It sounds like your Yukon XL AT4 is not engaging its cruise grade braking or normal grade braking properly, which should help control speed on downhill sections even without adaptive cruise control. Both cruise grade braking and normal grade braking are standard features on GMC Yukons, including the AT4.
Here are a few potential reasons this might be happening:
1. Transmission Settings: Make sure the transmission is in the correct mode. Some driving modes might behave differently when it comes to grade braking. Check if you are in a mode like "Tow/Haul," which tends to make these features more aggressive.
2. Brake System Calibration: Sometimes, the grade braking system might need recalibration, especially if it's not engaging when it should. This could be something a dealer needs to look into.
3. Firmware or Software Updates: There may be updates to the vehicle's software that affect cruise control behavior. It’s worth checking with your dealer to see if there are any available updates that address this issue.
4. Brake Wear: Though less likely in a new vehicle, if the braking system is not functioning optimally (e.g., brake pads are wearing unevenly), it could affect how the grade braking is triggered.
It would be a good idea to have the dealer perform a diagnostic to check if the cruise control and grade braking systems are functioning as intended. They can also verify if any system updates are needed.
