Functional Operation
The electronic components control their corresponding functions in the hydraulic unit (brake torque control circuit) and the functions of the electronic accelerator system (drive torque control circuit).
The voltage for the ABS/ASR control unit is supplied via the overvoltage protection relay.
The ABS/ASR control unit receives input signals from the following components:
^ 4 wheel speed sensors,
^ Stop lamp switch,
^ Electronic accelerator control unit, for throttle valve actual position,
^ Snow chain switch (ASR) with LED indicator,
^ Pressure reservoir switch.
The ABS/ASR control unit processes these input signals into output signals for the following components:
^ Solenoid valves of the hydraulic unit,
^ Relays (solenoid valves, return/pressure pump, ASR charging pump),
^ ASR charging pump,
^ Electronic accelerator control unit (Throttle valve nominal position signal),
^ ASR function indicator lamp,
^ LED indicator in the snow chain switch,
^ ABS warning lamp,
^ ASR warning lamp.
On the basis of the input signals, the ABS/ASR control unit distinguishes which of the 3 operational modes to engage:
^ Normal operation,
^ ABS control operation,
^ ASR control operation.
NORMAL OPERATION
In this case no wheel speed differences are present requiring control operations. All the solenoid valves in the hydraulic unit are switched to normal (non-energized) position. The system is ready for service brake use.
If the control unit receives a low pressure signal from the pressure switch in the ABS/ASR hydraulic unit, it activates the ASR charging pump and the return/pressure pump. Charging the pressure reservoir continues until the pressure switch sends a signal that the pressure reservoir is fully charged.
The throttle valve actual position signal is constantly monitored in the ABS/ASR control unit, to control operation of the electronic accelerator system.
ABS CONTROL OPERATION
From the wheel speed signals, the ABS/ASR control unit recognizes the need for an ABS control operation (wheel starting to lock up). In the ABS/ASR control unit, the ABS engagement circuits operate the corresponding solenoid valves in the ABS/ASR hydraulic unit. As on the previous ABS system, left and right front axle solenoid valves operate separately in the ABS/ASR hydraulic unit. The rear axle left and right solenoid valves operate together for ABS operation.
ASR CONTROL OPERATION
The ABS/ASR control unit's microcomputer continuously processes the wheel speed sensor signals. As soon as a drive wheel starts to spin, the ABS/ASR control unit activates switchover solenoid valve and the left and/or right rear axle solenoid valves in the hydraulic unit. The right and left rear axle solenoid valves activate independently. The return/pressure pump changes to pressure release position. Refer to HYDRAULIC SYSTEM. During ASR control operation the ASR function indicator lamp illuminates. If the ABS/ASR control unit's microcomputers detect excessively high drive torque, the throttle valve output signal to the electronic accelerator control unit is changed, which sends a signal to the throttle valve actuator reducing the throttle valve opening from what the driver calls for with the accelerator pedal. During ASR control operation, no pressure reservoir charging occurs. Actuation of the brake pedal interrupts ASR control operation. The ABS/ASR control unit recognizes malfunctions that affect ASR or ABS operation, indicating them via the ASR and ABS warning lamps. If the malfunctions affect the ABS system, both systems turn off.