Diagnosing Transmission Control Failures in the Case 621B Loader

[MikePhua]

The Case 621B and Its Mechanical Legacy
The Case 621B wheel loader was introduced in the early 1990s as part of Case Corporation’s mid-size loader lineup. With an operating weight of approximately 12,000 kg and powered by a turbocharged 6BT Cummins diesel engine producing around 130 horsepower, the 621B was designed for municipal work, aggregate handling, and general construction. Its Z-bar linkage, four-speed powershift transmission, and robust frame made it a popular choice across North America and Europe.
Case, founded in 1842, had already built a reputation for durable agricultural and construction machinery. The 621B continued that tradition, offering mechanical simplicity and ease of service. Thousands of units were sold globally, and many remain in operation today, especially in small fleets and rural municipalities.
Intermittent Transmission Failure and Neutral Behavior
A recurring issue with aging 621B units involves the transmission suddenly disengaging—acting as if the machine has been shifted into neutral or the parking brake has been applied. Operators report that the loader pulls normally, then abruptly loses drive. Restarting the engine sometimes restores functionality, but the problem often returns unpredictably.
This behavior suggests an electrical or control system fault rather than a mechanical failure. When the machine freewheels without locking up, and no brake pressure warning light is illuminated, the issue likely resides in the transmission control circuit.
Terminology Clarification

• FNR Controller: The Forward-Neutral-Reverse switch that sends signals to the transmission control unit (TCU).
  
• Solenoid Pack: A group of electrically actuated valves that control hydraulic flow to the transmission clutches.
  
• Declutch Switch: A device that disengages the transmission when the brake pedal is pressed, allowing braking without engine load.
  
• TCU (Transmission Control Unit): The onboard computer that manages gear selection and clutch engagement.
  

Common Failure Points and Diagnostic Strategy
Several components can cause the transmission to disengage intermittently:

• Declutch Switch Malfunction: Located either mechanically above the brake pedal or hydraulically beneath the cab, this switch can fail or send false signals, causing the TCU to disengage drive.
  
• Solenoid Resistance Variance: Solenoids should measure between 100–115 ohms. A faulty solenoid with resistance outside this range can cause the TCU to shut down.
  
• Grounding Issues: Poor ground connections, especially between the cab and frame or beneath the instrument panel, can interrupt signal flow.
  
• TCU Self-Shutdown: When the TCU detects an open circuit or faulty input, it may click three times on startup—indicating a shutdown sequence.
  

In one documented case, a technician traced the issue to a corroded ground wire near the right side of the engine. After cleaning and resecuring the connection, the transmission operated normally.
Electrical Panel and Relay Access
The fuse and relay panel is located behind a cover on the right-hand side of the operator’s seat. Accessing it may require partial removal of the dashboard. Technicians should inspect:

• Fuse integrity, especially the 5A fuse powering the TCU
  
• Relay function and socket corrosion
  
• Wiring harness continuity between the FNR controller and solenoid pack
  

In Oregon, a mechanic diagnosed a loader that clicked three times on startup and found a broken wire inside the harness connector. After repair, the machine resumed normal operation.
Speed Sensor and Gear Limitation Issues
The 621B uses speed sensors to regulate gear shifting. A failed sensor can prevent access to higher gears. The left-hand sensor governs basic speed input, while the right-hand sensor (on auto-shift models) controls advanced shifting logic.
Sensor resistance should measure between 1350–1590 ohms. If readings fall outside this range, the TCU may restrict gear access or shut down entirely. In Alberta, a loader lost 3rd and 4th gear due to a rotted wire on the right-hand sensor. Replacing the sensor and repairing the harness restored full functionality.
Recommendations for Technicians and Fleet Managers

• Use an ohmmeter to test solenoid resistance and speed sensor values
  
• Listen for TCU startup clicks—two is normal, three indicates shutdown
  
• Inspect and clean all ground points, especially near the cab and engine
  
• Check for power at the solenoid plug; absence may indicate TCU failure or open circuit
  
• Replace aging wiring harnesses proactively to prevent intermittent faults
  

Preventive Measures and Long-Term Reliability

• Schedule electrical inspections every 1,000 hours
  
• Use dielectric grease on connectors to prevent corrosion
  
• Label and document wiring repairs for future reference
  
• Avoid bypassing fuses with higher ratings, as this risks TCU damage
  
• Train operators to report early signs of transmission disengagement
  

Conclusion
The Case 621B remains a dependable workhorse, but its aging electrical systems require careful attention. Transmission disengagement is often a symptom of solenoid, grounding, or sensor faults—not mechanical failure. With methodical diagnostics and preventive maintenance, technicians can keep these loaders running smoothly and safely. In the world of mid-size loaders, the 621B proves that even legacy machines can deliver modern performance—if their circuits are kept clean and their signals strong.