Brief description, TCS and ABS function
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Brief description, TCS and ABS function
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Traction control and anti-lock brakes have been developed to utilize road grip to the full during braking and acceleration.
The ABS is a control system that gives modern brake systems the ability to harness the maximum braking effect of the vehicle in critical situations, regardless of the road conditions.
The main advantages of the ABS system are:
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No loss of directional stability on braking.
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Steering control retained even during heavy braking.
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Shortest possible braking distance.
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The Saab 9-5 had ESP 8.0; a 2-circuit, 4-channel braking system comprising the following components:
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Speed sensors, one at each wheel.
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ESP control module with the following integrated components:
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Valve block with one inlet and one outlet valve per wheel for the ABS function.
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One pressure increase and one pressure relief valve per front wheel for the TCS function.
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Return pump comprising an electric motor and pump unit (common to ABS, TCS and ESP).
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Control module with integrated main relay and pump relay. The control module controls the ABS, TCS and ESP.
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A wheel speed sensor at each wheel generates an alternating current with a frequency that increases as the speed increases. The ESP control module, which receives the information on wheel speed, calculates wheel acceleration (speed increase), wheel retardation (speed reduction), vehicle speed and wheel skid (degree of lock). If any of the wheels exceeds a certain degree of lock during braking, the control unit will activate inlet and outlet valves and the return pump to regulate the wheel in question and attain maximum braking force and retain steering ability.
The control module incorporates a function known as electronic brake-force distribution (EBD), which controls the rear inlet valves so that maximum braking effect is achieved on the rear wheels under different load conditions, without the risk of their locking up before the front wheels.
The TCS function in the ESP control module works by reducing engine torque (addressing a request to the Trionic) and applying the brakes on the driven wheels during TCS modulation.
Rear wheel speed is used as a reference for comparing the speed of the front wheels individually. When one of the drive wheels rotates faster than the rear wheels, the wheel is said to spin. The magnitude of this wheelspin and the speed of the car are decisive to how the system operates. Traction is given priority when wheelspin exceeds a limit value at low speeds. The system operates by applying the brakes first and limiting engine torque as a secondary measure.
The transfer of lateral forces to maintain steering ability is given priority when wheelspin exceeds a limit value at high speeds. The system then limits engine torque first and applies the brakes as a secondary measure.
A certain degree of wheelspin is always allowed so that the sporty feel and handling of the car will still be retained. This varies with the speed of the car, the friction between tyres and road surface, and how “aggressively” the car is being driven (position of accelerator pedal).
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Speed information for other systems
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Several other systems in the car need to know the vehicle speed. The ESP control module transmits the information on wheel speed on the P-bus. This information is corrected in the main instrument unit with a factor dependent on the programmed tyre size. The main instrument unit transmits information on the vehicle speed on the I-bus.
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Note
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In cars with a 2.2 l diesel engine, vehicle speed data is sent directly from the ESP control module (pin 40) to PSG 16 (pin 34).
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Refer to
System overview
for more information on bus communication.
