Antilock Brakes / Traction Control Systems: Description and Operation

Fig. 1 Anti-lock Brake System Wiring Circuit. 1991-92 300 Series:

The Anti-Lock Brake System (ABS) consists of a control unit, hydraulic unit, speed sensors and wiring harness. Each speed sensor has a gear wheel which runs past the permanently magnetized edge of the speed sensor installed in the wheel hub. The rotary motion of the wheels is recorded by inductive sensors and an electric signal is sent to the electronic control unit.
This control unit inputs acceleration, deceleration and slip factors from the electronic speed sensors, which are proportional to wheel velocity. The unit then issues control commands to the electronically operated valves in the hydraulic unit. If the control unit detects a defect in the wiring harness or system electronics, the monitoring circuit will switch off the ABS, which assures normal use of the brake system during an ABS malfunction. The ABS indicator lamp will then go On to notify the driver of a system malfunction.
The hydraulic unit, located in the engine compartment, is integrated with the rest of the conventional brake system. The unit has three-way valves which control hydraulic pressure to the wheel cylinders. Three pressure phases, pressure holding, pressure build-up and pressure drop adapt themselves to road conditions based on information from the control unit.
The electronically driven return delivery pump returns brake fluid from the wheel cylinders while dropping pressure to the appropriate brake circuit. The pump is designed as a two-piston pump so the circuits of the dual brake circuit remain separated.