Below is an overview of the Aux. Vacuum pump(AVP) or (SBA) noted below attached to 03 Tahoe and 03 Yukon. This post is from SHADAMS on the GM Inside News forum, so that full credit is given to this outstanding explanation this person should be commended. it is the best explanation I have found.
I have and 03 tahoe with 139K and 03 Yukon with 90k. The 03 Yukon has "SERVICE BRAKE BOOSTER" (SBB) light on when i bot the truck 6 months ago. I have sourced every thread I could find. The below explanation of what caused the light to come on helps but does not address what issues I have and will share. I have purchased a used (AVP or SBA pump) which I have plugged into my 03 tahoe which does not have a problem with SBB light. When connected to the tahoe the used pump purrs and seems to work just fine sucking from the inlet hole that attaches to the brake booster. This test was done with the used AVP or SBA pump sitting on the cowl as there is no need for ground on the pump, it has a male female connection of 6 wires I believe at the top of the AVP. When attached to the 03 Yukon, the pump will not purr much less run when the vehicle is started and the SBB light appears again on the cluster. I know I have a working pump but evidently there is no power running to the pump to turn it on, so after a few seconds of the testing outlined below in Shadams post, the SBB light comes on again. Evidently there is no power to this AVP unit upon startup, maybe a wire compromised, a Ground not tight somewhere, a elect. switch on a board not performing. I have taken both grounds off below the driver's side frame rail, cleaned them and coated battery paste to keep corrosion down in the future. I can not find a WIRING DIAGRAM FOR EITHER VEHICLE. This would help a lot to maybe source the lack of power to the unit. Is there any other ground any of you know about that may need cleaning? Has anyone else had the issue and solved it. There has been some discussion about the instrument cluster needing to be re-flashed or programed, along with changing the Alternator with more or less amps or maybe a ground on the alternator? This little pump was only used for a couple of years on these models and the pump has now been discontinued. It is not avail from any dealer I could find or after market source. I found a used one on EBAY, got lucky and it seems to work fine on the tahoe but won't run on the yukon.
Re: Service Brake Booster on DIC in '03 Tahoe
That vehicle has whats called a supplemental brake assist motor, it is a extra vacuum pump to supply extra vacuum in low vacuum conditions such as accel or if the booster failed. It runs a self test every time you start the vehicle for 1 second, and if the IPC (instrument panel) does not see that the test completes 5 times in a row it turns on the "Service Brake Booster" message.
When you start the vehicle and get to around 10 mph you should hear a light buzzing noise, maybe even feel it in the brake pedal. If that isnt happening then the SBA motor may need to be replaced. If it is coming on and the message appears, it may need an instrument cluster, but there are other reasons it could come on, see following:
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.
If everything checks out with the dealer than it is hard to point at any one part thats bad, intermittant conditions are the worst. Good Luck!
