Anti-Lock Brake System (ABS) & Traction Control System (TRACS)
Control Module
The control module is located in the engine compartment, and handles both ABS and TRACS. The control module processes the signals from the different sensors and controls the electromagnetic hydraulic valves in the hydraulic system. The control module also works the hydraulic pump via a combination relay.
The control module has two microprocessors which process the signals from the different sensors in the car. The microprocessors do all their calculations in parallel and then calculate the result. If the results do not agree because of an internal fault, this is indicated by a DTC for the control module.
The control unit also chocks all input and output signals via built-in diagnostics functions. If the control module detects a fault, the ABS system will be partly or completely disabled or disconnected, depending on how serious the fault is. If the fault is serious enough to knock the whole ABS system out, the ABS warning indicator also comes on.
To minimize the effect of a fault on the ABS system disabling is limited to the front circuit (e.g. if there is any problem with the signal from the front wheel sensors) provided the road speed is less than 40 km/h (25 mph). If the speed is over 40 km/h (25 mph), the ABS system cuts out completely. If the speed is under 20 km/h (12 mph), no DTC is posted and no warning indicator comes on.
If there is a fault with the signal from the rear wheel sensors, the ABS system always cuts out completely, whatever the speed. Otherwise, there is a risk of the rear wheels locking and going out of control.
To cut out the ABS system if there is a fault, the control module disables the combination relay. This switches the ABS warning indicator on and cuts the power supply to the solenoid valves and pump motor. It also cuts out the TRACS function.
Disabling does not disable the combination relay, but the ABS light comes on.
In cars fitted with TRACS, the control module constantly records how much the brakes are being used, which enables it to calculate the temperature of the front wheel brake discs. If TRACS is on and the estimated temperature is too high, approx. 450°C (842°F), TRACS cuts out and the warning indicator comes on. This prevents the brakes overhearing. The light goes out once the estimated temperature falls below around 300°C (572°F) when TRACS can cut back in. The ABS system is still working, however.
Hydraulic System
The hydraulic system consists of a hydraulic pump of eccentric piston design with electric motor and rotation sensor and a valve block with solenoid valves and an overflow valve. The control module controls the hydraulic valves and electric pump motor via a combination relay. The hydraulic system cannot be dismantled for repairs, but must always be replaced as a complete unit if there is a fault.
The hydraulic pump (1) is powered by a 12 V DC motor and supplies brake fluid to the brake circuits under ABS control. The pressure in the brake circuits is governed by the brake pressure in the master cylinder, which varies directly with the force the driver exerts on the brake pedal. The primary circuit (8) controls the front wheels and the secondary circuit (9) the rear wheels. The hydraulic pump and pump motor are oversized to some extent to guarantee an adequate flow and pressure at all times. From the pump, two fluid return lines (10) and (11) run to the brake fluid reservoir.
The pump motor has a built-in rotation sensor (2). The rotation sensor is designed to tell the control module whether the hydraulic pump is rotating. Each time a winding of the pump motor passes the rotation sensor, it induces a voltage in the sensor coil. As windings go past in turn, this generates an AC signal whose frequency and voltage vary with the number of windings passing per second. Frequency and voltage increase with engine speed. By noting the frequency, the control module can tell whether the pump is rotating.
The valve block is integral with the hydraulic pump and controls the brake pressure to each brake caliper via the control module when braking under ABS control. The valve block has three circuits: one for the LH front wheel (3), one for the RH front wheel (4) and a common circuit for the two rear wheels (5). The front wheels are under individual control, while the rear wheels are controlled together, depending on which wheel threatens to lock first.
The valve block has six solenoid valves for controlling brake pressure: three inlet valves (7) and three outlet valves (8). There is a separate inlet and outlet valve for each front wheel and common inlet and outlet valves for the rear wheels. In the resting state, inlet valves are open and cutlet valves closed. The outlet valves act as return valves between the brake calipers and brake fluid reservoir.
Cars fitted with TRACS have a different hydraulic system. The differences are that:
- The connection from the pipe to the master cylinder primary circuit (12) is connected to the valve casing instead of the hydraulic pump. The primary circuit connection to the hydraulic pump is replaced by a bleed nipple.
- The valve block has an additional solenoid valve (13) which closes the connection between the hydraulic pump and master cylinder primary circuit when TRACS is on. The valve is normally open.
- The valve block has a pressure switch (14): if the brake light switch is not working, this disconnects TRACS when braking. When braking, the brake light switch trips before the pressure switch.
- The hydraulic system has an overflow valve which controls the maximum pressure in the system when under TRACS control. This is because the hydraulic pump is under control all the time.
Combination Relay
The combination relay is by the control module. It has two relay circuits, both controlled by the control module. The combination relay supplies power to the hydraulic unit solenoid valves and controls the hydraulic pump.
The relay also has two built-in diodes. One diode eliminates the power surges induced when the pump motor cuts out, the other switches the ABS warning indicator on if the combination relay is not on or the control module is disconnected.
Pedal Sensor
The pedal sensor is located on the power brake booster overload relay.
The pedal sensor sends a signal to the control module telling it what position the brake pedal is in (i.e. how hard the driver is braking). The control module uses the signal to indicate if there is a fault in the hydraulic system if the pedal travel is too great. The control module also uses the signal to decide when to start and stop the hydraulic system pump so as not to affect the brake pedal position too much when the solenoid valves are working. The pedal sensor consists of six resistors connected in series and a seventh open position read off by a sliding contact, which means that the sensor resistance can have seven different values.
With the brake pedal fully released (position 1), the sensor resistance is at minimum. With the pedal pressed right down to the floor (position 7), the resistance is infinite. Position 7 tells the control module if the pedal is travelling too far down, which means there may be a hydraulic fault in the system.
When the brake pedal is depressed and ABS cuts in, ABS makes the brake pedal drop slightly, which the pedal sensor records. The hydraulic pump then cuts in to keep the pedal in the right position, and keeps working until the pedal sensor notes that the brake pedal has been pushed back up a step. This is to stop the pedal being pushed back up too much, which would make the pedal movement uncomfortable for the driver.
If the control unit finds that the pedal sensor is in position 7 while ABS is on, it switches the hydraulic pump on and runs it for approx. 0.7 seconds. If this does not push the brake pedal back up at least one step (e.g. because there is a leak in the system) the hydraulic pump and ABS are switched off.
As power brake booster overload relay tolerances vary as made, they are color-coded. To ensure that the pedal sensor is in the right position relative to the power brake booster overload relay, we use different length spacers between sensor and servo. The spacers are color-coded the same as the power brake booster overload relays.
Pulse Wheel, Wheel Sensors
On the front wheels, the pulse wheel is pressed onto the outer driveshaft and the wheel sensor is fitted radially in the spindle housing relative to the pulse wheel. On the rear wheels, the pulse wheel is pressed onto the hub and the wheel sensors are located axially relative to the pulse wheel. The rear pulse wheels are interchangeable. Both front and rear pulse wheels have 48 teeth.
The wheel sensors are designed to tell the control module how fast the wheels are turning. When a wheel turns, the teeth on the pulse wheel induce a current in the wheel sensor coil. This generates an AC signal whose frequency and voltage vary with the number of teeth passing per second. The frequency and voltage vary directly with engine speed. The induced voltage is normally approx. 300 mV AC when the wheel is turning at 1 revolution per second. By recording the frequency, the control module can calculate whether the wheel is speeding up or slowing down. If there is a fault in any wheel sensor, ABS/TRACS will cut out if in use or not cut in in the first place.