2021-2025 Audi A8, Q7, Q8 Noises/Vibrations in Tight Bends TSB

Symptoms

Owners of affected Audi models may report experiencing "noises/vibrations in tight bends". Specifically, complaints include:

• "functionality -> irregular" related to transmission power distribution.
• "noises, vibrations -> noise" affecting the entire vehicle's driving performance and acceleration.
• "noises, vibrations -> rubbing" felt in the chassis, steering, or steering assist system.

These symptoms are often reproducible during workshop inspections and are noted to be more pronounced compared to similar vehicles.

Technical Background

This condition is linked to the self-locking center differential, which is integral to Audi's permanent all-wheel drive system, particularly the Torsen differential. This differential uses worm gears to achieve a high locking effect, allowing the axle with more traction to receive up to three times more torque. While this design provides superior driving dynamics, it can lead to a specific phenomenon in very tight bends.

When there's a significant speed difference between the front and rear axles, especially during high-speed turns, the friction between the worm gear flanks and axial thrust washers in the differential can become extremely high. This friction can cause a "stick-slip effect," leading to the noises and vibrations reported by owners. This stick-slip effect can also cause other vehicle components to resonate, generating secondary vibrations or hums.

Important Considerations

Audi emphasizes that the "noises/vibrations in tight bends" issue is considered a comfort-related complaint. The behavior is construction-related and no damage occurs to components as a result. This means the differential is functioning as designed, but the resulting noise is a characteristic of its operation under specific conditions.

Several external factors can exacerbate this condition:

• Wheels: Differences in tire tread depth, hardness, width, type (summer or winter), or manufacturer (AO code).
• Wheel Alignment: Additional tension in the drivetrain due to improper wheel alignment.
• Assembly Mountings: Possible noise transmission from assembly mountings.
• Loose or Incorrectly Installed Parts: Potential noise generation or increase due to loose parts, parts under tension, or incorrectly installed components in the vehicle's interior or chassis.

Repair Procedure Summary

For vehicles experiencing this condition, an improvement may be achieved by changing the gear oil in the transfer box. This procedure is detailed in the Workshop Manual. The degradation of friction modifiers in the original gear oil can lead to a more pronounced stick-slip effect over time and mileage. Replacing the gear oil replenishes these modifiers, significantly reducing the issue within approximately 125 miles of driving, depending on the driving profile.

Key steps and considerations for the repair include:

• Gearbox Oil Temperature: For accurate evaluation, the gearbox oil must be between 40°C and 70°C (104°F and 158°F). A workshop tester should be used to confirm the exact temperature.
• Transfer Box Gear Oil Change: This is the primary action to address the noises and vibrations. The full effect of this oil change will only become apparent after driving a distance of approximately 60 miles.
• Associated Labor Operations: The Technical Service Bulletin 2074857/2 outlines specific labor operations, including removing and installing the rear transmission mount (Service Number 3738 19xx), replacing the gear oil (Service Number 3990 5599), diagnostic time (GFF, Service Number 0150 0060), charging the battery (Service Number 2706 8950), and an extended road test after the measure (Service Number 0121 0199).

Warranty Information

This Technical Service Bulletin (TSB) 2074857/2 is informational if the vehicle is outside of its warranty period. For warranty claims, the following codes are relevant:

• Service Number: 3936
• Damage Code: 0020

All warranty claims must adhere to the Audi Warranty Policies and Procedures Manual and are subject to review by Audi Warranty.

NHTSA ID

11020353