Low Vacuum Mode

WARNING: This page does not describe the selected car, but rather 12 other vehicles, including the 2003 GMC Yukon XL, 2003 GMC Yukon, 2003 GMC Sierra, 2003 GMC Cab & Chassis Sierra, and 2003 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.

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 7 in/Hg (18 cm/Hg) the SBA will start a 5 second timer. If the vacuum exceeds 7 in/Hg (18 cm/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 6 in/Hg (15 cm/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 9 in/Hg (23 cm/Hg). The SBA will continue to display REDUCED BRAKE POWER until the booster vacuum exceeds 10 in/Hg (25 cm/Hg).
Low Vacuum Engine Off - The SBA will monitor the vacuum when the vehicle is above 5 MPH and an engine OFF condition is detected. If the vehicle is below 5 MPH and an engine OFF condition is detected, the SBA will monitor the vacuum for 60 seconds.