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Brief description

Brief description

The TC/ABS system has been developed to utilize the friction of the road surface optimally during braking and acceleration by controlling both the ABS and TCS functions.

ABS

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:


• No loss of directional stability on braking.

• Steering control retained even during heavy braking.

• Shortest possible braking distance.

• Reduced tyre wear.


The Saab 9-5 is equipped with TC/ABS 5.3, which is a dual-circuit, 4-port brake system containing the following principal components:


• Wheel sensors, one at each wheel.

• TC/ABS hydraulic unit with the following integrated components:

- Valve block with one inlet and one outlet valve per wheel for the ABS function.

- One pressure increase and one pressure relief valve per front wheel for the TCS function.

- Return pump consisting of an electric motor and pump unit, common to the ABS and TCS functions.

- Control module with integrated main relay and pump relay. It controls both the ABS and TCS functions.


A wheel sensor at each wheel generates an alternating current with a frequency that increases as the speed increases. The TC/ABS control module, which receives the information on the 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 breaking, the control unit will activate inlet and outlet valves and the return pump to regulate the wheel in question and attain maximum braking force with retained 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.

TCS

The TCS function in the TC/ABS control module works by reducing engine torque (addressing a request to the Trionic) and applying the brakes on the driven wheels during TCS modulation.

The rear wheel speed is used as a reference value to compare both drive wheels individually. One of the drive wheels rotating faster than the rear wheels is described as wheelspin. The amount of wheelspin and the vehicle speed determines how the system will function. Traction is given priority when wheelspin exceeds a certain limit at speeds below 22 km/h (13.5 mph), as the system then applies the brakes first and thereafter limits the engine torque.

Priority is given to the transfer of lateral forces so that steering control is maintained when wheelspin exceeds a certain limit at speeds above 22 km/h (13.5 mph), as the system then limits engine torque first and thereafter applies the brakes.

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).

Speed information for other systems

Several other systems in the car need to know the vehicle speed. The TC/ABS unit puts 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 puts information on the vehicle speed on the I-bus.

Refer to System overview for more information on bus communication.

Certain systems (non-Saab standard) may require the signal form the wheel speed sensors. TC/ABS therefore generates a square wave signal directly from both sensors on the rear wheels. See also

Diagnostics

The control module continuously monitors its own internal functions and the other system components. A diagnostic trouble code is set if a fault develops. If only the TCS function is affected by the fault it will be disengaged and the TCS OFF warning lamp will light up. Since faults affecting the ABS function also affect the TCS function, the warning lamps for ABS, brakes and TCS OFF will light up as well as the central warning lamp. The ABS and TCS functions are disengaged at the same time.