Brake Warning System Description and Operation
Brake Warning System Description and Operation
Brake Warning Indicator
- The instrument panel cluster (IPC) illuminates the brake warning indicator when one or more of the following occurs:
- The body control module (BCM) detects that the park brake is engaged. The IPC receives a class 2 message from the BCM requesting illumination.
- The electronic brake control module (EBCM) detects a low brake fluid condition. The IPC receives a class 2 message from the EBCM requesting illumination.
- The EBCM detects an ABS malfunction which disables dynamic rear proportioning (DRP). The IPC receives a class 2 message from the EBCM requesting illumination.
- The IPC performs the bulb check at the start of each ignition cycle. The brake warning indicator illuminates for approximately 3 seconds before turning OFF.
- The IPC detects a loss of class 2 communications with the BCM or with the EBCM.
Service Brake System Message
- The service brake system message is displayed whenever the red brake warning indicator is illuminated.
Supplemental Brake Assist
- The Supplemental Brake Assist (SBA) displays two messages through the Driver Information Center (DIC).
Reduced Brake Power
- The REDUCED BRAKE POWER message is displayed when the supplemental brake assist signal circuit is held low. This happens when the SBA is active and maintaining reduced brake power levels.
Service Brake Booster
- The SERVICE BRAKE BOOSTER message will be displayed when the supplemental brake assist signal circuit is toggled at 1.0 Hz with a 50 percent duty cycle. This message is also displayed if the instrument panel cluster (IPC) does not see an alive and well signal at key on. When this message is displayed it indicates a problem with the SBA and should be serviced or replaced.
Hydraulic Brake System Description and Operation
System Component Description
- The hydraulic brake system consists of the following:
- Hydraulic Brake Master Cylinder Fluid Reservoir
- Contains supply of brake fluid for the hydraulic brake system.
- Hydraulic Brake Master Cylinder
- Converts mechanical input force into hydraulic output pressure.Hydraulic output pressure is distributed from the master cylinder through two hydraulic circuits, supplying front-rear opposed wheel apply circuits.
- Hydraulic Brake Pressure Balance Control System
- Regulates brake fluid pressure delivered to hydraulic brake wheel circuits, in order to control the distribution of braking force.Pressure balance control is achieved through dynamic rear proportioning (DRP), which is a function of the ABS modulator. Refer to ABS Description and Operation (W/O JL4)ABS Description and Operation (W/JL4) in Antilock Brake System for specific information on the operation of DRP.
- Hydraulic Brake Pipes and Flexible Brake Hoses
- Carries brake fluid to and from hydraulic brake system components.
- Hydraulic Brake Wheel Apply Components
- Converts hydraulic input pressure into mechanical output force.
System Operation
- Mechanical force is converted into hydraulic pressure by the master cylinder, regulated to meet braking system demands by the pressure balance control system, and delivered to the hydraulic brake wheel circuits by the pipes and flexible hoses. The wheel apply components then convert the hydraulic pressure back into mechanical force which presses linings against rotating brake system components.
Brake Assist System Description and Operation (Vacuum Assist)
System Component Description
- The brake assist system consists of the following:
- Brake Pedal:
- Receives, multiplies and transfers brake system input force from driver
- Brake Pedal Pushrod:
- Transfers multiplied input force received from brake pedal to brake booster.
- Supplemental Brake Assist:
- The Supplemental Brake Assist (SBA) unit creates vacuum for the brake booster in case of a vacuum loss or low vacuum to the brake booster.
- Vacuum Brake Booster
- Uses source vacuum to decrease effort required by driver when applying brake system input force.When brake system input force is applied, air at atmospheric pressure is admitted to the rear of both vacuum diaphragms, providing a decrease in brake pedal effort required. When input force is removed, vacuum replaces atmospheric pressure within the booster.
- Vacuum Source
- Supplies force used by vacuum brake booster to decrease brake pedal effort.
- Vacuum Source Delivery System
- Enables delivery and retention of source vacuum for vacuum brake booster.
System Operation
- Brake system input force is multiplied by the brake pedal and transferred by the pedal pushrod to the hydraulic brake master cylinder. Effort required to apply the brake system is reduced by the vacuum brake booster.
Supplemental Brake Assist
- The Supplemental Brake Assist (SBA) unit creates vacuum for the brake booster in case of a vacuum loss or low vacuum to the brake booster. The SBA consists of:
- The sensor circuit board and pressure sensor
- The check valve manifold
- The vacuum pump
- The motor
- When the vehicle is keyed ON, the SBA performs a self test by toggling the Supplemental Brake Assist Signal Circuit low for one second. The instrument panel cluster (IPC) recognizes this as an indication that the SBA is functional. If the IPC does not see this signal for five consecutive ignition cycles the Service Brake Booster Message is displayed. The SBA performs diagnostics at 16 Km/h (10 mph) under normal conditions, and before 16 Km/h (10 mph) if a low vacuum condition exists. The SBA then spins the motor and runs diagnostics to determine if the pump is operating and the SBA is functional. The SERVICE BRAKE BOOSTER message will display until the next ignition cycle if the unit is unable to maintain reduced power brake levels.
- The SBA unit utilizes the ignition on signal from the generator to determine the engine is running and monitors speed using an input from the vehicle speed sensor. It also has an internal pressure sensor that compares the pressure inside the booster to the ambient pressure. If the vacuum drops below 18 cm/Hg (7 in/Hg) the REDUCED BRAKE POWER message is displayed and a 5 second timer is started. If the vacuum stays below 18 cm/Hg (7 in/Hg) for 5 seconds the SBA starts the pump. If the vacuum drops below 15 cm/Hg (6 in/Hg) the pump comes on immediately. Once the vacuum level increases to 23 cm/Hg (9 in/Hg) The pump shuts off. The REDUCED BRAKE POWER message continues to be displayed until the vacuum level reaches 25 cm/Hg (10 in/Hg). If the pump runs for one minute without the vacuum reaching 23 cm/Hg (9 in/Hg) the pump shuts off and the SERVICE BRAKE BOOSTER message is displayed.
- The SBA assembly operates in two modes:
- Low Vacuum Mode - The low vacuum mode can occur with the engine ON or OFF. The SBA determines the ON/OFF status by monitoring the regulator output on the engine ON circuit. This mode involves SBA pump activation and deactivation due to changes in vacuum detected by the pressure sensor.
- Low Vacuum Engine ON Mode - The SBA will operate in reaction to changes to brake booster vacuum as monitored by the pressure sensor. This mode is normally active on power up if a valid engine ON signal is detected and a low vacuum condition is indicated for more than 5 seconds.
- Reduced Brake Power Mode - When the booster vacuum drops below 18 cm/Hg (7 in/Hg) the SBA will start a 5 second timer. If the vacuum exceeds 18 cm/Hg (7 in/Hg) the timer stops and resets. After 5 seconds the pump activates and the REDUCED BRAKE POWER message is displayed.
- Immediate Pump Activation Mode - When the booster vacuum drops below 15 cm/Hg (6 in/Hg) the REDUCED BRAKE POWER message is displayed and the pump is turned on.
- Pump Deactivation Mode - The pump will be turned off when the booster vacuum exceeds 23 cm/Hg (9 in/Hg). The SBA will continue to display REDUCED BRAKE POWER until the booster vacuum exceeds 25 cm/Hg (10 in/Hg).
- Low Vacuum Engine Off - The SBA will monitor the vacuum when the vehicle is above 8 Km/h (5 mph) and an engine OFF condition is detected. If the vehicle is below 8 Km/h (5 mph) and an engine OFF condition is detected the SBA will monitor the vacuum for 60 seconds
- Vacuum Augment (Engine OFF) Mode - If the engine ON state changes to OFF after reaching 8 Km/h (5 mph) the SBA activates the pump for up to 60 seconds. After 60 seconds the pump shuts off and the SERVICE BRAKE BOOSTER message is displayed. If the engine ON signal is detected or the vehicle slows to below 8 Km/h (5 mph) before the pump times out. The pump shuts off and no message is displayed.
Brake Assist System Description and Operation (Hydraulic Assist)
System Component Description
- The brake assist system consists of the following:
Brake Pedal
- Receives, multiplies and transfers brake system input force from driver.
Brake Pedal Pushrod
- Transfers multiplied input force received from brake pedal to brake booster.
Hydraulic Brake Booster
- Uses power steering fluid pressure to decrease effort required by driver when applying brake system input force.When brake system input force is applied, pressurized power steering fluid is admitted to the rear of an apply piston, providing a decrease in brake pedal effort required. When input force is removed, pressurized power steering fluid is blocked off from the piston.
- Power Steering System
- Supplies pressurized power steering fluid used by hydraulic brake booster to decrease brake pedal effort.
System Operation
- Brake system input force is multiplied by the brake pedal and transferred by the pedal pushrod to the hydraulic brake master cylinder. Effort required to apply the brake system is reduced by the hydraulic brake booster.