John Deere 755/755C Crawler Loader: Common Issues, Diagnostics & Repair Insights

[MikePhua]

The John Deere 755 and its later 755C variant are compact crawler loaders prized for their durability and versatility. Owners often encounter issues related to starting, power loss, hydraulic movement, safety switches, and electronics. Understanding these common faults, their causes, and troubleshooting strategies can help operators and technicians keep these machines running reliably.
1. Starting and No‑Start Issues

• Diesel units rarely “spark”—starting depends on proper fuel delivery and glow plug function.
  
• Cold weather worsens starting; failing glow plugs or fuel starvation can prevent ignition .
  
• Fuel system faults: blocked pickup screen in the tank, a clogged pre-filter at tank inlet, faulty electric transfer pump, or contaminated fuel can disrupt flow .
  
• Electrical issues: bad grounding or battery leads under starter load can mimic starter failure .
  
• Diagnostic steps:
  • Check power to glow plugs under cold conditions.
    
  • Verify fuel pump output and bleed system up to injectors.
    
  • Inspect for fuel tank debris or settlement blockage.
    
  • Test battery and starter voltage under load at starter terminal.
    

2. Sudden Power Loss or Engine Hesitation

• Operators reported abrupt loss of power or engine stalling, followed by short spurts of high RPM, often due to fuel delivery interruptions or injector/filter blockages .
  
• Remedies include flushing filters, inspecting fuel lines, cleaning tank internals, and ensuring transfer pump functionality .
  

3. Hydrostatic Control and Movement Issues

• Differential steering or travel may act inconsistently if pedals or control valves are misadjusted.
  
• Some 700‑series loaders exhibit unexpected reverse movement on one track when braking due to misadjusted transmission control valves—a delicate setup requiring proper linkages and gauge calibration .
  
• Cases involving hydraulic or computer lockouts: unexpected immobilization, brake warning lights, and electronic shutdowns often relate to park brake sensors, faulty relays, or TCU fault codes (especially on 755C models) .
  

4. Safety Seat Switch & Inching Control

• Seat‑weight safety switches or “dead‑man” controls can prevent starting or interrupt controls if miswired or faulty .
  
• Hydrostatic inching pedal responsiveness depends heavily on wiring integrity and sensor function—worn or taped-in switches can cause erratic motion or control loss .
  

5. Electrical Faults & Diagnostic Tools

• The 755C introduced electronic hydrostatic control and safety interlocks; failures here often mimic mechanical issues .
  
• Red flashing brake indicator or transformer light on dash signals TCU fault or over-voltage condition.
  
• Excess alternator voltage (e.g., 28.6V) can trigger electronic shutdowns; tapping control boxes sometimes temporarily restores function but does not fix root cause .
  

Terminology Clarified

• Hydrostatic Drive: Uses pressurized fluid to transmit power from pump to travel motors, enabling smooth speed control.
  
• Transfer Pump: Low-pressure fuel pump drawing diesel from the tank to the high-pressure injection pump.
  
• Deck Safety Switch (Dead‑Man Switch): Prevents machine operation when no operator is seated.
  
• Transmission Control Unit (TCU): Electronic controller managing hydrostatic transmission and safety interlocks.
  
• Counter‑rotation: Mechanism for tight-turn steering where one track reverses while the opposite moves forward.
  
• Park Brake Sensor/Relay: Electrical components signaling brake status to the control unit.
  

Maintenance & Diagnostic Steps Checklist

• Starting test:
  • Verify fuel pump operation and adequate fuel delivery.
    
  • Inspect and bleed fuel lines up to injectors.
    
  • Check glow plug activation in cold weather.
    
  • Test battery/starter voltage directly at the starter terminal.
    
• Power loss troubleshooting:
  • Replace filters, clean tank, check transfer pump.
    
  • Ensure no blockage or contamination in fuel system.
    
• Hydraulic/travel issues:
  • Confirm pedal sensor alignment and linkage adjustment.
    
  • Bleed hydrostatic control valve as per service manual specs.
    
  • Test counter‑rotation behavior for smoothness.
    
• Electronic faults:
  • Use diagnostic tools to read fault codes.
    
  • Check park brake sensors, TCU wiring, and battery charging voltage.
    
  • Address overvoltage by testing alternator and voltage regulator.
    

Real‑World Experience Highlights

• One owner revived a JD 755 that wouldn't start in winter. A frozen fuel pickup screen and bad wiring at glow plugs were culprits; correcting both resolved the no‑start issue.
  
• Another operator suffered jerky inching and erratic movement traced back to a taped-down seat switch and misadjusted hydrostatic pedals—correct wiring and new switches restored control.
  
• A 755C experienced hydraulic lockouts and flashing brake lights; diagnostics revealed over-voltage from alternator and a faulty pedal sensor. Replacing both restored full travel capability.
  

Summary of Common Issues & Fixes

• No‑Start: likely fuel starvation, glow plug wiring fault, or starter/ground issue.
  
• Power Loss: clogged filters or blocked pick-up screen leading to fuel interruption.
  
• Erratic Hydrostatic Control: misadjusted pedals, seat safety switches, or control valve settings.
  
• Computer Lockout: often tied to park brake sensors, high voltage, or TCU error codes.
  

Conclusion
The John Deere 755 and 755C series offer reliable performance when properly maintained and diagnosed. Common faults often arise from fuel delivery problems, safety interlock wiring, hydrostatic control misadjustment, or electrical/voltage anomalies. Systematic troubleshooting—starting with proper tools, fluid filters, wiring checks, and pedal calibration—helps resolve most issues. Investing in the official service manual and diagnostic equipment can save time and prevent guesswork. With careful attention and methodical repair, these machines can serve reliably for many years.