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Home / DTC / U0102 — Lost Communication with Transfer Case Control Module

U0102 — Lost Communication with Transfer Case Control Module

Detailed page for trouble code U0102.

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U0102

MERCEDES-BENZ U — Network/User

Lost Communication with Transfer Case Control Module

Brand: MERCEDES-BENZ
Type: U — Network/User
AI status
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ready
Completed 100%
Page language: EN

Causes

  • Blown fuse or relay supplying power to the transfer case control module
  • Poor or missing ground at the transfer case control module
  • Open, short or high-resistance wiring in CAN bus or power circuits
  • Corroded, loose or damaged connector at the transfer case control module
  • Failed transfer case control module
  • CAN bus termination fault or short between CAN_H and CAN_L

Symptoms

  • Transfer case/4MATIC/4WD warning lamp illuminated or flashing
  • Loss of 4WD transfer case functions or inability to change drive modes
  • Vehicle may enter limp or restricted drive mode for driveline systems
  • Related traction/ESP/ABS warnings if networks share bus
  • Diagnostic tool reports no response from Transfer Case Control Module or communication timeout
  • Intermittent operation of transfer case functions

What to check

  • Verify battery voltage (key on and cranking) is within 11–14 V
  • Check transfer case module fuses and relays in fuse boxes (engine and passenger compartments)
  • Perform a system scan for other U-codes on the same CAN network
  • Attempt communication with the transfer case module using factory-level diagnostic tool (e.g., XENTRY)
  • Visually inspect connectors and wiring at the transfer case module for corrosion, water ingress, bent pins, or damage
  • Check for proper ground connection continuity from module ground to chassis/engine

Signal parameters

  • Battery / supply pin: ~11–14 V with ignition ON (nominal 12 V)
  • Module ground: near 0 Ω continuity to chassis ground
  • CAN bus idle voltages (recessive): CAN_H ≈ 2.5 V (typically 2.3–2.7 V), CAN_L ≈ 2.5 V (typically 2.3–2.7 V)
  • CAN dominant state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (when messages are transmitted)
  • Differential voltage during dominant state: ~2 V
  • Bus termination: ~60 ohms measured between CAN_H and CAN_L (two 120Ω terminators in parallel)

Diagnostic algorithm

  1. Confirm the fault: connect a factory-level diagnostic tool and verify U0102 and any related U-codes. Note whether the fault is current or intermittent.
  2. Check vehicle battery state and charging system. Low voltage can cause modules to sleep or not respond.
  3. Inspect fuses/relays that supply the transfer case module; replace any blown fuses and re-test communication.
  4. Visually inspect the transfer case module connector and wiring harness for corrosion, bent pins, water entry or mechanical damage. Repair or replace damaged connectors/wires.
  5. With the ignition ON, verify module power and ground at the module connector using a DMM. If power or ground missing, trace wiring back to the source and repair.
  6. Measure CAN_H and CAN_L voltages at the module connector with ignition ON. Compare with expected idle values (≈2.5 V). If voltages are out of range, isolate and repair wiring shorts/opens.
  7. Check termination resistance between CAN_H and CAN_L (~60 ohms). If abnormal, inspect for missing or shorted terminators or a short to ground/battery on the bus.
  8. If CAN voltages and wiring are OK but the module still does not respond, inspect other modules on the same CAN network for bus flooding or causing errors. Disconnect suspect modules one at a time (follow safety and manufacturer procedures) and re-check communication.
  9. If wiring and bus are good, consider a failed transfer case control module. Before replacing, verify availability of required coding/programming and immobilizer/security restrictions for Mercedes-Benz.
  10. If module is replaced, program/cod e it per manufacturer specifications and clear stored codes. Re-test system for proper communications and operation.
  11. If intermittent, perform wiggle tests on harness and reproduce conditions (road test) to isolate intermittent wiring faults. Document and repair accordingly.

Likely causes

  • Damaged/loose connector or wiring to the transfer case module
  • Blown fuse, missing power or faulty ground at the module
  • CAN bus wiring fault (open, short to ground or battery, poor termination)
  • Failed transfer case control module

Fault status

⚠️ Status
No response from Transfer Case Control Module on CAN network; CAN messages absent or module not present/responding to diagnostic requests.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 1.0-4.0 hours

Similar codes

U0100 U0101 U0103 U0121 U0140
856

Browse 856 MERCEDES-BENZ manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.

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