Vacuum Supplemental Brake Assist System Operation: Notes

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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:

Sensor Circuit Board And Pressure Sensor
Check Valve Manifold
Vacuum Pump
Motor

When the ignition is turned 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 10 MPH under normal conditions, and before 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 7 in/Hg (18 cm/Hg) the REDUCED BRAKE POWER message is displayed and a 5 second timer is started. If the vacuum stays below 7 in/Hg (18 cm/Hg) for 5 seconds, the SBA starts the pump. If the vacuum drops below 6 in/Hg (15 cm/Hg), the pump comes on immediately. Once the vacuum level increases to 9 in/Hg (23 cm/Hg), the pump shuts off. The REDUCED BRAKE POWER message continues to be displayed until the vacuum level reaches 10 in/Hg (25 cm/Hg). If the pump runs for one minute without the vacuum reaching 9 in/Hg (23 cm/Hg), the pump shuts off and the SERVICE BRAKE BOOSTER message is displayed.

The SBA assembly operates in two modes.