Principles Of Operation
Principles of Operation
Conventional Brake System
Applying the brake pedal uses lever action to push a rod into the brake booster which, through the use of vacuum, boosts the force of the rod and then transmits this force into the master cylinder. This produces hydraulic pressure in the master cylinder. On vehicles not equipped with ABS, the hydraulic pressure is transmitted by brake fluid through the brake tubes to the individual brake calipers. On vehicles equipped with ABS, the hydraulic pressure is transmitted by brake fluid through the brake tubes to the ABS Hydraulic Control Unit (HCU), which then distributes that pressure through brake tubes to the individual brake calipers. The brake calipers use hydraulic pressure to apply the pads. The application of the brake pads will cause the rotation of the wheels to slow or stop depending on how much brake pressure is applied. The parking brakes carry out the same function except that they are mechanically actuated by a cable that connects only to the rear brakes.
Regenerative Braking System (Hybrid Vehicles)
Instead of just using the brakes to stop the vehicle, the electric motor that drives the hybrid electric vehicle can also slow the vehicle. In this mode, the electric motor acts as a generator and charges the batteries while the vehicle is slowing down. To achieve this, the ABS module calculates the amount of speed reduction requested by the driver through the accelerator pedal position and brake pedal travel sensor. The ABS module measures the inertia of the vehicle through the longitudinal accelerometer located in the Restraints Control Module (RCM) and determines if the required deceleration has been achieved. During deceleration/braking, the PCM partially engages the Electronically Controlled Continuously Variable Transmission (eCVT) to be turned by the vehicle wheels and slow it down. The ABS module requests the powertrain more or less dependent upon the driver request through the pedals. If the desired deceleration is not achieved through regenerative braking, the ABS module applies the friction brake pads to accommodate the driver request.
Electric Vacuum Pump (Hybrid Vehicles)
When the combustion engine is not running, an electric vacuum pump is used to maintain vacuum inside the brake booster. The ABS module turns the electric vacuum pump on and off through the use of a mechanical relay and a solid state relay. If the ABS module detects a vacuum pump motor circuit concern, a vacuum system leak or a vacuum pump performance concern, a DTC will be set in the ABS module and the combustion engine will run to provide brake vacuum for the brake booster.
Brake-By-Wire System (Hybrid Vehicles)
Hybrid vehicles are equipped with a brake pedal assembly that utilizes driver input and electronic components to apply the brakes. The driver does not, under normal circumstances, have direct control over the amount of brake pressure that is applied to the friction brakes. In the event that the ABS module detects a fault which requires the deactivation of this system, the ABS module relinquishes control of braking and the driver will still have the use of conventional hydraulic brakes. The HCU will still provide power assisted braking even if the ABS system is disabled. The brake booster push rod has an elongated slot that attaches to the brake pedal with a clevis pin. The elongated slot allows for a small amount of pedal travel (freeplay) to occur without the brake pedal applying pressure on the booster push rod. This freeplay allows regenerative braking to occur. To compensate for the freeplay, the pedal assembly is equipped with a hydraulic brake pedal feel simulator that provides the same feel and resistance as a conventional brake pedal. On light or normal brake applications, sensors attached to the brake pedal assembly measure brake pedal angle and the amount of pressure applied to the brake pedal. The sensors send signals to ABS module which activates a solenoid inside the brake booster. The solenoid draws the booster push rod into the booster, applying pressure to the master cylinder piston which then applies hydraulic pressure to the brake system. On hard brake applications, the brake pedal will travel to the end of the elongated slot, eliminating the freeplay and physically engaging the booster push rod, operating the brakes similar to a conventional brake actuation system.
Red Brake Warning Indicator
The red brake warning indicator alerts the driver to certain conditions that exist in the brake system. The Instrument Cluster (IC) performs a bulb check when the ignition key is turned to the RUN position. The conditions that cause the indicator to illuminate are low brake fluid level, the parking brake is applied or there is a fault in the brake system. To diagnose red brake warning indicator concerns, refer to Instrument Panel, Gauges and Warning Indicators.