Troubleshooting - Conventional and Hybrid Brake System

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BRAKE SYSTEM - HYBRID

Principles of Operation

The hydraulic control unit (HCU) controls the brake assist function as well as the anti-lock functions. The HCU contains an accumulator, driven by an electric motor, to supply pressurized brake fluid to the brake calipers during brake application. The electric motor begins to build pressure in the accumulator and initiates the brake system self-test whenever the ignition switch is turned to the ON position, the brake pedal is applied, the vehicle doors are opened or when the dome light is activated. If the dome light has been deactivated using the dimmer switch, the electric motor and the brake self-test will not activate. During the pressure build and self-test, 12 mm (0.47 in) of brake fluid is drawn from the master cylinder reservoir into the accumulator. The accumulator discharges 4 minutes after the ignition switch is turned to the OFF position and the vehicle dome light turns off, returning the brake fluid used during the charging of the accumulator to the master cylinder reservoir. The brake calipers use the hydraulic pressure from the HCU 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

The regenerative braking system uses the electric drive motor to provide some level of charging to the high-voltage traction battery (HVTB) during braking. There are 2 levels of regenerative braking: the first level is when the driver releases the accelerator pedal, the second level is obtained when the brake pedal is applied. Whenever the ABS is activated, the affects of regenerative braking are reduced until the ABS event is over and the accelerator is applied.
In addition to providing a level of charging to the HVTB, the electric drive motor also aids in stopping the vehicle though the use of negative torque requests from the ABS module to the PCM. When the driver releases the accelerator pedal, the ABS module compares several items such as vehicle speed, brake pedal input, accelerator input and then determines if a negative torque request is necessary. If the request is necessary, the ABS module sends a message to the PCM over the HS-CAN bus to reduce torque in the electric drive motor to slow the vehicle. The PCM has a predetermined limit of negative torque, once this limit is reached, the ABS module will use the HCU to apply the brakes. When the torque request is sent to the PCM, this will also engage the regenerative braking system.
Regenerative braking logic is as follows:

- The ABS module determines the total torque request from accelerator pedal and brake pedal input.
- The ABS module compares the drivers total request for a reduction of speed, to the PCM ability to slow the vehicle or negative "torque limit".
- The ABS module requests the reduction in speed up to the torque limit from the PCM, via a torque modification request message sent to the PCM.
- The ABS module applies the friction brakes beyond the torque limit.

By using the electric drive motor to aid in stopping the vehicle, front brake pad wear is reduced. However, the rear brake pads do not benefit from the regenerative braking and will wear as normal.

Red Brake Warning Indicator

The red brake warning indicator alerts the driver to certain conditions that exist in the brake system. The instrument cluster carries out 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 ABS (if the yellow ABS warning indicator is also illuminated). The brake fluid level switch is part of the brake fluid reservoir and is wired directly to the ABS module, when the fluid is below the predetermined level, the ABS module sends a message to the instrument cluster along the high speed controller area network (HS-CAN) bus to illuminate the indicator. The parking brake switch is mounted on the parking brake control and is wired to the smart junction box (SJB). When the parking brake is engaged, the SJB sends a message to the instrument cluster along the HS-CAN bus to illuminate the indicator. When there is one or more DTCs in the ABS module, the module sends a message to the instrument cluster along the HS-CAN bus to illuminate the red brake warning indicator and the yellow ABS warning indicator.