Steering and Brake Systems

When there is no load on the vehicle, and the front tires are pointed straight ahead, the steering wheel spokes should be centered, ±10 degrees, as shown in Fig. 6.1 . See Group 46 of the Columbia® Workshop Manual for steering adjustment procedures.

The power steering system consists of an integral steering gear (which includes a manual steering mechanism, a hydraulic control valve, and a hydraulic power cylinder), hydraulic hoses, power steering pump, reservoir, and other components. Some models are also equipped with a separate hydraulic power cylinder on the right side of the front axle. The power steering pump, driven by the engine, provides the power assist for the steering system. If the engine is not running, there is no power assist. If the power-assist feature does not work due to hydraulic fluid loss, steering pump damage, or some other cause, bring the vehicle to a safe stop. Do not drive the vehicle until the cause of the problem has been corrected.

Drivers should carefully use the power available with a power steering system. If the front tires become lodged in a deep chuckhole or rut, drive the vehicle out, instead of using the steering system to lift the tires from the hole. Also, avoid turning the tires when they are against a curb, as this places a heavy load on steering components and could damage them.

A dual air brake system consists of two independent air brake 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.

Before driving your vehicle, allow time for the air compressor to build up a minimum of 100 psi (689 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. If this happens, check the dual system air pressure gauge 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, causing a longer stopping distance. 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 (242 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.

Before the vehicle 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.

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

IMPORTANT: 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 all passengers are wearing seat belts.

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. Apply the spring parking brakes if the vehicle is to be parked.

The trailer brake hand control valve ( Fig. 6.2 ) actuates the trailer service brakes independently from the vehicle chassis service brakes. The valve can be partially or fully applied, but in any partially-on position it will be overridden by a full application of the foot brake control pedal. Moving the valve handle clockwise actuates the trailer brakes, while moving it counterclockwise releases the trailer brakes. The valve handle is spring-loaded and returns to it's original, no-air application position when released.

The red octagonal-shaped knob ( Fig. 6.3 ) in 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 (242 to 310 kPa), the red knob automatically pops out, exhausting the trailer air supply, and applying the trailer service or spring parking brakes.

The yellow diamond-shaped knob ( Fig. 6.3 ) in the control panel actuates the parking brake valve. Pulling out the knob applies both the tractor and trailer spring parking brakes and automatically causes the trailer air supply valve knob to pop out.

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 are 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 65 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 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 standard air brake system. ABS passively monitors vehicle wheel speed at all times, and controls wheel speed during emergency stops. If equipped with Automatic Traction Control, wheel spin is controlled during reduced-traction situations. In normal braking applications, the standard air brake system is in effect.

ABS includes signal-generating tone wheels and sensors located in the wheel hubs of each sensed wheel. The sensors transmit vehicle wheel speed information to an electronic control unit (located on the frame rail). The control unit's main circuit interprets the speed sensor signals and calculates wheel speed, wheel retardation, and a vehicle reference speed. If the calculations indicate wheel lockup, the main circuit signals the appropriate solenoid control valve to reduce braking pressure. During emergency braking, the solenoid control valve alternately reduces, increases, or maintains air pressure supply in the brake chamber to prevent front and rear wheel lockup.

If equipped with Automatic Traction Control, an additional solenoid valve is installed. During reduced-traction situations, the valve increases or reduces air pressure supply in the brake chamber to provide better traction whenever wheel spin occurs.

The electronic control unit also has a safety circuit that constantly monitors the wheel sensors, traction control valve (if equipped), solenoid control valves, and the electrical circuitry.

After the ignition switch is turned on, the tractor warning light (TRACTOR ABS) and, if equipped with Automatic Traction Control, the wheel spin (WHEEL SPIN) indicator light come on for about three seconds. After three seconds, the warning lights go out only if all of the tractor's ABS components are working.

Vehicles with electronic engines and ABS may have automatic traction control (ATC). On these vehicles, the ATC system automatically limits wheel spin during reduced-traction situations.

If the vehicle has ATC, there will be a momentary contact rocker switch on the dash labeled NORM/SPIN and ATC.

When the ATC system is in the NORMAL mode, it will apply gentle braking to the spinning wheel, to feed power to the wheel(s) with better traction. If both wheels are spinning, the system will signal the electronic engine to reduce power.

Pressing NORM/SPIN will temporarily allow more drive wheel spin to help burn through a thin layer of ice, or to help throw off accumulated mud or snow. SPIN mode is indicated by a flashing WHEEL SPIN light. Pressing NORM/SPIN again will cycle the system back to normal operation.

The Meritor WABCO ABS system combines one front-axle control channel with one rear axle to form one control circuit. For example, the sensor and solenoid control valve on the left-front axle form a control circuit with the sensor and solenoid control valve on the right rear axle. 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 (TRACTOR ABS) comes on and the control circuit where the failure occurred is switched to normal braking action. The remaining control circuit will retain the ABS effect. Even if the ABS system is completely inoperative, normal braking ability is maintained. An exception would be if a solenoid control valve (or combination solenoid control valve) is damaged and inoperative. As these components are an integral part of the air 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.

The Meritor WABCO Electronic Braking System (EBS) is designed to continuously monitor and coordinate all aspects of service braking.

The EBS pairs an electronic control system with a pneumatic application system. Key braking data such as driver brake demand, wheel speed, and vehicle load is gathered by a network of electronic sensing devices (including load sensors, and signal generating tone wheels and sensors located in the wheel hubs of each sensing axle).

An electronic control unit, the central module, oversees the translation of electronic signals into pneumatic actuation. During a braking application, electronic-pneumatic modulators control the brake pressure of each wheel, as well as at the trailer coupling head. As a result, the same amount of vehicle deceleration will occur for a given brake pedal travel, regardless of vehicle load. During emergency stops (i.e. wheel lockup), the EBS also activates the Antilock Braking System (ABS).

The EBS includes a system check for all modulators, sensors, brake system electric circuits and data lines and is thereby able to detect malfunctions in the braking system. When a fault is detected, the central module will disable the affected part of the EBS. Under this safe status (i.e. fail-safe) mode, the nonaffected parts will continue to function. For example, in the case of a sensor malfunction, the system works without the sensor information but at a reduced performance level.

If a serious malfunction causes a total loss of one or more electronic braking circuits, a pure pneumatic backup system is available.

The following functions can be deactivated in the event of a malfunction:

This system has built-in diagnostics to ensure that all components are operating properly. Under normal conditions, the system performs a three second self-check when the ignition is turned on. The two warning lamps (EBS, ABS) and the information lamp (ATC) should illuminate for three seconds and then go out if all EBS components are working properly. If all three lamps do not switch on or if all three lamps do not switch off after three seconds, the system should be checked.

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.