About I-CAR - Current Events/News: e-newsletters: Advantage Online: June 17, 2002

June 17, 2002 -Power steering using hydraulics was introduced in the early 1950s. As vehicles grew larger and heavier, steering wheels had gotten bigger also. Power steering eliminated those large steering wheels and parking was suddenly much easier. As with most innovations, power steering was first sold on luxury class vehicles. The first hydraulic pumps were large, heavy devices requiring a lot of power from the engine. Today almost every vehicle comes standard with power steering.

Now, electric power steering has entered the field, thanks to the innovation of smaller, lighter electric motors. Electronic control systems give engineers the ability to design sophisticated programs, allowing variable assist, power steering without the engine running, and tuning the steering system to the vehicle without changing mechanical parts. The coming 42-volt electrical systems will provide more powerful steering systems, applicable to heavy vehicles with larger tire contact patches. Already in demonstration at several equipment makers and vehicle makers are steering systems with individual controls at each wheel.

Advantages

The advantages of electric power steering are many in that the complex system of pump, hoses, cooling lines, and metering valves is replaced with one motor, mounted on the rack or on the column. Assist can continue while the vehicle is moving, even if the engine stops, as long as battery voltage is above 9 volts. Using electricity means engine horsepower is free to motivate the vehicle. Additional horsepower is needed to produce the required power from the alternator, but an electric steering system is up to 80% more efficient than a hydraulic system.

Saturn Vue EPS

Figure 1–The EPS control module (on the bottom), motor, and torque sensor are all located within this assembly.

In 1993, Acura introduced the first electric power steering system on the NSX. The first electric power steering system from a domestic vehicle maker is on the 2002 Saturn Vue. The system uses a column-mounted motor/control system. There are three parts that make up the Electric Power Steering (EPS) system: the motor, the EPS control module, and the torque sensor. All these are contained in one assembly (see Figure 1). The assembly is mounted under the instrument panel, below the upper steering column. The motor is mounted on the side and drives the column through a reduction and worm gear.

The torque sensor has two rings that are fed alternating current. The upper and lower steering column, referred to as the input and output shafts, are connected by a torsion bar, much like the spool valve in hydraulic power steering systems. The magnetic detection rings sense any twist between the input and output shafts and send a signal to the EPS control module. The control module uses several signals to compute the needed assist: vehicle speed, temperature, and driver torque.

A tuning profile is programmed into the EPS control module when It’s installed. This will determine the amount of assist at various speeds. Different profiles can be supplied by engineers, so the same control module can be used on several vehicles. Also, the same vehicle with a larger engine and tires could be supplied with a little more assist. At this time, there are nine programs for this control module.

The ignition voltage is used to determine if the engine is running or not. With the engine not running, assist will be maintained until some preset time interval after the vehicle speed reaches zero. If the EPS motor is overheated, from excessive parking maneuvers for example, the amount of assist will be reduced until the motor temperature is lowered.

Damage Analysis Of The Vue EPS

With all of the hydraulic pump system removed from the engine compartment, the number of items to inspect after a collision is reduced. The motor will be either on the rack or under the instrument panel and is much less likely to be damaged than the power steering pump mounted on the front of the engine.

Figure 2–Collapsing of the steering column is measured at this capsule nut location on both sides of the steering column.

Saturn has strict inspection guidelines for collision-damaged vehicles. Any collision resulting in sheet metal damage or deployment of the driver airbag requires inspection for electrical system integrity and column collapse.

Figure 3–One side of this steering column was intentionally collapsed to show what it looks like when measuring at the capsule nut location. Click on the left image to see a larger view of the capsule nut location not shifted. Click on the right image to see a larger view of a measurement that is out of specification (beyond 3 mm)

A Tech 2 service tool can be used to check the electrical system. If the electrical system checks within specification, but the column collapse mechanism is not, or the steering wheel has been distorted, the steering column upper jacket assembly must be replaced. Inspecting the upper steering column capsule nuts for movement is done by removing the knee bolster and column shrouds. This reveals the upper steering column capsule nuts (see Figure 2). If more than 3 mm movement is observed, the column has collapsed too far and the steering column upper jacket assembly must be replaced (see Figure 3). The replacement steering column upper jacket assembly is available as a kit (see Figure 4).

Figure 4–The steering column upper jacket kit includes replacement bolts and grease for the splines.

The EPS motor, torque sensor, and control module are mounted below this upper column. Replacement of the entire steering column, including the motor and control module assembly and the upper jacket, is required if the electrical system integrity is not within specification. See the Video for this steering column replacement procedure.

While replacing the entire column or the upper jacket is a relatively simple mechanical procedure, there are several things to note. The control module has a separate ground that must be attached. All bolts need to be replaced with supplied parts. If the control module is dropped it must be replaced, and the system must be programmed after installation.

Conclusion

With the obvious advantages of fewer parts, flexible design, and fuel savings, electric power steering will be showing up on more vehicles in the near future. This may simplify collision repairs by eliminating power steering hydraulic fluid from the engine bay. But it will require specific vehicle repair manuals when service is required.

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