F. Electronics

Wheel speed sensors

Rod-shaped wheel speed sensors or impulse sensors are used for measuring wheel speeds. In the three-channel system with three wheel speed sensors installed in our vehicles (excluding vehicles with ASR), the wheel speed of each wheel is measured separately on the front axle.

Fig 1: Locating Wheel Speed Sensors

The left/right front wheel-speed sensors (L6/1 and L6/2) are installed in the steering knuckles.

Fig 2: Identifying Left/Right Front Wheel-Speed Sensors

The wheel speed sensor (L6) is located on the rear axle casing (67). The drive pinion serves to measure the mean speed of both rear wheels.

Fig 3: Identifying Wheel Speed Sensor And Rear Axle Casing

The wheel speed sensors sense wheel speeds by way of the rotor teeth. On the front axle, the rotor teeth (69a) are machined into the front wheel hub (69).

The wheel speed sensors (L6/1, L6/2) on the front axle have a diameter of 18 mm.

Fig 4: Identifying Front Wheel Hub Components Location

On the rear axle, the rotor is a toothed wheel (121) and pressed onto the drive pinion gear (120). For each rear axle ratio there is a corresponding gear wheel with a different number of teeth. Refer to □ RA 42-0714 Removal and installation of wheel speed sensor on rear axle" for assignment of rotor to rear axle.

Fig 5: Sectional View - Drive Pinion Gear

Wheel speed sensors consist of a magnetic core and a coil. Rotation of the toothed wheel or rotor, which is located at a given distance in relation to the wheel speed sensor, will change the magnetic field, so that an alternating voltage is induced in the coil. This alternating voltage changes its frequency in accordance with the wheel speed and the number of teeth, i.e. the frequency is proportional to the wheel speed.

Fig 6: Identifying Toothed Wheel (Rotor)

The cable (27) of the wheel speed sensor (L6/1 or L6/2) from the steering knuckle to the coaxial plug in the engine compartment is routed via the bracket (29, 32) in the wheelhouse.

Fig 7: Identifying Cable

Location - front axle models 124, 201

Fig 8: Identifying Steering Knuckle And Wheel Speed Sensor - Front Axle Models 124, 201

The wheel speed sensor on the rear axle is connected to the cable harness under the rear seat via the plug connection (L6x).

Location - rear axle models 124, 201

Fig 9: Identifying Plug Connection - Rear Axle Models 124, 201

ABS control unit

Model 124 (except 124.034), 129 (except 129.058/063), 201, 202, located on the right side of the engine compartment.

Fig 10: Identifying ABS Control Unit

The control unit installed in Models 124.034, 129.058/063, 140 and 202 includes a fault memory. On Models 124, 129 and 140 this is located in the module box. For information on diagnosis and test program, see Chassis Diagnosis Manual, Volume 2, Section 6.

Fig 11: Identifying ABS Control Unit

The control unit processes the signals from the wheel speed sensors and controls the valves in the hydraulic unit.

The entire signal conditioning and signal processing is digital.

The control unit is connected to the ABS main cable harness via a 35-pole plug connection. The electronic control unit is functionally divided into:

the signal conditioning section
the logic section and
the safety circuit.
fault memory (Model 124.034, 129.058/063, 140, 202)

Signal conditioning section

In the signal conditioning section the signals supplied by the wheel speed sensors are converted into a form suitable for use by the logic section.

To prevent problems while measuring the wheel speed, which may be caused by production tolerances and by movements in the steering knuckle, the input signals are filtered prior to use. Deceleration and acceleration signals obtained from the wheel speed signals are processed in the logic section.

Logic section

The logic section of the ABS control unit employs the following input signals for each controlled wheel or the controlled rear wheels:

wheel slip

wheel speed acceleration

wheel deceleration

Output signals from the logic section control the solenoid valves of the hydraulic unit. As a result, the following hydraulic functions can be generated in the brake calipers of the wheel brakes:

pressure build-up

pressure holding

pressure reduction

Safety circuit

The safety circuit has the job of recognizing faulty signals in the ABS control unit and faults outside the ABS control unit in the electrical installation. In addition, the safety circuit intervenes in the control sequence during extreme driving conditions such as aquaplaning. When a fault is recognized, the system should be switched off, a condition which is indicated to the driver by the ABS malfunction indicator lamp coming on.

The safety circuit continuously monitors the battery voltage. If the voltage is below specific requirements (11 V), the system is also switched off until the voltage returns to the specified range.

In addition to monitoring operation, the safety circuit also incorporates an active component, a test cycle with the designation BITE (B uilt I n T est E quipment).

The test cycle begins as soon as the wheel speed in all three speed channels is higher than 5-7 km/h. The cycle is activated by the wheel speed sensor voltage, which is monitored automatically at the same time. The test cycle itself checks parts of the monitoring circuit as well as the logic section. For this purpose, the electronic control unit is fed with the specific test sample signals to check whether the correct output signals are available.

Fault memory

The control unit installed on Models 124.034, 129.058/063, 140 and 202 incorporates a fault memory and is thus suitable for fault diagnosis. Recorded errors remain stored even when the battery cable is disconnected.

Relay with overvoltage protection

To guarantee the function of the ABS system under all operating conditions, the power supply is switched via a relay, which is activated by terminal 15 (ignition lock).

The overvoltage protection, which protects the electronic control units against overvoltages is connected in series between the battery and relay (ABS, CIS-E, ASD or ASR).

Fig 12: Identifying Relay With Overvoltage Protection (5 And 7 Pole)

The relay and the overvoltage protection form one unit. The overvoltage protection has a replaceable fuse. A 4-pole overvoltage protection was fitted on models 124, 201 up to December 1984. On vehicles as of January 1985, depending on the vehicle model and special equipment, a 5-, 7- or 9-pole overvoltage protection relay has been fitted.

On Models 124.034, 129.058/063 and 140 the electrical current is supplied by the basic module (N16/1). This is located along with the control unit (N30) in the module box.

Fig 13: Identifying Basic Module And Control Unit

ABS hydraulic unit (A7)

Two relays are located under the cover of the hydraulic unit. Relay (A7K2) activates the return pump and current flows to the solenoid valves via relay (A7K1).

A diode soldered into the plug socket (13a) controls the ABS malfunction indicator lamp in the instrument cluster when the multipin plug is disconnected from the electronic control unit. Phased in at the beginning of 1986, the diode is no longer located in the plug socket but in the solenoid valve relays. The new relay has 6 contact pins (previously 5).

Fig 14: Identifying Hydraulic Unit And Solenoid Valves Relay

In connection with the appropriate relay and fixing clamps the revised design of hydraulic unit can also be installed in vehicles produced earlier. It can be recognized on the outside by the inclined cover with the letters ABS (item A).

Fig 15: Identifying Plug Connectors

Electric wiring diagrams

For electric wiring diagrams and information on the layout of wires and plug connectors as well as relay and control unit locations, consult the manuals: "Electric wiring diagrams"