Brakes

A dual air brake system uses one set of brake controls to operate two independent air brake systems. Each system has its own reservoir, 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. The trailer brakes operate from both systems, or either one if the other malfunctions.

The rear service brakes will not work if the primary system loses air pressure; however, the front brakes will continue to be operated by secondary system air pressure. The secondary system will also continue to operate the trailer brakes.

The front brakes will not work if secondary system air pressure is lost. The primary system will continue to operate the rear service brakes and trailer brakes.

Before driving your vehicle, allow time for the air compressor to build up a minimum of 95 psi (655 kPa) pressure in both the primary and secondary systems. Monitor the air pressure system by observing the dual system air pressure gauge and the low air pressure warning light and buzzer. The warning light and buzzer shut off when air pressure in both systems reaches 64 to 76 psi (441 to 524 kPa).

The warning light and buzzer come on if air pressure drops below 64 to 76 psi (441 to 524 kPa) in either system. 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, causing a longer stopping distance. If this happens, check the air system pressure gauges to determine which system has low air pressure. Bring the vehicle to a safe stop, and have the air system repaired before continuing.

On tractor-trailer vehicles, if both the primary and secondary systems become inoperative, the trailer service brakes or spring parking brakes will automatically apply when air pressure drops below 35 to 45 psi (241 to 310 kPa). The tractor spring parking brakes will automatically apply when air pressure drops below 20 to 30 psi (138 to 207 kPa). On straight trucks, spring parking brakes will apply when air pressure drops below 20 to 30 psi (138 to 207 kPa). Do not wait for the brakes to apply automatically. When the warning light and buzzer first come on, immediately bring the vehicle to a safe stop. Before continuing operation of the vehicle, correct the cause of the air loss.

After correcting the brake system problem, uncage the spring parking brakes before resuming normal vehicle operation.

Before driving the vehicle, secure all loose items in the cab so that they will not fly forward during a full brake application. Make sure that all occupants are wearing seat belts.

During normal brake stops, push the foot brake control pedal until braking action slows down the vehicle. Increase or decrease the pressure on the pedal so that the vehicle comes to a smooth, safe stop. As the vehicle nears full stop, push the clutch pedal in and shift the transmission into neutral. Apply the spring parking brakes if the vehicle is to be parked.

The yellow diamond-shaped knob ( Fig. 7.1 ) on the control panel actuates the parking brake valve. Pulling out the knob applies both the tractor and the trailer spring parking brakes, and automatically causes the trailer air supply valve knob to pop out.

The red octagonal knob ( Fig. 7.1 ) on the control panel actuates the trailer air supply valve. After the vehicle's air hoses are connected to a trailer, and the pressure in both air systems is at least 65 psi (448 kPa), the red knob must be pushed in. It should stay in to charge the trailer air supply system and to release the trailer spring parking brakes; it must be pulled out before disconnecting a trailer. It must also be pulled out when operating a vehicle without a trailer. If pressure in both air systems drops to 35 to 45 psi (241 to 310 kPa), the red knob automatically pops out, exhausting the trailer air supply and applying the trailer service or spring parking brakes.

If the brakes are wet, drive the vehicle in low gear and lightly apply the brakes to heat and dry them.

Allow hot brakes to cool before using the spring parking brakes.

If the trailer is not equipped with spring parking brakes, pulling out the yellow knob applies the tractor spring parking brakes and the trailer service brakes. When the tractor and trailer parking brakes (or trailer service brakes) are both applied, the trailer brakes can be released by pushing in the red knob, leaving the tractor parking brakes applied. Air pressure in the primary or secondary reservoir must be at least 64 psi (447 kPa) before the tractor spring parking brakes, or the trailer service or spring parking brakes, can be released.

On trailers not equipped with spring parking brakes, chock the trailer tires before disconnecting the truck or tractor when parking just the trailer. When parking a truck or tractor with a trailer (combination vehicle), and when the trailer is not equipped with spring parking brakes, apply the truck or tractor spring parking brakes.

The Meritor WABCO® Antilock Braking System (ABS) is an electronic wheel speed monitoring and control system that works with the air brake system. ABS passively monitors vehicle wheel speed at all times, but controls wheel speed during an emergency or reduced-traction stop. In normal braking applications, the standard air brake system is in effect.

ABS uses sensors reading signal-generating tone rings in the wheel hubs of each sensed axle. The sensors transmit vehicle wheel speed information to an ECU located behind the passenger seat. The control unit's main circuit interprets the speed sensor signals and calculates wheel speed, wheel retardation, and a vehicle reference speed. If the ECU calculates that a wheel is about to lock, the main circuit signals the appropriate solenoid control valve to reduce braking pressure. During emergency braking, the solenoid control valve regulates air pressure supply in each brake chamber to avoid locking that wheel.

The ECU also has a safety circuit which constantly monitors the wheel sensors, solenoid control valves, and the electrical circuitry. If only the tractor has an ABS system, the tractor warning light (TRAC ABS) comes on after turning on the ignition switch. See Fig. 7.2 . Once the vehicle moves faster than about 4 mph (6 km/h), the warning light will go out if all of the tractor's ABS components are working.

A wheel spin indicator light (WHL SPIN) comes on if one of the drive wheels spins during acceleration. See Fig. 7.2 . The light goes out when the wheel stops spinning. When the light comes on, partially release the throttle pedal until the light goes out. If slippery road conditions continue, engage the axle lock.

If the vehicle engine is electronically equipped, an automatic traction control (ATC) system may be installed. On these vehicles, the ATC system automatically reduces wheel spin during reduced-traction starts.

An "ATC Function" switch (if equipped), may be used under more adverse conditions. Turning on this switch illuminates the DEEP SNOW/MUD indicator light ( Fig. 7.2 ). The ATC system now allows some wheel spin (for example, to help burn through a thin layer of ice) during reduced-traction starts.

The Meritor WABCO® ABS system combines one front-axle control channel with either one rear axle (the 4-sensor system), or two rear axles (the 6-sensor system) to form one control circuit. For example, the sensor and solenoid control valve at the left-front axle form a control circuit with the sensor(s) and solenoid valve(s) on the right rear axle(s). If, during vehicle operation, the safety circuit senses a failure in any part of the ABS system (a sensor, solenoid control valve, wiring connection, short circuit, etc.), the tractor warning light (TRAC ABS) comes on and the control circuit where the failure occurred switches to normal braking action. ABS will continue to work on the unaffected system. Even if the ABS system is partially or completely disabled, normal braking ability is maintained. A damaged solenoid control valve or combination solenoid control valve can create an exception; since they are an integral part of the air brake system, normal braking may be impaired or may not work at all.

During emergency or reduced-traction stops, fully depress the brake pedal until the vehicle comes to a safe stop; do not pump the brake pedal. With the brake pedal fully depressed, the ABS system will control all wheels to provide steering control and a reduced braking distance.

Although the ABS system improves vehicle control during emergency braking situations, the driver still has the responsibility to change driving styles depending on the existing traffic and road conditions. For example, the ABS system cannot prevent an accident if the driver is speeding or following too closely on slippery road surfaces.

Antilock Braking Systems (ABS) on tractors are designed to communicate with trailer ABS systems, if they are compatible. Compatibility will result in the illumination of the trailer ABS lamp during vehicle start-up and fault detection.

The dash-mounted lamp will operate as follows when a compatible trailer is properly connected to a tractor:

The Trailer ABS lamp will not illuminate unless a compatible trailer is connected to the tractor.

Automatic slack adjusters are required on all vehicles equipped with air brakes manufactured after October 20, 1994. Automatic slack adjusters should never be manually adjusted except during routine maintenance of the foundation brakes (e.g., replacing shoes), during slack adjuster installation or in an emergency situation.

When the brake pushrod stroke exceeds the legal brake adjustment limit on a vehicle, there is likely a mechanical problem with the foundation brake components or the adjuster is improperly installed.

Visit a repair facility as soon as possible when brakes equipped with automatic slack adjusters are determined to be out of adjustment.

The Bendix hydraulic brake system includes a power booster, master cylinder, reservoir, hydraulic lines, a brake rotor on each wheel hub, and a brake caliper and pad assembly at each rotor.

The master cylinder controls braking power to the front and rear brakes. The Bendix Hydro-Max® power booster is attached to the rear of the master cylinder and is connected to the power steering system (which provides pressurized power steering fluid). An electrically powered "reserve" pump operates if there is inadequate fluid flow from the power steering pump to the power booster. The brake system warning light comes on if there is a problem within the system. Bring the vehicle to a safe stop and correct the problem before continuing operation of the vehicle. See Group 42 of the Business Class® Trucks Service Manual for hydraulic brake system troubleshooting and adjustment procedures.

Before driving the vehicle, secure all loose items in the cab so that they will not fly forward during a full brake application. Make sure that all occupants are wearing seat belts. Check that the brake system warning light is off after releasing the hand brake. If the warning light does not go off, correct the problem before continuing operation of the vehicle.

During normal brake stops, depress the foot brake control pedal until braking action slows down the vehicle. Increase or decrease the pressure on the pedal so that the vehicle comes to a smooth, safe stop. When the forward speed of the vehicle has decreased to almost the idling speed of the engine, push the clutch pedal in, and shift the transmission into Neutral. Apply the hand parking brake if the vehicle is to be parked.

The Meritor WABCO® Antilock Braking System (ABS) is an electronic wheel speed monitoring and control system that works with the hydraulic brake system. ABS passively monitors vehicle wheel speed at all times, but controls wheel speed during an emergency or reduced-traction stop. In normal braking applications, the standard hydraulic brake system is in effect.

ABS includes signal-generating tone wheels and sensors located in the wheel hubs of each sensed axle. The sensors transmit vehicle wheel speed information to the ECU located behind the passenger seat which interprets these signals to calculate wheel speed and a vehicle reference speed. If the calculations indicate wheel lockup, the appropriate control circuit signals the brake pressure modulator to increase or decrease braking pressure.

The electronic control unit also has a safety circuit that constantly monitors the wheel sensors, brake pressure modulator, and the electrical circuitry. The dash warning light (TRAC ABS) comes on after the ignition is switched on. Once the vehicle moves faster than about 4 mph (6 km/h), the warning light goes out only if all of the tractor's ABS components are working.

If, during vehicle operation, the safety circuit senses a failure in any part of the ABS system (a sensor, brake pressure modulator, wiring connection, short circuit, etc.), the warning light (TRAC ABS) comes on and the control circuit where the failure occurred is switched to normal braking action. Even if the ABS system is partially or completely inoperative, normal braking ability is maintained. An exception would be if the brake pressure modulator, or hydraulic fluid line is damaged. As these components are an integral part of the hydraulic brake system, normal braking may be impaired or inoperative.

During emergency or reduced-traction stops, fully depress the brake pedal until the vehicle comes to a safe stop; do not pump the brake pedal. With the brake pedal fully depressed, the ABS system will control all wheels to provide steering control and a reduced braking distance.

Although the ABS system improves vehicle control during emergency braking situations, the driver still has the responsibility to change driving styles depending on the existing traffic and road conditions. For example, the ABS system cannot prevent an accident if the driver is speeding or following too closely on slippery road surfaces.

Antilock Braking Systems (ABS) on tractors are designed to communicate with trailer ABS systems, if they are compatible. Compatibility will result in the illumination of the trailer ABS lamp during vehicle start-up and fault detection.

The dash-mounted lamp will operate as follows when a compatible trailer is properly connected to a tractor:

The Trailer ABS lamp will not illuminate unless a compatible trailer is connected to the tractor.