Hill Start Assist

When the vehicle is stopped on an incline the ABS module holds the brake pressure for approximately 1.5 seconds while the driver transitions from the brake pedal to the accelerator pedal. This is accomplished by monitoring several HS-CAN messages and several sensors to determine if the vehicle is stopped and not parked, and if the vehicle is on an appropriate incline. The brake pedal message sent by the PCM and the wheel speed sensor inputs allow the ABS module to determine the vehicle has come to a complete stop. The transmission selector lever message sent by the PCM informs the ABS module the vehicle is not parked. The stability sensor messages sent by the RCM enable the ABS module to determine if the vehicle is on an incline greater than 1.5 degrees (approximately a 3% grade). Once the above conditions have been met, the hill start assist function automatically engages. As the driver releases the brake pedal, the ABS module commands the HCU to close the isolation valves which maintain the current brake system pressure, preventing the vehicle from rolling down the incline. Once the driver presses the accelerator pedal and the torque produced by the engine reaches a specific level, the ABS module gradually releases the brake pressure to make sure the vehicle is neither rolling back nor driving off until there is sufficient driving torque to move the vehicle forward (or backward if reversing up the incline). For vehicles with an automatic transmission the incline must be greater than 3.5 degrees (approximately a 6% grade), for vehicles with a manual transmission the incline must be greater than 1.5 degrees (approximately a 3% grade) and for vehicles equipped with the stop/start feature (regardless of transmission) the incline must be greater than 0.5 degrees (approximately a 1% grade).