cat 315—performing reliably season after season.