Table of contents: General information ↓ Features of ceramic brake discs ↓ Security measures ↓ Notes on repair work on the ABS/ESP… ↓
General information
Depending on the vehicle modification, different brake mechanisms may be installed (see Specifications). The type of brakes installed can be determined by the PR codes on the vehicle data sticker (see illustration 1.0), which is located in the spare wheel niche, as well as in the service book (see Introduction).
1.0. Vehicle data sticker:
A - Model (Q7);
B - Front brakes (1LF);
C - Rear brakes (1KF).
PR codes indicate the required combination of brake calipers, discs/drums and pads.
The brake system is divided diagonally into two independent hydraulic circuits and consists of a master brake cylinder (MBC) with a vacuum booster, a vacuum hose, brake hydraulic lines and brake mechanisms. If one of the circuits fails (for example, as a result of a breach in the seal) the second one continues to function in normal mode, providing adequate braking of the car. Fluid pressure in both circuits is generated by a tandem-design master cylinder (MC). The MC is activated when the brake pedal is depressed. The brake fluid reservoir is attached to the master cylinder.
The brake booster accumulates part of the vacuum created in the engine intake manifold. Additional vacuum for reliable operation of the vacuum booster is created by a vacuum pump.
The parking brake is cable-operated from an additional pedal, acting on the rear brakes.
The procedures for bleeding and checking the brake system, as well as replacing the brake fluid, are described in Sections 8-9 of Chapter 1.
When checking the brake system, the wheels are driven by a single-axis roller dynamometer at a maximum speed of 6 km/h (otherwise, the Torsen differential may be damaged; if the rollers are not engaged simultaneously, the electronic differential lock may be activated). The speed of rotation of the wheels must be the same. During the check, the transmission must be in neutral position.
ABS or ESP systems with various additional functions are installed as standard. To monitor the operation of the brake system and the ABS/ESP system, the corresponding indicator lamps in the instrument cluster are used (see chapter "Controls and operating techniques").
Features of ceramic brake discs
The features of the ceramic brake disc are shown in Figure 1.1a.
1.1a. Features of a ceramic brake disc:
1 - Cooling channel jumpers;
2 - Friction layer;
3 - Load-bearing material.
The friction ring and the brake disc hub are connected to each other with bolts that must not be loosened. Already during the production of brake discs, surface cracks form in the bridges of the cooling channels (see illustration 1.1b) and relaxation cracks of different lengths on the friction surface (see illustration 1.1c).
1.1b. Surface cracks on the cooling channel bridges.
1.1c. Relaxation cracks.
These cracks do not indicate a faulty disc.
Replacing both discs of one axle is required when:
- Detection of cracks in the area of the bolted connection extending to the friction surface (see illustration 1.1d);
11.1d. Cracks in the area of the bolted connection of the friction ring to the disc hub.
- If there are chips on the edges of the disc (see illustration 1.1e) exceed 2 mm in width/depth, or 10 mm in length, or there are more than three chips on one disc;
1.1e. A chip on the edge of the disc.
- If there are pits on the friction surface with an area of more than 1 cm² (see illustration 1.1f);
1.1f. Chip on the friction surface.
- If there are cracks from the friction surface to the cooling channel or through it (see illustration 1.1g);
1.1g. Cracks from the friction surface to the cooling channel or through it.
- If at least one wear indicator appears (see illustration 1.1h): Each disk has three indicators on each side, spaced 120° apart.
11.1h. Wear indicator.
Description of the ceramic disc markings is shown in Figure 1.2.
1.2. Ceramic disc marking:
1 - Direction of rotation;
2 - Supplier;
3 - Serial number;
4 - Audi part number (depends on the model);
5 - Production date;
6 - Minimum allowable disc thickness (depends on the model);
7 - Weight of a new disc assembled with a hub (depends on the model).
Security measures
Brake fluid is a highly toxic and chemically aggressive compound and when in contact with body panels it destroys the paintwork.
Dust generated during the wear of brake pads may contain asbestos, which is harmful to human health. Do not inhale it under any circumstances when cleaning brake mechanisms!
Working with the brake system requires special cleanliness and strict adherence to instructions. If you don't have the necessary experience, it's advisable to contact a service station.
Note: When driving on wet roads, periodically press the brake pedal to remove moisture from the brake discs; As the wheel rotates, moisture is removed from the brake discs by centrifugal force, but a film of silicone, rubber abrasion products, grease, and other contaminants remain, reducing braking efficiency!
Corroded disc brakes create a shaking effect when braking that does not disappear over time. In this case, the disks should be replaced.
Dirt burning onto the surface of the brake pads causes grooves to form on the surface of the brake discs, which reduces braking efficiency.
Notes on repair work on the ABS/ESP system
When performing any repair work on the ABS/ESP, the first step is to determine the cause of the malfunction using a diagnostic tool connected to the vehicle's diagnostic connector (see Chapter 5).
Observe general safety precautions when working with brake fluid. After performing work in which the brake hydraulic circuit was open, air should be removed from the hydraulic drive (pump it up, see Chapter 1). Maintain impeccable cleanliness, never use additional parts containing mineral oils.
Clean the connections and surrounding area thoroughly before disconnecting the brake lines. Do not use aggressive detergents, gasoline, solvents, etc.
Place the removed parts on a clean surface and cover them.
After separating the control unit from the hydraulic modulator, use transport protection for the connector contacts.
If repairs cannot be made immediately, cover or seal exposed components.
Use only lint-free cloth.
Unpack spare parts only immediately before installation. Do not work with compressed air or move the vehicle when the system is open.
When painting a vehicle, the ABS/ESP electronic control unit may be exposed to a maximum temperature of 95°C for a short period only. Exposure to a temperature of 85°C is allowed for approximately 2 hours.
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