Diagnosing SCR Heater Failures on Bobcat T770

[MikePhua]

Bobcat’s Compact Loader Evolution
Bobcat Company, founded in 1947 in North Dakota, revolutionized compact equipment with the introduction of the skid-steer loader in the 1960s. The T770, part of Bobcat’s track loader lineup, was launched in the early 2010s as a Tier 4-compliant machine equipped with a diesel particulate filter (DPF) and selective catalytic reduction (SCR) system. With a rated operating capacity of 3,475 lbs and a 92 hp turbocharged diesel engine, the T770 quickly became a favorite in construction, landscaping, and agriculture. By 2020, Bobcat had sold over 100,000 units globally across its compact loader range.
Terminology Note

• SCR (Selective Catalytic Reduction): An emissions control system that injects diesel exhaust fluid (DEF) into the exhaust stream to reduce nitrogen oxide (NOx) emissions.
  
• DEF Line Heater: An electrically powered heating element embedded in DEF hoses to prevent freezing in cold climates.
  
• DCU (Diesel Control Unit): The electronic module that manages engine and aftertreatment functions, including DEF dosing and heater activation.
  

Common Fault Codes and Heater Circuit Issues
Operators of the Bobcat T770 have reported fault codes such as U004344-05, U004346-05, and U003511-24, all related to SCR heater circuits. These codes typically indicate a short circuit to ground (SCG) or an open load condition in the DEF line heaters. Specifically:

• U004344-05 refers to a fault in DEF Line Heater 4, which controls the backflow line.
  
• U004346-05 flags an issue in DEF Line Heater 5, associated with the suction line.
  
• U003511-24 points to a short in DCU Actuator Relay 2, which powers the heater circuits.
  

These faults often appear during cold starts or prolonged idling in sub-zero temperatures, when the DEF system attempts to activate heaters to maintain fluid flow.
Diagnostic Steps and Repair Strategy
To resolve these faults, technicians should follow a structured approach:

• Step 1: Park the machine in a safe area and power down the key switch.
  
• Step 2: Inspect all DEF line heater connections for corrosion, loose terminals, or damaged insulation.
  
• Step 3: Check the heater relays and fuses. Replace any blown fuses and test relays for continuity.
  
• Step 4: Use a multimeter to verify voltage at the DCU-related pins. Look for voltage drop or erratic readings.
  
• Step 5: Perform a continuity test on the heater circuits. Repair or replace any compromised harnesses.
  
• Step 6: If all components test correctly but faults persist, replace the DCU and recheck system status.
  

Field Experience and Lessons Learned
In New Jersey, a road mechanic reported a recurring U004344-05 fault on a T770 used for snow removal. After replacing the suction line heater twice, the issue remained. The root cause was traced to a cracked connector housing that allowed moisture ingress, shorting the circuit intermittently. Sealing the connector with dielectric grease and replacing the harness resolved the fault permanently.
Preventive Measures and Design Considerations
To reduce heater-related faults:

• Use weatherproof connectors and heat-resistant wiring during repairs.
  
• Install insulated DEF hoses in regions with prolonged freezing.
  
• Update software on the DCU to the latest version to improve fault detection logic.
  
• Log heater activation cycles and ambient temperatures to identify patterns.
  

Bobcat’s SCR system, while effective in emissions control, requires precise electrical integrity. The heaters are critical in maintaining DEF fluidity, especially in Tier 4 Final machines where inducement strategies may limit engine power if faults persist.
Conclusion
The Bobcat T770’s SCR heater system is vital for cold-weather reliability and emissions compliance. Fault codes like U004344-05 and U004346-05 often stem from wiring degradation, connector failure, or relay faults. With a methodical diagnostic approach and preventive upgrades, operators can maintain system integrity and avoid costly downtime. As emissions regulations tighten and DEF systems become more complex, understanding the nuances of heater circuits becomes essential for field technicians and fleet managers alike.