Code
U1026
HUMMER
U — Network/User
Loss of ATC Class 2 Communication
AI status
Completed
Completed
100%
Causes
- Open or short in Class 2 serial data wiring
- Poor power or ground to the ATC module
- Corroded or damaged connector at the transfer case control module
- Blown fuse supplying ATC or bus power
- Failed ATC (transfer case) control module
- Faulty BCM or other module on Class 2 bus pulling the network down
Symptoms
- Malfunction indicator lamp (MIL) or ABS/traction warning on
- 4WD/AWD functionality disabled or transfer case stuck in a default mode
- No communication with ATC module shown by scan tool
- Related drivability/traction systems degraded or in limp/default state
- Intermittent or permanent loss of features controlled by the transfer case
What to check
- Read and record all stored and pending DTCs and freeze frame data
- Attempt to communicate with ATC module using a factory scanner; note response/no response
- Verify battery voltage (key on, engine off and cranking) and try after a fully charged battery
- Inspect fuses and fusible links related to transfer case and Class 2 power
- Visually inspect connectors and wiring at the ATC module and along harness for damage, corrosion, rodent damage, or pin push-out
- Check module grounds for cleanliness, tightness, and continuity to chassis
Signal parameters
- Class 2 serial data — single-wire digital communication on the vehicle data bus
- Idle bus level typically near battery voltage with periodic digital pulses when modules communicate
- When functioning, repeated messages and activity should be visible on a scan tool or oscilloscope with ignition on
- A hard short to ground will pull the bus low and stop communication; a short to battery may hold it high and corrupt messages
- Intermittent wiring faults produce sporadic data bursts or dropouts observable with a scope
Diagnostic algorithm
- Verify customer complaint and note when/where issue occurs. Retrieve and record all DTCs and freeze frame data.
- Clear codes and attempt to reproduce. If code returns, continue diagnosis.
- Attempt to communicate with the ATC module via a diagnostic scan tool. If the module does not respond, suspect wiring or module power/ground first.
- Verify supply voltage and ground at the ATC module connector with ignition ON. Check related fuses and fusible links.
- Visually and physically inspect connectors and wiring from the ATC module to the main harness and GST/BCM area. Repair any damaged insulation, corroded pins, or loose terminals.
- Check continuity of the Class 2 data circuit between the ATC module connector and the BCM/diagnostic connector. Also check for shorts to ground or battery.
- Use an oscilloscope or high-quality scan tool to monitor Class 2 bus activity at multiple points. Compare signals at the ATC connector to another known-good module location.
- Disconnect suspect modules (except required network masters) one at a time to see if the bus restores, isolating a device that may be loading the bus.
- If wiring, connectors, and power/grounds are good and the bus is healthy at the harness but the ATC still doesn’t communicate, consider replacing or bench-testing the ATC module per manufacturer procedures.
- After repairs, clear codes and verify normal network communication, proper transfer case operation, and that the code does not return.
Likely causes
- Damaged connector or wiring to transfer case (common near chassis/tunnel area)
- ATC module not powered due to blown fuse or bad ground
- Class 2 bus shorted to ground or battery through damaged insulation
- Failed ATC module that no longer transmits messages
- Intermittent connection (corrosion, loose pin) causing sporadic loss of messages
Fault status
Status
Loss of communication with the Automatic Transfer Case (ATC) over the Class 2 serial data bus. ATC messages not received.
Repair difficulty: Medium
Diagnostic time: 1.0 - 3.0 hours
Repair manuals
Manual library for HUMMER
138
Browse 138 HUMMER manuals: repair procedures, diagnostics, wiring diagrams, component locations, service data and Labor Times by year, model and trim.
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HUMMER: 1993
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