Diagosing and repairing a steering system that has been damaged in a collision requires a basic understanding of these mechanical systems. Regardless of model-specific challenges, steering systems share many basic techologies and components necessary to make the vehicle obey the steering input of the driver. The following details common steering system concepts, components, and diagnostic techniques.
Regardless of the specific type of steering system, getting control of the vehicle's direction involves the steering wheel, the steering shaft, and linkage that connect the shaft movement to the front wheels. The steering system may be either manual or power-assisted. A manual steering system relies on the driver to provide the force needed to change wheel direction while a power-assist steering system uses hydraulic pressure as an assist, reducing the input force required.
The basic operation is the same for both manual and power steering. As the driver turns the steering wheel, the steering shaft (located within the steering column) rotates. The steering shaft is connected to a steering gear (either a separate gearbox or the pinion gear of a rack and pinion unit). The movement of this input gear causes linkage to move left or right. Linkage connects the steering gear to the steering arms, which results in the wheels turning to the right or left as needed.
Mechanical Advantage and Steering Ratio
"Mechanical advantage" is the ratio of the output force to the input force applied to a mechanical device. Using mechanical advantage, a small input force can result in a larger output force. As the driver applies a relatively small input force to the steering wheel, a much greater output force is applied to the wheels.
A 10-lb. force applied to the steering wheel may produce up to 270 lbs. or more force at the wheels. When the steering wheel is turned, this results in movement of the tire/wheel assembly. This means that whatever force is used by the driver must result in moving the steering gear, which applies movement to the steering arms.
When the steering arms move, this force must be great enough to move the tires. This means overcoming the frictional force of the tires against the ground. Considering the weight of the vehicle that is applied to the tires, this represents a great amount of weight that must be overcome.
Realizing this, you can gain an appreciation of the work that must be done by the steering system. Mechanical advantage produced by the steering gear system allows wheel movement to take place. This mechanical advantage is due to the steering ratio of the steering gear.
Steering ratio is the number of degrees that the steering wheel must turn in order to pivot the front wheels by one degree. Steering ratios vary, depending on the type of vehicle and its intended use. Typical automotive steering ratios range from about 24:1 with manual steering to about 14:1 with power steering assist. The higher the steering ratio, the easier it is to turn the wheel and steer the vehicle.
The lower the ratio is, the more effort is needed at the steering wheel. A quicker-ratio steering may need higher force at the steering wheel but may pivot the wheels faster for faster steering response. Steering ratio is determined by steering gear ratio and by steering linkage ratio.
Steering gear ratio in the pitman arm steering gear depends on the angle and pitch of the worm and sector gear teeth. In a rack and pinion steering assembly, the steering ratio is determined by the number of teeth on the pinion gear. The fewer teeth on the pinion gear, the higher the ratio. The more teeth on the pinion gear, the lower the ratio.
Types of Steering Systems
Some steering gears provide variable ratio steering. This allows the steering ratio to change, or vary, as the steering wheel is turned away from its straight-ahead position. A typical change might be from 16:1 to 13:1.
For the first 40 degree of steering wheel movement in either direction, the steering ratio remains constant. This faster steering ratio offers more control for highway driving. As the steering wheel is turned beyond 40 degrees from the straight ahead position, the steering ratio decreases. This lower and slower steering ratio helps the driver in city driving when cornering or parking. The steering wheel doesn't have to be turned as far in order to pivot the wheels.
A small number of cars use a four-wheel steering system. In this type of system, the rear wheels turn as well as the front wheels. The purpose is to provide greater handling control and greater ability to make tight turns. The rear wheels of a four-wheel steering system can be controlled either mechanically or electronically.
In order to provide more control at low speeds, the rear wheels may pivot in the opposite direction as the front wheels, allowing the rear of the car to pivot more. This can be useful in tight spaces or when parking. At higher speeds, the rear wheels will steer in the same direction as the front wheels, providing greater stability during cornering and high-speed lane changes.
The manual steering system has four major components:
- The steering wheel and steering shaft are the "input" portion of the steering system. This allows the driver to input steering commands.
- The steering gear connection changes the rotary motion of the steering shaft into linear motion (left and right motion). The steering gear translates the input commands of the steering wheel and shaft into "output" commands.
- Another component is the steering linkage, which connects the steering gear to the steering arms. Since the wheels are attached rigidly to the steering arm assemblies, the wheel turns whenever the steering arms move to the right or left. The steering motion that connects the steering gear to the wheels involves linkage that handles steering "output."
- Finally, the steering arms are part of the wheel mounting assembly. Steering arms are basically extended lever arms that connect the wheel to the steering linkage.
A power steering system uses a device that assists the steering effort. This makes it easier for the driver to turn the steering wheel. A power-assist steering system multiplies the force that the driver applies at the steering wheel.
The most common type of power assist is the hydraulic type. This features a hydraulic pump (a power steering pump) that pressurizes hydraulic fluid to the steering gear system, helping the driver to move the gears. In some cases, a vehicle might be equipped with electronic power steering, where an electric motor provides the power assist.
All hydraulic power steering systems basically work in the same way. A hydraulic pump pressurizes the hydraulic fluid. Through hydraulic hoses and tubes that attach the power steering pump to the steering gear, this pressurized fluid is then made available to the steering gear when it's needed.
When the steering wheel is turned, the control valve in the power steering gear opens and closes various fluid passages inside the gear housing. The pressurized fluid enters the gear housing, causing the piston to move, which places hydraulic force at the gears, helping them to move. This power assist reduces the amount of steering effort needed by the driver.
Many vehicles today use speed-sensitive power steering. This is a variable-assist system that uses an electronic control to decide how much power assist is needed at any given time. Additional parts in this type of system may include a special electronic control module (ECU) called a power steering control module, a steering angle sensor, vehicle speed sensor, and solenoid valve. Some cars will not use a steering angle sensor, and will only use the vehicle speed sensor signal to provide input to the ECU.
If a steering angle sensor is included, this sensor detects the rate of steering wheel movement, and the vehicle speed sensor detects vehicle road speed. These signals are inputs that go to the electronic controller. In turn, the controller decides how much steering assist is needed and sends a signal to the solenoid valve.
Cars equipped with a variable assist power steering system will feature a separate diagnostic connector at the steering ECU, allowing the technician to retrieve diagnostic codes for this system. Electronic variable power assist can be found on vehicles with either rack and pinion steering or recirculating ball steering.
A mechanical four-wheel steering system features a mechanical connection between the front steering gear and rear steering gear. This may be done with a long steering shaft that connects to a pinion output shaft at the front rack and to a special rear steering gear. The rear steering gear is then connected to each rear wheel by a rear tie rod.
During small input turns of the steering wheel, such as during highway driving, the rear wheels are steered by a small amount, in the same direction as the front wheels. When the steering wheel is turned more during low speed operation, the rear wheels will turn in the opposite direction as the front wheels.
An electronic four-wheel steering system uses an additional power steering pump for the rear steering gear, and a separate rear control valve assembly for the rear. When the steering wheel is turned, the hydraulic pressure that is applied to the front steering gear acts on the rear steering system's control valve. This directs pressurized fluid to move from the rear power steering pump to the rear power cylinder.
The power cylinder then forces the rear trailing arms to move right or left. In this type of system, the rear wheels always move in the same direction as the front wheels, up to a maximum of 1.5 degrees. The rear wheel steering angle changes in proportion to vehicle speed, wheel traction and steering wheel input.
Some cars may use electromechanical four-wheel steering. This features a mechanical connection, using a steering shaft that connects the front steering gear to the rear steering gear. In addition, the ECU sends control signals to the rear steering gear. At low speeds, the rear wheels will turn in the opposite direction of the front wheels, and at high speeds, the rear wheels turn in the same direction as the front wheels.
Electronic Rack and Pinion
Some cars may be equipped with an electronic rack and pinion power steering gear. Instead of using a flat rack with straight teeth as found in a conventional rack, this type of steering gear uses a helical-gear rack driven by a moving ball nut.
The ball nut is acted upon by a fast-acting electric motor. A magnet and a magnetic sensor mounted on the pinion shaft works as a torque sensor, which signals the ECU, telling the computer how much steering torque is being applied by the driver, and in what direction. As the magnetic sensor detects greater movement, the signal sent to the ECU becomes stronger.
This causes the ECU to send a varying voltage to the electric motor inside the rack housing. The ball nut, which is directly attached to the electric motor, then runs over the helical grooves on the rack, causing the rack to move right or left as needed. An electric power steering rack does not use a hydraulic system, so there is no power steering pump, hoses, reservoir, or fluid.
Pressure Testing
Before performing a pressure test, inspect all hoses and fittings for condition. If any hoses are soft and spongy, hard and brittle, cracked, reveal worn rub spots, etc., the hose must be replaced immediately. Tubes that are rusted, dented, cracked or otherwise damaged must also be replaced.A fluid pressure test can be performed to determine if the pump, hoses, tubes and steering gear is operating under proper internal pressures. A fluid pressure gauge is installed at the high pressure (feed) tube at the steering gear. Using a power rack and pinion as the example, disconnect the pressure feed tube from the rack and pinion steering gear housing and attach the feed tube that runs from the pump to the gear to the "in" position of the gauge.
Connect the pressure inlet port of the steering gear to the OUT position of the gauge. With the power steering system bled and the engine idling, and the gauge valve open, turn the steering wheel from lock to lock several times. With the engine idling, close the gauge valve and note the gauge reading. A pressure reading of about 925 psi may be noted, though this may vary depending on the system (always check the service manual).
With the engine idling, open the valve fully. Measure the fluid pressure at engine speeds of 1,000 rpm and 3,000 rpm. The difference in fluid pressure should be about 71 psi or less.
With the engine idling and the valve fully open, turn the steering wheel to a full-lock position. Fluid pressure should be at least 925 psi (again, specific pressure specification may vary). Turn the engine off and allow the system to cool to room temperature before disconnecting the gauge. Once the tubes are reconnected to the steering gear, bleed the system and check fluid level.
Power Rack and Pinion Diagnosis
If vibration is felt in the steering wheel and/or the dash during parking or low speed operation, it is possible that air is trapped in the power steering fluid. The vibration should go away after a few miles of operation. However, a loose or worn tie rod or tie rod end might also be the cause.
If the steering catches, binds or sticks in certain positions, or is difficult to turn, there are a number of possibilities:
- Low power steering fluid
- Under-inflated tires
- Dry, non-lubricated front ball joints
- Worn lower ball joints
- Dry outer tie rod ends
- A loose power steering drive belt
- Faulty power steering pump
- Excessive friction in the steering column
- A binding lower ball joint
- A worn front strut spring seat and bearing
- Excessive friction in the steering gear
If the steering is hard to turn, or if the driver experiences momentary increases in effort when turning the steering wheel, the power steering pressure switch might be bad, or the steering gear assembly might have a leak in the high pressure circuit. Other causes might include under-inflated tires, low power steering fluid level, dry ball joints, worn lower ball joints, or low power steering pump pressure. If the rack and pinion unit was damaged in a collision, the toothed rack might be bent or chipped.
Steering Column Diagnosis
During a front-end collision, the steering column may have to be replaced because of the design, which may allow the column to collapse in order to absorb impact energy. In most cases, the steering column must be replaced if the vehicle was involved in a collision that deploys the air bag, even if the damage appears to be light. The steering shaft assembly may feature an intermediate coupler that connects the upper shaft to the lower shaft.
This is also easily damaged in a collision and may require replacement. If a chirp, squeak or rubbing sound exists, this may involve the steering column. Inspect for contact between the shroud, intermediate shaft, column and steering wheel. Re-align or clean and lubricate as needed.
If the steering catches, binds or sticks in certain positions, the bind may be in the steering column. Check the shaft for bind, and check the steering coupler. If excess play exists in the steering wheel, check the steering shaft universal joints for wear or damage
