Brake System Outline

BRAKE SYSTEM OUTLINE
The Hybrid Tribute has a new electro-hydraulic brake (EHB) system that replaces a portion of the conventional hydraulic brake system found on the Tribute.

The EHB system includes:
- A new actuation control unit (ACU), which is similar to a modified master cylinder.
- The EHB system has no conventional brake booster. The booster was deleted because the gasoline engine does not run constantly and cannot provide a sufficient vacuum to the braking system.
- A pedal feel emulator, which is a chamber built into the ACU, provides the familiar sensation of braking.
- A modified brake system control module (BSCM) assembly.
- The hydraulic control unit (HCU) portion of the BSCM assembly has two additional lines that provide fluid from the ACU reservoir to the HCU pump (source line) and return fluid to the ACU reservoir (return line).
- The brake control module assembly also has an additional fluid accumulator below the unit.
- The brake control module assembly filter is not replaceable.
- The brake control module assembly is located on the right hand side of the cowl.
- New heat shields protect the brake control module assembly from high catalytic converter temperatures.
- Sensor inputs to the BSCM include:
- A pedal travel sensor (a magnetic resonant travel sensor mounted on the ACU that measures how fast and how far the brake pedal has moved).
- A stability assist sensor cluster (although this vehicle does not have stability assist, it uses the same inputs for the braking system).

The EHB system uses sensor inputs to regulate the amount of hydraulic pressure, supplied by the HCU, that is applied to the conventional four wheel disc brakes (including the anti-lock braking system).

The EHB system is coordinated with the powertrain management to re-capture energy that would normally be lost to braking.






HYBRID BRAKE SYSTEM
The hybrid brake system consists of:
- Brake pedal travel sensor
- Diagonal split hydraulic system
- Fixed position foot pedal
- Four-wheel anti-lock brake system (4WABS)
- Front brake discs utilizing a single piston floating brake caliper
- Longitudinal accelerometer
- Master cylinder
- Mechanical parking brake system utilizing a drum-in-hat type brake within the rear brake discs
- Rear brake discs utilizing a single piston floating brake caliper

REGENERATIVE BRAKING SYSTEM
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 charge the batteries while the vehicle is slowing down.
- The ABS module calculates the amount of speed reduction requested by the driver through the accelerator pedal position and brake pedal travel sensor located on the master cylinder.
- The ABS module measures the inertia of the vehicle through the longitudinal accelerometer located in the stability control sensor cluster and determines if the required deceleration has been achieved.
- The PCM partially engages the electronically controlled continuously variable transaxle (eCVT) to be tuned by the vehicle s wheels and slow it down.
- The ABS module requests the powertrain dependent upon the driver request through the pedals.

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 driver s 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 sent to the PCM.
- The ABS module applies the friction brakes beyond the torque limit.






If the desired deceleration is not achieved through regenerative braking, the ABS module applies the friction brake pads to accommodate the driver request. The driver does not, under normal circumstances, have direct control over the amount of brake pressure that is applied to the rotors.

In the event that the ABS module detects a fault which requires the deactivation of the system, the ABS module relinquishes control of braking and the driver will still have use of the manual hydraulic brakes. The hydraulic control unit (HCU) will still provide power assisted braking even if the ABS system is disabled.

Since the regenerative braking uses the electric motor to slow the vehicle s front wheels, front brake pad wear is reduced. The rear brake pads wear at approximately twice the rate of the front brake pads.

RED BRAKE WARNING INDICATOR
The red brake warning indicator alerts the driver to certain conditions that exist in the brake system. The instrument cluster 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
- there is a fault in the ABS (if the yellow ABS warning indicator is also illuminated)