Code
U1168
HUMMER
U — Network/User
Loss Of TTM Communications
AI status
Completed
Completed
100%
Causes
- Blown fuse or lost power to the TTM module
- Poor ground or high resistance ground at the TTM
- Damaged, corroded, or disconnected connector or wiring to the TTM
- Open or short on the CAN/high-speed data bus (CANH/CANL)
- Failed TTM module or internal module fault
- Faulty termination resistor or improper bus termination
Symptoms
- Diagnostic scanner cannot communicate with TTM module
- Transfer case will not shift or 4WD functions unavailable
- Warning/indicator for 4WD, traction, or driveline displayed on cluster
- Intermittent or permanent loss of related drive-mode functionality
- Multiple U-codes present for modules that communicate with the TTM
What to check
- Read and record all stored codes and freeze frame with a capable scan tool
- Check battery voltage (key ON, engine OFF) — should be ~12.4–12.8 V
- Inspect fuses and power feeds for the TTM
- Visually inspect TTM connector and wiring for corrosion, damage, or looseness
- Check for other U-codes indicating broader network problems
- Backprobe TTM power and ground pins to verify proper supply and good ground
Signal parameters
- Battery supply to TTM (Key ON): ~12–14 V
- Ground: low resistance to chassis (
- CAN idle voltages: CANH ≈ 2.5V, CANL ≈ 2.5V (differential ~0V idle); recessive ~2.5V each
- CAN dominant state: CANH ≈ 3.5V, CANL ≈ 1.5V (approx differential 2.0V)
- Termination resistance: ~60 ohms measured between CANH and CANL at two-wire bus
Diagnostic algorithm
- Verify vehicle battery voltage and charging system health. Recharge or replace battery if low.
- Use a scan tool to capture U1168 and any related U-codes. Note when the code sets (key on, driving, after shock, etc.).
- Attempt to communicate with the TTM. If the module is not detected, confirm module identity and wiring location in service literature.
- Inspect TTM connector and harness for corrosion, bent pins, water intrusion, or physical damage. Repair or replace damaged connectors or wiring.
- Check relevant fuses and power feeds to the TTM. Repair open fuses or wiring faults.
- Backprobe the TTM power and ground pins with the ignition ON. Verify proper voltages and good ground continuity to chassis.
- With ignition ON, measure CANH and CANL at the TTM connector. Compare to expected idle/dominant values. If voltages are abnormal, trace wiring for shorts to battery/ground or opens.
- Measure termination resistance across CANH–CANL (approx 60 ohms). Remove connectors as necessary to isolate and locate short/termination fault.
- If wiring and power/ground are good but no communication, disconnect downstream nodes to isolate which segment is failing (if network has branches).
- If diagnostics point to the TTM as failed, verify latest software/calibration and reprogram or replace the module per manufacturer procedures. Clear codes and retest.
- After repair, road test to confirm communication is restored and no related codes reappear.
Likely causes
- Blown fuse or power feed to TTM
- Corroded connector or pinback at the TTM
- Open or shorted CAN bus wiring near the transfer case area
- Failed TTM module requiring replacement/programming
- Weak battery or charging system issue causing module dropout
Fault status
Status
Loss of communications with the TTM module detected on the vehicle network. Transfer case/torque transfer messages may be unavailable; related drive functions or warnings may be present. Check power, ground, connectors, and the CAN data bus.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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Repair manuals
Manual library for HUMMER
138
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