Steering and Brake Systems

Steering System

General Information

When there is no load on the vehicle, and the front tires are pointed straight ahead, the steering wheel spokes should be at the 4 and 8 o'clock positions, ±10 degrees. See Fig. 6.1 . See Group 46 of the Heavy-Duty Trucks Service Manual for steering adjustment procedures.

1. 8 o'Clock
2. 4 o'Clock

Fig. 6.1, Power Steering Wheel Position (with tires pointed straight ahead)

Never steam clean or high-pressure wash the steering gear. Internal damage to gear seals, and ultimately the steering gear, can result.

Manual Steering Gear Lubrication

When operating in temperatures continuously below 30°F (–1°C), using the wrong type of lubricant in the steering gear could make the steering wheel difficult to turn. To prevent trouble in cold temperatures, be sure that the steering gear lubricant complies with the cold weather recommendations specified in Group 46 of the Heavy-Duty Trucks Maintenance Manual .

Failure to comply with the cold weather recommendations could cause inadequate steering control, which could result in an accident and possible physical injury.

Power Steering System

The power steering system consists of a 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.

Driving the vehicle without the power-assist feature of the steering system requires much greater effort, especially in sharp turns or at low speeds, which could result in an accident and possible injury.

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.

Brake System

General Information

A dual air brake system uses a single set of brake controls to operate two independent air brake systems. 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.

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.

The rear service brakes will not work if the primary system loses air pressure, but the front brakes and the trailer brakes will continue to work with secondary system air pressure. Loss of secondary system air pressure makes the front axle brakes inoperative, but rear service brakes and trailer brakes will operate through the primary system.

Before driving your vehicle, allow time for the air compressor to build up at least 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. The brake pedal will slow the vehicle but since one brake system will not be working, stopping distance will increase. 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.

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.

Brake System 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 passengers are wearing seat belts.

IMPORTANT: An air brake proportioning system is used in tractor air brake systems. When operating in bobtail mode, the rear brake chambers (whose axle load has been greatly reduced) receive reduced or proportional air pressure, while the front axle brake chambers receive full (normal) air pressure. This results in a different brake pedal "feel," as the pedal seems to require more travel and/or effort to slow or stop the vehicle. However, the air brake proportioning system actually improves vehicle control when the tractor is in the bobtail mode of operation. When the tractor is towing a trailer, the rear brake chambers will receive full (normal) application air pressure.

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 spring parking brakes if the vehicle is to be parked.

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.

NOTE: If equipped with main and auxiliary transmissions, do not shift both transmissions into Neutral while the vehicle is rolling. Shifting both transmissions back into gear would be difficult while the vehicle is rolling.

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 remains in the position selected until it is manually moved.

1. Turn Signal Lever
2. Trailer Brake Hand Control Valve
3. Hazard Warning Light Tab

Fig. 6.2, Steering Column-Mounted Controls

Do not use the trailer service brakes for parking; they are not designed for this purpose. If air bleeds out of the trailer air tank during parking, the vehicle could roll, causing serious personal injury or property damage.

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.

1. Trailer Air Supply Valve Knob
2. Parking Brake Valve Knob

Fig. 6.3, Trailer Air Supply/Parking Brake Control Panel

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 the trailer spring parking brakes and automatically causes the trailer air supply valve knob to pop out.

Do not use the spring parking brakes if the service brakes are hot, such as after descending a steep grade. Also, do not use the spring parking brakes during freezing temperatures if the service brakes are wet. To do so could damage the brakes if hot, or cause them to freeze during cold weather.

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. Always chock the tires.

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.

If a trailer or combination vehicle is not equipped with spring parking brakes, do not park it by pulling out only the trailer air supply valve knob. This would apply only the trailer service brakes. If air were to bleed from the trailer brake system, the trailer brakes would release, possibly causing an unattended runaway vehicle.

Never apply the service and spring parking brakes simultaneously. To do so transmits excessive input force to the brake components, which could damage or cause eventual failure of brake actuating components.

Meritor WABCO® Antilock Braking System (ABS)

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, but controls wheel speed during an emergency or reduced-traction stop. In normal braking applications, the standard air brake system is in effect.

IMPORTANT: For proper ABS system operation, do not change tire sizes. The sizes of the tires installed during production are programmed into the electronic control unit. Installing different sized tires could result in a reduced braking force, leading to longer stopping distances.

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 an electronic control unit (ECU) located behind the passenger seat. The ECU interprets the speed sensor signals and compares wheel speed, wheel braking and vehicle speed. If it senses wheel lockup, the ECU signals the appropriate solenoid control valve to reduce braking. During emergency braking, the solenoid control valve constantly adjusts air pressure supply in the brake chambers to prevent wheel lockup.

An accumulation of road salt, dirt, or debris on the ABS tone wheels and sensors can cause the ABS warning light to come on. If the ABS light does come on, the tone rings and sensors should be inspected for corrosion and serviced if necessary. The service should include cleaning of the tone rings and sensors. If any tone ring on a vehicle shows severe corrosion, all tone rings on that vehicle should be replaced.

During winter months in areas where corrosive materials are used on the highways, periodically clean the underside of the vehicle, including the tone rings and sensors, to ensure proper ABS function and to protect the components from corrosion. Clean more frequently when unusually corrosive chemicals are being used.

The electronic control unit also has a safety circuit that constantly monitors the wheel sensors, solenoid control valves, and the electrical circuitry. If only the tractor is equipped with an ABS system, the tractor warning light (TRAC ABS) comes on after the engine is started. See Fig. 6.4 . If all of the tractor's ABS components are working, the light should go out when the vehicle moves faster than about 4 mph (6 km/h).

1. Parking Brake Indicator Light
2. Tractor ABS Warning Light
3. Wheel Spin Indicator Light
4. Trailer ABS Warning Light

Fig. 6.4, Warning and Indicator Lights

If equipped with the tractor and trailer ABS system, the vehicle also has a trailer warning light labelled TRLR ABS ( Fig. 6.4 ).

After the engine is started, the TRLR ABS light comes on if the trailer is equipped with a compatible ABS system. Once the vehicle moves faster than about 4 mph (6 km/h), the TRLR ABS warning light goes out only if all of the trailer's ABS components are working.

For more information on trailer ABS lamp operation, see Trailer ABS Lamp Operation in this chapter.

A wheel spin indicator light (WHL SPIN) comes on if one of the drive wheels spins during acceleration ( Fig. 6.4 ). 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.

Do not engage the axle lock while the WHL SPIN light is on. To do so could damage the rear axle. See Chapter 9 , "Rear Axles," for axle lock instructions.

If the vehicle is equipped with an electronic engine, 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), allows the driver to select from two levels of drive axle traction-control assistance:

NORMAL—reduces drive axle wheel spin on icy, wet, or sand covered roads.

DEEP SNOW/MUD—allows a higher threshold of drive axle wheel spin to help burn through a thin layer of ice, or to help throw off accumulated mud or snow.

The DEEP SNOW/MUD mode is indicated by a flashing WHL SPIN light. To engage this mode, the ATC function switch must be in the NORMAL position when the vehicle is initially powered up. Once the vehicle is started, the ATC function switch can be set to the DEEP SNOW/MUD position. The ECU indicates this change by a constant flashing of the WHL SPIN lamp (or by illumination of the DEEP SNOW/MUD light ( Fig. 6.4 ) on vehicles built before July, 1994).

If the ATC function switch is in the DEEP SNOW/MUD position when the vehicle is powered up, the ECU will not accept this function change and will remain in the NORMAL mode. Indication of this condition will be the absence of the flashing WHL SPIN light (on vehicles built before July, 1994, the absence of an illuminated DEEP SNOW/MUD light). To engage the DEEP SNOW/MUD mode in this situation, change the position of the ATC function switch to the NORMAL mode. After 2 seconds, move the switch to the DEEP SNOW/MUD position. When this occurs, the indicator light will activate as previously described.

IMPORTANT: ABS and ATC blink code diagnostics should only be performed when the vehicle is stopped. If the vehicle is equipped with ATC, turning "On" the ABS CHK switch will reduce engine speed to idle for 3 seconds and affect ATC function performance. If the vehicle is driven with the ABS CHK switch "On" (in the "Up" position), the WHL SPIN light will illuminate continuously. Under this condition, turning the ABS CHK switch "Off" (in the "Down" position), while the vehicle is still in motion, will only turn off the WHL SPIN light, but will not restore ATC function performance. To fully restore engine and ATC function capability, the vehicle must be brought to a complete stop. Then, move the ABS CHK switch to "Off" (in the "Down" position).

The Meritor WABCO ABS system combines one front-axle control channel with either one rear axle (the 4-channel system), or two rear axles (the 6-channel 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 control 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 is switched to normal braking action. The remaining control circuit will retain the ABS effect. Even if the ABS system is partially or completely inoperative, normal braking ability is maintained. The vehicle is not disabled.

IMPORTANT: If any of the ABS warning lights do not work as described above, or come on while driving, repair the ABS system immediately to ensure full antilock braking capability.

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 drive appropriately for 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.

Trailer ABS Lamp Operation

Antilock braking systems 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:

When the ignition key is turned to the ON position, the trailer ABS lamp will illuminate momentarily, then turn off.

If the lamp comes on momentarily during vehicle operation, then shuts off, a fault was detected and corrected.

If the lamp comes on and stays on during vehicle operation, there is a fault with the trailer ABS. Repair the trailer ABS system immediately to ensure full antilock braking capability.

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

IMPORTANT: If a compatible trailer is connected and the lamp is not illuminating momentarily when the ignition key is turned to the ON position, it is possible that the lamp is burned out.

Automatic Slack Adjusters

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.

Manually adjusting an automatic slack adjuster to bring the pushrod stroke within legal limits is likely masking a mechanical problem. Adjustment is not repairing. In fact, continual adjustment of automatic slack adjusters may result in premature wear of the adjuster itself. Further, the improper adjustment of some automatic slack adjusters may cause internal damage to the adjuster, thereby preventing it from properly functioning.