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Bobcat T86 SCR/DEF Reductant Heater Circuit Diagnostic

Bobcat·T86·Bobcat D34·Advanced·1-3 hours·0 views

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During cold weather operations, the Bobcat T86 registers SCR or DEF (Diesel Exhaust Fluid) system faults. The engine starting cycle is slow and soon triggers DEF system derates, locking out auxiliary hydraulic activity.

Symptoms

Emission level error lights illuminated in orange/red
DEF freeze-prevent system alerts logged instantly on morning startup
Engine performance drops to limited-service RPM range

Possible Causes

Burned-out inline DEF heating element or supply line element failure
Blown defroster/reductant heater circuit power relay
Corroded pins inside the main DEF pump bottom harness plug

Diagnostic Procedure

1
Verify DEF Quality Concentration and Temperature
Use a refractometer to verify the DEF concentration is precisely 32.5 percent. A distorted fluid concentration can cause false heater performance codes. Check real-time DEF tank temperature readings in Bobcat Service Analyzer to see if the sensor is reporting realistic thermal values.
2
Inspect DEF Lines Electrical Connectors
Locate the heated DEF lines running from the DEF tank unit to the doser injector on the engine bracket. Locate the electrical terminal harness connections. Look for signs of dirt ingress, green corrosion, or damaged wire routing pins. Clean pins with electric aerosol spray.
3
Conduct DEF Heater Element Resistance Check
Unplug the harness at the DEF tank heater, suction block heater, and pressure hose line heaters. Use a digital multimeter on low Ohms scale to measure element resistance. If any of the heater elements show a complete open circuit (infinite resistance), that specific line or component must be replaced.
4
Verify DEF Heater Relay and Fuse Integrity
Locate the diesel emissions fuse center. Test continuity of the DEF heater fuses. Using a fused jumper wire or swapping with a known good cabin relay, verify the DEF heater supply relay switches on and delivers 12 Volts when commanded by the engine ECM during a cold start simulation.

Common Repairs

Replacing the damaged DEF freeze-resistant heated pressure line
Installing a replacement DEF tank internal heater element assembly
Replacing the DEF system primary power relay

Safety Precautions

Avoid contact with exhaust components. Let the system cool completely before handling.
Store and handle DEF carefully; if spilled on electrical connectors, wash immediately as it corrodes wiring pins quickly.

Required Tools

Digital Multimeter (DMM)
DEF Refractometer
Bobcat diagnostic hardware tool

Related Fault Codes

E5405 (DEF Line Heater Circuit Fault)E5406 (DEF Tank Heater Performance Malfunction)E5410 (SCR Controller Communication Dropout)

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