Active Brake Booster

The active brake booster functions like a conventional brake booster with the added feature that it can be actuated electrically by the ABS module. The actuation of the brake booster by the ABS module is necessary in severe stability assist events as it makes sure that the HCU can generate enough brake pressure to maintain vehicle stability. The brake assist from the brake booster is especially useful in cold weather because the HCU cannot draw the brake fluid from the reservoir quickly at cold temperatures.

The ABS module controls a solenoid/air valve attached to the active brake booster to actuate the booster. The ABS module uses the brake pedal travel sensor (integral to the booster) and the brake pressure transducer (integral to the HCU) to determine if booster actuation is necessary. When actuation is necessary, the ABS module applies increasing current to the solenoid which opens the air valve which allows more air in and increases the boost provided by the brake booster. When actuation is no longer necessary, the ABS module decreases the current to the solenoid which closes the air valve and decreases the boost provided by the brake booster. When the ABS module is actuating the brake booster, the brake pedal will be pulled toward the front of the vehicle as the brake booster input rod moves. The ABS module will continue to actuate the brake booster until it receives brake pedal input from the driver.

Brake pedal input from the driver is determined using the brake pedal travel sensor and the brake pedal position (BPP) switch when all of the following conditions exist:

• The ignition key is in the RUN position
• The ABS module is not  actuating the active brake booster
• There are no DTCs present in the ABS module

Brake pedal input from the driver is determined using only  the BPP switch when one or more of the following conditions exist:

• The ignition key is in the OFF position
• The ABS module is actuating the active brake booster
• There are DTCs present in the ABS module

When the ignition switch is in the OFF position, failures cannot be detected. If there are failures, several layers of redundancy attempt to keep the driver brake pedal input signal accurate. 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 changed states and determine which are actually due to driver brake pedal input. Similarly, the stability assist system can take advantage of other sensors to detect BPP switch failures.