SYSTEM DESCRIPTION
Anti-spin regulator (ASR) mode of operation
If one or more driven wheels spin when the vehicle moves off or accelerates, the ABS/ASR ECU detects
this based on the speeds of the driven wheels (normally the rear wheels) and the vehicle speed and
reduces the speed of the spinning driven wheels to the optimum level.
ASR has two control circuits for this purpose:
• Brake control circuit
• Engine control circuit
ASR control is indicated by the yellow ASR check lamp.
Brake governor
If the coefficient of friction on the left wheel is not the same as the coefficient of friction on the right wheel
(μ-split), the brake governor intervenes to brake the spinning driven wheel. The ASR solenoid valve and the
ASR shuttle valve allow reservoir air to be introduced into the driven axle's brake circuit for this purpose.
The driven axle's ABS pressure control valves are used to control the pressure in the brake cylinders so
that the two driven wheels have practically the same speed.
The reference speed variable selected for brake control is the speed of the driven wheel on the same axle.
The ASR solenoid valve is activated if a driven wheel exceeds a certain permitted spin threshold, compared
to the opposite wheel. In this case, the reservoir pressure is available at the driven wheels' ABS pressure
control valves. The pressure control valve on the non-spinning driven wheel is switched between "pressure
reduction" and "pressure maintaining" so as to prevent this wheel from being fully braked.
The brake governor is only active within a defined vehicle speed range. If it is already active at a low speed
during acceleration, it remains in effect until the driver stops accelerating.
Once braking control has ended, the line to the brake cylinders is fully bled once again.
Engine governor
If both driven wheels are spinning equally when starting off, the engine governor responds and reduces the
engine drive torque. An adaptive, non-linear integral governor reduces the engine power until the average
speed of the driven wheels is higher than the average speed of the non-driven front wheels. The reference
speed variable is the average speed of the non-driven front wheels.
The ABS/ASR control unit informs the engine control unit of the necessary torque reduction via the CAN
databus.
ASR slip increase (ASR function switch)
ASR slip increase allows better traction under certain conditions (e.g. driving in deep snow or sand dunes).
The increased slip is indicated by the flashing ASR check lamp.
Brake lamp signal evaluation
The brake lamp switch (BLS) signal is required in order to obtain additional information regarding ABS
control and in order to prohibit the ASR function when the brake pedal is pressed.
Correct functioning of the BLS (brake lamp switch) is checked each time the ignition is switched on. In this
case, the control unit waits for the brake pedal to changed from "pressed" state to "non-pressed" state. If
this change does not occur after the ignition is switched on, ASR is prohibited and the ASR check lamp
remains lit.
T50 - A1 2.01 - 3
