Resurrecting a CAT 955K That Refuses to Start: In-Depth Troubleshooting Guide

[MikePhua]

This article delves into the gritty details behind starting issues on the Caterpillar 955K track loader, drawing on technical insights, field experiences, and expert wisdom. You’ll find clear guidance segmented into sections with defined terms, helpful narratives, and a structured checklist—all in flowing English, with no references to the original forum or link-heavy clutter.

1. Electrical System and Starter Impediments
Starting troubles often stem from power delivery issues.

• Weak battery voltage, corroded terminals, or undersized battery banks (commonly using two Group 31 units instead of the recommended four 6-volt batteries) can drastically reduce cranking power. Voltage should be measured under load to rule this out.
  
• A compromised or stuck starter solenoid or Bendix (drive gear) can impede engine engagement. Tapping lightly on the starter during a failed crank may free the mechanism.
  

2. Fuel Delivery and Pressure Control
Even with good power, fuel must flow correctly for the engine to fire.

• A defective transfer (low-pressure) pump may fail to maintain adequate feed to the injection system, particularly critical on CAT 330-series engines.
  
• The bypass or pressure relief valve in the injection pump housing can clog or stick—pressure should be verified with a proper gauge; the maximum expected is around 15 psi.
  
• Air in the fuel lines or a stuck governor rack may prevent the injection pump rack from advancing. On the 955K, the “rack stop” feature inhibits movement until oil pressure builds, giving the illusion of a non-responsive rack.
  

3. Air Intake and Fuel Quality Concerns
A deprived air supply or contaminated fuel can block ignition even when fuel is flowing:

• Clogged air filters or precleaners inhibit combustion, even causing heavy black smoke before shutdown. Removing them has helped many units start more readily.
  
• Fuel contaminated with water or sediments leads to injector line blockages and inconsistent pressure. One owner swapped out filters, injectors, and the transfer pump after finding debris in injectors. Still, only a dribble of fuel surfaced at injectors during cranking—indicative of deeper delivery faults.
  

4. Governor, Injection Rack, and Limit Stop Interactions
The governor-rack system controls fuel delivery during startup and operation:

• A stuck injection rack, particularly one held by the internal governor rack stop, may prevent fueling; loosening the governor end cover and gently manipulating the rack while cranking can help dislodge it—*but injector lines must be loosened during this test to prevent runaway. *
  
• After bleeding, fuel should spurt freely from injector lines; if only dribbling, pressurization or mechanical movement remains inadequate.
  

5. Real-World Diagnostics and Fix Sequence
Here’s how a seasoned mechanic might systematically approach the issue:

• Battery check: Under-crank voltage measurement.
  
• Starter test: Tap to free the Bendix; inspect solenoid and brushes.
  
• Air intake: Remove filters to test free airflow during crank.
  
• Fuel system: Replace filters, prime system, replace the transfer pump if pressure remains low.
  
• Check bypass valve: Ensure it isn’t stuck and is relieving as designed.
  
• Rack operation: Remove governor cover, attempt gentle movement of the rack while cranking (with injector lines loosened).
  
• Bleed process: Crack injector lines, pump until clear fuel emerges.
  

Glossary

• Bendix: Starter gear that engages the flywheel; can bind over time.
  
• Transfer Pump: Low-pressure pump that feeds fuel to the injection pump.
  
• Bypass Valve: Pressure relief within the injection pump to protect from overpressure.
  
• Governor Rack Stop: Safety mechanism that prevents rack movement until oil pressure stabilizes.
  
• Injection Rack: The movable component that meters fuel volume into the engine.
  
• Bleeding: Purging air from the fuel lines to restore pressurization.
  

Anecdote: Better Fuel Flow—and a Working Machine
One owner replaced filters and injectors clogging with debris, yet still saw minimal fuel flow. After priming and careful rack manipulation—ensuring lines were cracked open—the rack freed, pressurization returned, and the engine jerked to life. The lesson: successful starting often hinges on overcoming rack and pressure hurdles, not just clean filters.

Troubleshooting Checklist

• Verify 24 V battery circuit under start load.
  
• Inspect and service starter/solenoid assembly.
  
• Test engine start with air filters removed.
  
• Replace fuel filters and prime the fuel system.
  
• Test fuel system pressure; address transfer pump or bypass valve faults.
  
• Manipulate rack with governor cover off and injector lines cracked.
  
• Bleed injectors until fuel flow is clear and consistent.
  

In combining these diagnostic steps—backed by understanding of mechanical interlocks and real-world experiences—you dramatically increase the odds of reviving a stubborn 955K. Let me know if you’d like wiring diagrams, rack alignment specs, or a deep dive into governor assembly mechanics!