GM (TCCM) Transfer Case Computer Module

GM (TCCM) Transfer Case Computer Module Post-Repair Troubleshooting Guide

Important: Use this guide if you reinstalled your repaired GM Transfer Case Computer Module (TCCM) and the problem is still happening.
Important: Please do not open/disassemble the module. Opening the module can damage it and may void warranty coverage.

Common symptoms this guide helps with:

• 4WD system does not engage (stays in 2HI / won’t shift to AUTO/4HI/4LO)
• No communication with the TCCM
• 4WD selector lights flashing or not matching the selected mode

Step 1: Confirm the exact symptom

• Does the truck shift modes at all (2HI ↔ AUTO ↔ 4HI ↔ 4LO)?
• Are the 4WD buttons responsive?
• Do the mode lights flash, go out, or show the wrong mode?
• Is the issue constant or intermittent (only cold, only after rain, only after driving)?

Step 2: Battery / voltage check

• Low voltage during crank can cause TCCM resets and shift failures.
• If the battery is weak or the vehicle has starting/charging issues, correct that first.

Step 3: Check the 4WD/TCCM fuses (do this before anything else)

• Use the fuse box legend and check all fuses related to:
  • TCCM / 4WD / Transfer Case
  • IGN power feeds
• If a fuse is blown, don’t keep replacing it—inspect wiring to the transfer case and front axle actuator for shorts.

Step 4: Key cycle reset

• Key OFF, wait 60 seconds.
• Key ON (engine can stay off), do not press buttons for 10 seconds.
• Then try selecting a mode once (AUTO/4HI), wait and watch the lights.

Step 5: Verify the 4WD switch operation (dash selector)

• If the buttons don’t respond or feel dead:
  • Confirm the connector at the switch is seated.
  • Check for sticky/spilled liquid damage around the switch area.
• A failed switch can mimic a failed TCCM.

Step 6: Inspect the TCCM connector and mounting area

• Key OFF.
• Unplug/reseat the TCCM connector until the lock fully clicks.
• Inspect for:
  • Corrosion/green tint on pins
  • Moisture/water marks
  • Bent or pushed-back pins
• If corrosion is present, the vehicle-side connector/harness must be repaired or the problem can continue.

Step 7: Scan for codes (critical step)

• Perform a full vehicle scan and record codes from:
  • TCCM (Transfer Case Control Module)
  • ABS / BCM
  • PCM
• If there are multiple communication codes across modules, focus on vehicle network/power/ground issues first.

Step 8: Clear codes and re-test shifting

• Clear codes.
• With vehicle stopped:
  • Try 2HI → AUTO → 4HI (wait for lights to finish flashing each time)
• For 4LO testing:
  • Stop vehicle, transmission in Neutral, foot on brake
  • Command 4LO and wait (do not “spam” the buttons)

Step 9: If 4WD does NOT engage but the TCCM communicates
This usually points to a vehicle-side component, not the TCCM:

• Transfer case shift motor/encoder issue
• Wiring to the transfer case (chafing, broken wires)
• Front axle actuator issue (won’t lock in)
• Ground or power drop under load

Step 10: If the scan tool shows NO communication with the TCCM
Work the basics first:

• Recheck Step 3 (fuses)
• Recheck Step 6 (connector/pins)
• Check for water intrusion at the module location
• Inspect harness for rub-through near the module and along the chassis
  No-comm is commonly caused by power/ground or harness faults on the vehicle.

Step 11: If the issue is intermittent
Intermittent 4WD problems are often caused by:

• Moisture intrusion/corrosion at connectors
• Harness movement (broken wire inside insulation)
• Low voltage events
• Transfer case motor/encoder starting to fail
  Try to note the trigger (rain, cold start, bumps, long drive) and inspect wiring/connectors accordingly.

Step 12: What to send UpFix if issues continue

• VIN
• Exact symptom + which modes fail (AUTO/4HI/4LO)
• Photos of any flashing mode lights or dash messages
• Full code list (module + code numbers)
• Confirm whether the TCCM communicates on a scan tool