For posterity - internet research - Re-set Transmission Control Module (TCM) (2008 Yukon L76/4L70E):
Method #1:
- Disconnect both battery poles (negative first);
- Leave disconnected for 30 minutes minimum;
- Reconnect battery poles;
- Start;
- Check dash indications;
- Road test for smooth shifting.
Method #2:
- Disconnect both battery poles (negative first);
- Disconnect TCM (blue multi-pin connector driver side of ENG); leave disconnected for 30 minutes minimum; reconnect TCM.
- Reconnect battery poles;
- Start;
- Check dash indications;
- Road test for smooth shifting.
Method #3; Reset TCM:
- Ignition switch to ON (no start);
- Dash light ON
- Press full down on gas pedal; hold full down for minimum 15 seconds;
- Ignition switch to OFF;
- Release gas pedal;
- Wait approx. 5 minutes;
- Start ENG;
- Check dash indications;
- Road test for smooth shifting;
- Owner: 20-30 minute drive for TCM to re-learn driver habits;
Method #4: R&R TCM:
- Preferred: GM/ACDelco #24252114; approx. $148 + $25 core; Other: Dorman #599-120; approx. $150 + $50
core;
- Start ENG;
- Check dash indications;
- Road test for smooth shifting;
- Owner: 20-30 minute drive for TCM to re-learn driver habits.
Method #1:
- Disconnect both battery poles (negative first);
- Leave disconnected for 30 minutes minimum;
- Reconnect battery poles;
- Start;
- Check dash indications;
- Road test for smooth shifting.
Method #2:
- Disconnect both battery poles (negative first);
- Disconnect TCM (blue multi-pin connector driver side of ENG); leave disconnected for 30 minutes minimum; reconnect TCM.
- Reconnect battery poles;
- Start;
- Check dash indications;
- Road test for smooth shifting.
Method #3; Reset TCM:
- Ignition switch to ON (no start);
- Dash light ON
- Press full down on gas pedal; hold full down for minimum 15 seconds;
- Ignition switch to OFF;
- Release gas pedal;
- Wait approx. 5 minutes;
- Start ENG;
- Check dash indications;
- Road test for smooth shifting;
- Owner: 20-30 minute drive for TCM to re-learn driver habits;
Method #4: R&R TCM:
- Preferred: GM/ACDelco #24252114; approx. $148 + $25 core; Other: Dorman #599-120; approx. $150 + $50
core;
- Start ENG;
- Check dash indications;
- Road test for smooth shifting;
- Owner: 20-30 minute drive for TCM to re-learn driver habits.