Precautionary measures
When working with anti-lock braking system (ABS) the following precautions should be observed:
Checking electrical circuits should be done using an LED probe. This probe can be used to test the hydraulic modulator, ABS control unit, wheel speed sensors and wires related to the ABS system. To avoid damage, a check must be carried out each time the brake lines or pressure regulators have been disconnected.
When disconnecting and connecting the ABS control unit connector. the ignition must be off.
When performing welding work on the vehicle, the ABS control unit must be disconnected.
When charging the battery or replacing the hydraulic modulator, disconnect the battery wires.
When drying the car in the chamber after painting, the ABS control unit must be removed if the temperature in the chamber exceeds 85°C and the car has been in the chamber for more than two hours.
On vehicles equipped with ABS, an oversized spare tire cannot be used. Tires and wheels of the correct size should be used.
The vehicle must not be driven with an anti-lock brake tester connected.
Do not fabricate brake lines yourself. Only factory parts should be used. The union nuts of the pipelines are tightened with a torque of 15 Nm.
The hydraulic modulator cannot be repaired, only the relay can be replaced. If the hydraulic modulator breaks down, it should be replaced with a new one.
Bleeding pressure in the anti-lock system
To relieve pressure in the anti-lock system, it is recommended to use a special tool. It is a set of two manometers for measuring high and low pressures. The operation is performed as follows.
Raise and support the vehicle securely. Remove one of the front wheels.
Make sure the ignition is off. Remove the bleed valve from the caliper.
Connect the special tool and install the tool to hold the brake pedal in the depressed position.
The brake pedal must be depressed until the instrument shows that the pressure in the system has dropped.
System check
After performing repair work on the brakes, it is necessary to test the operation of the ABS system. These repairs include replacement of lines and/or discs, hoses, brake booster or master cylinder, wires, and parking brake parts.
This simple test allows you to find out if the ABS system was damaged during the repair. Perform the following steps:
Switch on the ignition, the ABS warning light should come on.
Start the engine, the signal lamp should go out.
With the engine running, turn off the ABS system. the signal lamp should light up.
Switch the ignition off and on again: the ABS warning light should turn off, indicating that manual mode has been selected and the system has been reset.
Drive the vehicle at a speed of more than 30 km/h, the ABS warning light should come on. The differential lock on the Quattro should not be on during this test.
Carry out a test drive at a speed of more than 30 km / h, while turning on the ABS system. Check system operation and vehicle stability.
Removal and installation of the hydraulic modulator
Disconnect the negative cable from the battery.
Relieve pressure in the system.
Remove the modulator cover.
Remove the wiring harness holder.
Detach hydraulic modulator from mountings.
Disconnect the brake fluid reservoir from the mountings.
Unscrew the brake lines. Mark the pipelines so that they can be connected without confusion. Plug all openings with plugs immediately.
Connect the pipes to the modulator, but do not fully tighten the nuts.
Install the modulator, then tighten the pipe nuts. Install tank and connect wires.
Top up with DOT4 brake fluid. Remove air and check system operation.
Removing and installing wheel speed sensors
Front wheels
Raise and support the vehicle securely.
Remove wheel.
Find the sensor on the side of the body and pull it out.
The left and right sensors are the same.
To disconnect the wire, remove the lower engine cover.
Disconnect the plug and remove the sensor.
Installation is carried out in the reverse order of removal. Before installing the mounting sleeve, the hole in the wheel bearing housing must be filled with grease.
Press in bushing if necessary.
Press the sensor all the way in by hand.
Rear wheels
The rear wheel speed sensor is pressed by hand into the rear wheel bearing housing. The bushing is inserted into the hole on the front side of the housing and holds the speed sensor.
Raise and support the vehicle securely.
Remove wheel.
Locate the sensor on the front side of the rear wheel bearing housing and pull it out. The left and right sensors are the same. Disconnect the plug and remove the sensor.
Installation is carried out in the reverse order of removal. Before installing the mounting sleeve, grease the hole in the rear wheel bearing housing.
Press in bushing if necessary. Push the sensor in by hand until it stops.
Removal and installation of the switch triggered by acceleration
The ABS acceleration switch is located under the rear seat on the left side. This is a mercury switch that works when braking. If braking is carried out using the ABS system. the switch provides additional stabilization during braking.
Switch off the ignition.
Loosen the fastening screws.
Disconnect wire.
When installing, make sure that the arrow on the cover points forward in the direction of travel.
Removal and installation of the electronic control unit
The ABS electronic control unit is located on the left side under the rear seat.
Switch off the ignition.
Raise the rear seat and find the electronic control unit. Loosen the fastening nuts.
The plug of the electronic control unit has locking tabs. Disconnect the plug by pressing the spring on the narrow end of the plug. When connecting, insert the protrusion of the plug into the recess and press the plug. The plug should click into place.
Installation is made in an order, the return to removal.
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