Input Signals
ABS, EBD and TRACSInput signals
Wheel sensors and pulse wheel
On the front wheels, the pulse wheel is pressed onto the outer driveshaft and the wheel sensor is located in the spindle casing radially relative to the pulse wheel.
On the rear wheels, the pulse wheel is pressed onto the hub and the wheel sensor is located axially relative to the pulse wheel. The rear pulse wheels can be exchanged. Both the front and rear pulse wheels have 48 teeth.
The wheel sensor's task is to provide the control module with information on the wheel rotation speed. When a wheel rotates, the teeth on the pulse wheel induce a voltage in the wheel sensor coil. This generates an AC voltage signal whose frequency and voltage depend on the number of teeth passing per second. The frequency and voltage increase as the speed of rotation increases. The voltage induced is normally approx. 3OOmVAC when the wheel is rotating at 1 turn per second. By recording the frequency, the control module can calculate the wheel's acceleration and retardation.
If there is a fault in any wheel sensor, ABS and TRACS will be disconnected or not engaged in the first place. However, EBD (Electric Brake force Distribution) remains on the whole time.
Using the signals from the wheel sensors, the control module calculates the car's road speed.
EBD pressure sensor
The EBD pressure sensor tells the control module when the brakes are being used, to enable the control module to activate EBD. The EBD pressure sensor takes power from the control module at a no-load voltage of approx. 11V. When the primary circuit is not under pressure (no braking), the pressure sensor is unaffected and the contacts closed, which means that the voltage is reduced to approx. 5V by the control module. When the contacts are closed, the voltage is pulsed to ground. When the primary circuit is under pressure (braking in progress), the contacts break and the signal goes to high, which the control module interprets to mean that braking is in progress and that EBD can be used. The EBD pressure sensor is connected to the master cylinder primary circuit.
Brake light switch
The brake light switch sends a signal to the control module telling it that braking has started, so that the control module disengages TRACS, if it is in use, and stands by to use ABS/EBD if necessary, which means that the control module makes all calculations more precisely. If the control module does not receive any signals from the brake light switch, it will engage the ABS system, although the control will be rougher when ABS first cuts in.
The brake light switch is powered from the battery. When contact is made, the brake light switch comes on and the control module receives a signal telling it braking has started.
Power supply and ground points
The control module is powered continuously at 2 x 30-supply from the battery and a 15I-supply from the ignition when the key is turned to position 11. The control module has two ground connections.
TRACS switch
The TRACS switch (5) switches TRACS on and off (cars with TRACS only). Once the ignition is switched on, TRACS is on at all times. TRACS only disengages when the switch is pressed. The switch is spring-loaded and always returns to its normal position with the contacts broken.
The signal lead between the control module and TRACS switch is common to both the warning light and TRACS switch. The signal from the instrument panel (dashboard) warning light (4) to the control module is normally approx. battery voltage. When the switch is pressed, contact is made and the signal is grounded, causing the warning light to come on. The control module then disconnects TRACS and keeps the signal grounded, so the warning light stays on even when the switch is released.
The control module switches oft the TRACS warning light on the instrument panel (dashboard) when TRACS is engaged via the switch.