Deere 750C Series II Speed Control Faults and Fixes

[MikePhua]

Unpredictable Speed Control Stutter
Operators of the 750C Series II often experience sudden loss or fluctuation in speed—machines dropping to low speed “1,” jumping to “3,” or completely refusing to move—while fault codes flash simultaneously. One case reported error codes F654 and F658, which correspond to issues like:

• F654: Transmission Speed Control Switch voltage falling below 0.5 V
  
• F658: Decelerator/Speed Control Sensor signal shorted to ground
  
  

The root cause often isn’t the joystick switch but rather voltage signal loss or poor continuity in the wiring harness, causing the Transmission Control Unit (TCU) to interpret abnormal readings.
Voltage Checks and Signal Diagnostics
A reliable method is to back-probe the joystick switch’s signal wire and confirm two key voltages:

• A stable 5 V reference supply to the switch
  
• A signal output above 0.5 V when the switch is operated
  

If both values are correct, but the reading at the TCU is lower, the wiring harness is likely faulty or misrouted. This indicates the issue is not mechanical but electrical in the harness or connectors.

Common Fault Codes and Sensor مشکلات
Other frequent fault codes on this model and related Deere machines include:

• F627 / F635: Speed sensor/pickup alignment or wiring issues, typically causing steering faults or speed inconsistencies
  
• F640: Hydrostatic Pump Speed sensor malfunction—often speed drops or “return to neutral” messages appear; cleaning, repositioning, or replacing sensor resolves it
  
  

Mistakes in mounting or adjusting these sensors (even off by a fraction of a millimeter) can trigger codes or intermittent faults.

Moisture and Connector Corrosion – A Hidden Culprit
Several owners found speed sensor failures following pressure washing or wet-season operations. A tiny amount of moisture in the sensor or corroded connector can derail readings instantly. Inspecting and cleaning connectors behind the shift lever or relevant control box often resolves sudden faults. This phenomenon is particularly noted in 850C models but applies to 750C as well.

Transmission Controller Diagnostics (Fault Code Patterns)
According to Deere’s diagnostic documentation:

• F3 + 6x codes relate to transmission speed sensor or hydrostatic pump speed feedback issues
  
• Codes ending in 0, 2, or 5 signal open or short circuits—meaning voltage signal loss
  
  

Understanding this coding helps pinpoint issues systematically: if the TCU can’t see a valid signal or detects grounding, it defaults to safe mode or refuses to operate.
Quick Reference Overview

• F654: Transmission Speed Control voltage < 0.5 V
  
• F658: Switch circuit short to ground
  
• F627/F635: Misaligned or wrong speed sensor or wiring
  
• F640: Hydrostatic pump speed sensor failure
  
• Sensor issues often show up after water exposure
  
• Deere codes (F3x) provide insight into specific circuit failures
  

Recommendations for Troubleshooting

• Use a multimeter/back-probe testing to trace voltage from switch through harness to TCU
  
• Visually inspect connectors for moisture, corrosion, or misalignment—especially after pressure washing
  
• Ensure sensors are correctly positioned relative to reluctor discs—too close or too far can trigger errors
  
• Replace suspect sensors incrementally—not all at once—to avoid unnecessary expense or failure
  
• Re-clean, secure, and protect connectors to prevent recurrence, and confirm readings post-repair
  

Real-World Fix Scenario
An operator faced intermittent speed loss and F654/F658 codes that reoccurred after cleaning the joystick switch. Back-probing showed correct output voltage at the switch, but readings dropped at the TCU connector. Tracing the harness revealed a pin misaligned in a sealed multiplug. Reseating the pin restored voltage continuity; the dozer ran smoothly all week afterward with no further codes.

In summary, the 750C II’s speed-control issues often trace back to electrical signal integrity—either through bad voltage, misaligned sensors, or wet/corroded connectors. Systematic voltage checks, harness tracing, and protecting connections against moisture dramatically improve reliability and reduce needless parts replacement.