Brake System General Information
A dual air brake system consists of two independent air systems that use a single set of brake controls. Each system has its own reservoirs, plumbing, and brake chambers. The primary system operates the service brakes on the rear axle; the secondary system operates the service brakes on the front axle. Service brake signals from both systems are sent to the trailer.
Warning: Do not operate the vehicle with the front brakes backed off or disconnected. Backing off or disconnecting the front brakes will not improve vehicle handling and may lead to loss of vehicle control, resulting in property damage or personal injury.
If air pressure drops below approximately 70 psi (483 kPa) in either system, a warning telltale illuminates and the driver display shows a warning pop-up. See the first line of Table. for examples of the telltale and ICU warning.
Note: An audible alert also sounds if the park brake is not set.
If the low air pressure warning is activated, check the air pressure gauges to determine which system has low air pressure. Although the vehicle's speed can be reduced using the foot brake control pedal, either the front or rear service brakes will not be operating at full capacity, causing a longer stopping distance. Bring the vehicle to a safe stop and have the air system repaired before continuing.
Note: The warning buzzer for low air pressure is suppressed on some vehicles when the vehicle is stationary.
Table 1, Brake System Warnings, Indicators, and Messages
|
Brake System Warnings, Indicators, and Messages |
||||
|---|---|---|---|---|
|
Telltale |
Color |
Description |
Driver Display |
|
 |
Red |
Low Air Pressure (EPA10 and Newer) |
Indicates air pressure in the primary or secondary reservoir has dropped below approximately 70 psi (483 kPa). A buzzer will also sound if the park brake is released or if the vehicle is moving--depending on vehicle options. |
 |
 |
Red |
Parking Brake (EPA10 and Newer) |
Indicates the parking brake is engaged. |
N/A |
 |
Amber |
Vehicle ABS |
Momentary illumination indicates the vehicle ABS is engaged. Solid illumination indicates a problem with the vehicle ABS. Repair the ABS immediately to ensure full braking capability. |
N/A |
 |
Amber |
Trailer ABS |
Momentary illumination indicates the trailer ABS is engaged. Solid illumination indicates a problem with the trailer ABS. Repair the ABS immediately to ensure full braking capability. |
N/A |
 |
Amber |
Stability Event |
Flashing indicates a stability event has been detected, and the ESC system is active. Solid illumination indicates a problem with the stability system. Repair the ESC system immediately to ensure full stability capability. For more information on the ESC system, see General Description, Standard Adaptive Cruise Control. |
N/A |
 |
Amber |
Wheel Spin Event |
Flashing indicates a wheel spin event has been detected, and the ATC system is active. |
N/A |
 |
ATC Deactivated |
Indicates the ATC SPIN button has been pressed to allow wheel slip. |
 |
|
 |
Green |
Engine Brake |
Indicates the engine brake is enabled. |
N/A |
Table 1, Brake System Warnings, Indicators, and Messages
Important: In the event of a total loss of service brakes with full system air pressure, use the parking brake control valve (yellow knob) to bring the vehicle to a complete stop in the safest location possible.
Before a vehicle with insufficient system air pressure can be moved, the spring parking brakes must be released by applying an external air source at the gladhands, or by manually caging the parking brake springs.
Warning: Do not release the spring parking brakes and then drive the vehicle. There would be no means of stopping the vehicle, which could result in serious personal injury or vehicle damage. Before releasing the spring parking brakes, make the connection to a towing vehicle or chock the tires.
After correcting the brake system problem, uncage the spring parking brakes before resuming normal vehicle operation.
Loss of air pressure in the primary air system causes the rear service brakes to become inoperative. The secondary air system will continue to operate the front brakes and, if equipped, the trailer brakes.
Loss of air pressure in the secondary air system causes the front axle brakes to become inoperative. The primary air system will continue to operate the rear service brakes and, if equipped, the trailer brakes.