Anti-lock braking system (ABS) (Audi 100 C4)

Precautionary measures

When working with the anti-lock braking system (ABS), the following precautions must be observed:

Electrical circuits should be checked using a LED tester. This tester can be used to check the hydraulic modulator, ABS control unit, wheel speed sensors and wires related to the ABS system. To avoid damage, it is necessary to check each time the brake lines or pressure regulators have been disconnected.

When disconnecting and connecting the ABS control unit plug, the ignition must be off.

When performing welding work on a vehicle, it is necessary to disconnect the ABS control unit plug.

When charging the battery or replacing the hydraulic modulator, it is necessary to disconnect the battery cables.

When drying a car in a chamber after painting, the ABS control unit must be removed if the temperature in the chamber exceeds 85°C and the car is in the chamber for more than two hours.

On vehicles equipped with ABS, a spare tire of a reduced size must not be used. The correct size tires and wheels must be used.

Do not drive your vehicle with the anti-lock brake tester connected.

Do not manufacture brake lines yourself. Use only factory parts. The union nuts of the lines are tightened to a torque of 15 Nm.

The hydraulic modulator cannot be repaired, only the relay can be replaced. If the hydraulic modulator breaks, it should be replaced with a new one.

Bleeding the Anti-lock Braking System

To bleed the pressure in the anti-lock braking system, it is recommended to use a special device. It is a set of two pressure gauges for measuring high and low pressure. The operation is performed as follows.

Raise and securely support the vehicle. Remove one of the front wheels.

Make sure the ignition is off. Remove the air bleed valve from the caliper.

Connect the special device and install the device to hold the brake pedal in the pressed position.

The brake pedal should be pressed 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. Such types of repairs include replacing pipes and/or discs, hoses, brake booster or master cylinder, wires and parking brake components.

This simple test will help you find out if the ABS system has been damaged during repairs. Follow these steps:

Turn on the ignition, the ABS system indicator light should come on.

Start the engine, the indicator light should go out.

With the engine running, turn off the ABS system. The indicator lamp should light.

Turn the ignition off and on again: the ABS warning light should go out, indicating that manual mode has been selected and the system has been reset.

Drive the vehicle at a speed of over 30 km/h, the ABS warning light should come on. The differential lock on the Quattro vehicle should not be engaged when performing this test.

Perform a test drive at a speed of over 30 km/h, turning on the ABS system. Check the system operation and vehicle stability.

Removal and installation of the hydraulic modulator

Disconnect the negative cable from the battery.

Release the pressure in the system.

Remove the modulator cover.

Remove the wiring harness holder.

Disconnect the hydraulic modulator from the mountings.

Disconnect the brake fluid reservoir from its mountings.

Unscrew the brake pipes. Mark the pipes so that they can be connected without getting mixed up. Immediately plug all holes with plugs.

Connect the pipes to the modulator, but do not tighten the nuts completely.

Install the modulator, then tighten the pipeline nuts. Install the tank and connect the wires.

Fill with DOT4 brake fluid. Remove air and check the system operation.

Removal and installation wheel speed sensors

Front wheels

Raise and securely support the vehicle.

Remove the wheel.

Find the sensor on the body side 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 performed 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 the bushing if necessary.

Press the sensor in with your hand until it stops.

Rear wheels

The rear wheel speed sensor is pressed into the rear wheel bearing housing by hand. The bushing is inserted into the hole on the front side of the housing and holds the speed sensor.

Raise and securely support the vehicle.

Remove the wheel.

Find 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 performed in the reverse order of removal. Before installing the mounting bushing, lubricate the hole in the rear wheel bearing housing with grease.

Press the bushing in if necessary. Press the sensor in by hand until it stops.

Removing and installing the acceleration switch

The ABS acceleration switch is located under the rear seat on the left side. It is a mercury switch that is activated when braking. If braking is performed with the ABS system, the switch provides additional stabilization during braking.

Turn off the ignition.

Loosen the mounting screws.

Disconnect the 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 electronic control unit of the ABS system is located on the left side under the rear seat.

Turn off the ignition.

Lift the rear seat and locate the electronic control unit. Unscrew the mounting nuts.

The ECU plug has locking tabs. Disconnect the plug by pressing the spring on the narrow end of the plug. When connecting, insert the plug's projection into the recess and press the plug. The plug should click into place.

Installation is carried out in the reverse order of removal.

(The original material is located on the website: «audimanual»)