Brake Booster

The brake booster is a normal brake booster which has the added function that it can be actuated electrically by the stability assist module. This is sometimes necessary in severe stability assist events, to make sure that the HCU can generate enough brake pressure to improve vehicle stability. The help from the brake booster is especially needed in cold weather when the HCU cannot draw the brake fluid from the reservoir when it becomes very viscous at cold temperatures. Within the booster is a solenoid for electrical actuation and a release switch to indicate when the driver is stepping on the brake. The solenoid provides electrical actuation of the brake booster. Without the force of the input rod, the air valve is directly opened due to the movement of the energized solenoid. With increasing current applied to the solenoid, the air valve opens and output force is created. With decreasing current applied to the solenoid, the air valve is closed and the vacuum valve opens reducing output force.

The release switch indicates when the brake pedal has been pressed. It is integrated into the booster key that normally sits against the rear shell of the boot in its rest position and it adjoins the valve body when in its balance position. The switch itself provides indication from two positions. In its rest position, it supplies two signals, normally open (NO) and normally closed (NC). Upon application of sufficient force to initiate movement of the key away from the valve body, the brake booster switch changes state, signifying a driver application. When the stability assist module is activating the brake booster, the brake pedal is pulled forward as the valve body moves. The release switch is held in position by the input rod spring and remains against the valve body. Although the brake pedal position (BPP) switch changes state when the brake pedal drops, the release switch remains inactive until such time as it encounters an external force from the driver, whether in a stability assist event controlled by the stability assist module, or in a standard mechanical braking event.

The master cylinder attached to the booster carries out the same functions as master cylinders found on non-booster brake systems. It differs in that there are two pressure transducers that are installed in spare outlet ports. The values measured by each pressure transducer should be similar. The pressure transducer is a diaphragm type unit. When a pressure is seen the diaphragm is stretched, causing a voltage to be output. As the pressure increases or decreases, the voltage also increases or decreases with it. The configuration of the pressure transducers, which transducer is connected to the primary port and which is connected to the secondary port, is not known by the stability assist module. Since the stability assist module cannot differentiate between primary and secondary port transducers, diagnosis of pressure faults requires some care to identify exactly which transducer is failed.

Modules which are normally connected to the BPP switch are connected to a signal from the stability assist module. This signal is called driver brake application. This signal only changes state when the driver has applied the brake. If the ignition switch is in the RUN position, the brake booster is not electrically actuated, and no failure has been detected, the driver brake application signal will indicate a brake applied when either the BPP switch or the release switch built into the brake booster indicate that the driver has applied the brake pedal. If the ignition switch is in the OFF position, the driver brake application signal will indicate a brake apply only when the BPP switch indicates that the driver is applying the brake pedal.

When the ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy will try to keep the driver brake application signal accurate. If the stability assist system detects faults on the driver brake application signal line it will also attempt to maintain correct stoplamp operation by communicating on the SCP network. Control of the stoplamp operation is also required because the BPP switch can change states during a stability assist event when the driver is not applying the brake pedal. The switch can also change states during the system check of the brake booster. Only the stability assist system can interpret these state changes and determine which ones are actually due to the driver applying the brake. Similarly, the stability assist system can take advantage of its other sensors to detect failures in the BPP switch.