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Hydraulic Control Assembly - Antilock Brakes: Service and Repair


WM 453019 Removing and installing hydraulic unit

Tools





Information

The brake components (brake booster, brake master cylinder, hydraulic unit) are installed under the luggage compartment cover -2-.





Luggage compartment cover

Remove hydraulic unit -D- together with the support and the four brake lines, which are fitted on the intermediate piece -E-.





Brake components







Preliminary work

1. Remove cover over hydraulic unit (front luggage compartment cover) -> 703019 Removing and installing front luggage compartment cover 70301900 Removing and installing front luggage compartment cover.

There are two types of luggage compartment covers -1- (drawings below).





Removing luggage compartment cover





Removing luggage compartment cover

Removing hydraulic unit

1. Remove hydraulic unit -D-.
1.1. Hold down brake pedal with pedal holder so that the brake fluid (in the reservoir) does not drain completely.
1.2. Loosen the brake lines -1- on the brake master cylinder.
1.3. Loosen the brake lines -2- from the hydraulic unit -D- on the intermediate piece -E-.
1.4. Close the brake lines and connections with suitable plugs.
1.5. Release control unit connector on the hydraulic unit and pull it off. Cover the plug connection and connector (to shield them from brake fluid or water).
1.6. Loosen support on hydraulic unit -Arrows- (3-point mount).
1.7. Remove support/hydraulic unit assembly.





Hydraulic unit

Installing hydraulic unit

Information

First connect the electric plug connections on the hydraulic unit. This will protect the plug connections from brake fluid or water.

After bleeding, check the assignment of the brake lines to the correct brake calipers. Carry out a swap test using the PIWIS Tester.

1. Install hydraulic unit -D-. Observe tightening torques -> 4700TW Tightening torques for the brake hydraulic system.





Hydraulic unit

1.1. Insert hydraulic unit with support and tighten fastening nuts -arrows- (3-point mount).
1.2. Connect control unit connector to hydraulic unit and lock it.
1.3. Fit brake lines -2- in correct position. Observe marking on the hydraulic unit -D- and intermediate piece -E-. RL = rear left/RR = rear right/FR = front right/FL = front left.
1.4. Fit the brake lines -1- on the hydraulic unit -D- and then at the correct position on the brake master cylinder. HZ 1 = Brake master cylinder push-rod circuit (close to fastening flange). HZ 2 = Intermediate piston circuit.
1.5. Fit the brake line -1- on the hydraulic unit first and then on the brake master cylinder and tighten to 25 Nm/19 ftlb. (increased from 20 Nm/15 ftlb.).





Brake lines/hydraulic unit

When tightening the brake lines -1- on the brake master cylinder (25 Nm/19 ftlb.), use special tool retaining device 9763 to counter at the crimping point of the hose (close to the rigid line) -3-. This prevents the hoses from twisting away from the new type of brake lines -1- that are used (modified material at the cap screws/legal requirement).

Information

First bleed the brake system in the normal way - with the bleeding device at the reservoir and approx. 2.0 bar pressure (primary circuit).

Then, use the PIWIS Tester to bleed the brakes as well (secondary circuit). Only necessary if the hydraulic unit was replaced and if brake pedal travel is excessive after bleeding in the normal way (primary circuit). The process is menu-driven. Always observe the specified sequence rear left, front left, front right, rear right.

To bleed the secondary circuit, the bleeding device remains connected to the brake fluid reservoir. Bleeding pressure is approx. 2.0 bar.

Throughout the entire bleeding process, the brake pedal must be operated cyclically (keep depressed for 2 to 3 seconds, then release slowly). After operating the pedal for the last time, hold the brake pedal down and close the bleeder valve.

2. Bleed the brake system and adjust the brake-fluid level. -> 470107 Bleeding the brake system Service and Repair.
3. When replacing the hydraulic unit, code the control unit (manual transmission/Tiptronic).
4. Carry out the following tests:
- Check the system for leaks.
- Read out and erase the fault memory of the ABS/PSM control unit using the PIWIS Tester. Carry out a static test as well.
- Check electric and hydraulic lines for incorrect assignment using the PIWIS Tester. This test checks that the brake lines/solenoid valves are assigned to the corresponding brake calipers.
- Then start the engine. The ABS/PSM-related lights in the instrument cluster must go out when the engine is running.
- Carry out a function check on the brakes. Then take a short test drive and perform at least one ABS control operation, taking the road conditions into consideration. Read out the ABS/PSM fault memory again. There must not be any stored faults.

Subsequent work

Install cover over hydraulic unit (luggage compartment cover) -1- , -> 703019 Removing and installing front luggage compartment cover 70301900 Removing and installing front luggage compartment cover.





Installing luggage compartment cover





Installing luggage compartment cover