Steering Systems & Electric Power Steering (EPS)

Section 6.3: Steering Systems & Electric Power Steering (EPS)

Steering Gear Configurations and Principles

Automotive steering systems convert the steering wheel's rotary motion into the linear motion needed to steer the front wheels. The most common configuration is the rack and pinion steering gear. The steering column shaft connects to a pinion gear inside the steering housing. The pinion meshes with a geared rack that moves side-to-side, translating motion to the steering knuckles via inner and outer tie rod ends.

Another configuration is the recirculating ball steering gear, common in heavy-duty trucks and older SUVs. The steering shaft turns a worm gear, which recirculates steel balls to move a nut along the shaft. This nut rotates a sector shaft connected to a pitman arm. The pitman arm moves a steering linkage assembly consisting of a center link, an idler arm, and tie rods.

The steering ratio is the ratio of steering wheel rotation degrees to the steering angle of the front tires. A ratio of $15:1$ means the steering wheel must be turned $15^\circ$ to turn the front wheels $1^\circ$. Lower ratios provide faster, more responsive steering.

Electric Power Steering (EPS) Architectures

Electric Power Steering (EPS) has replaced hydraulic systems due to its efficiency. Hydraulic systems use an engine-driven pump that draws power constantly, whereas EPS systems use an electric motor that only draws current when steering assistance is active, improving fuel economy by up to $3%$. EPS also integrates with advanced driver assistance systems like Lane Keep Assist (LKA).

EPS configurations are classified by assist motor placement:

1. Column-assist EPS: The motor and ECU are mounted on the steering column inside the cabin. This is common in smaller, lightweight vehicles.
2. Pinion-assist EPS: The motor is mounted on the steering gear housing, applying assist force directly to the steering gear pinion shaft.
3. Rack-assist EPS (belt drive/direct drive): The motor is mounted parallel to or coaxially around the steering rack, driving a ball-screw mechanism via a belt. This configuration provides the high force required for heavy-duty SUVs and trucks.

Critical EPS Sensors: Torque and Steering Angle

An EPS system determines the required assistance by monitoring driver steering inputs. Two sensors are critical: the torque sensor and the steering angle sensor.

Torque Sensor

The torque sensor measures the twisting force the driver applies to the steering wheel. It is the primary input the EPS ECU uses to calculate motor assistance. A torsion bar is positioned inside the steering gear, linking the steering wheel shaft to the pinion shaft. When the driver turns the wheel, the torsion bar twists by a small angle (usually under $5^\circ$).

Modern systems use magnetic Hall-effect torque sensors. For safety, the sensor uses a dual-channel design, generating a Main torque signal (TRQ1) and a Sub torque signal (TRQ2). When the steering wheel is at rest, both signals output a neutral voltage, typically $2.5\text{ V}$. When torque is applied, TRQ1 increases while TRQ2 decreases by an equal amount (e.g., $3.0\text{ V}$ and $2.0\text{ V}$). The EPS ECU verifies that the sum of the two signals remains constant ($TRQ1 + TRQ2 = 5.0\text{ V}$). If this sum deviates, the ECU immediately disables power assistance and sets a DTC, protecting against runaway steering assist.

Steering Angle Sensor (SAS)

The Steering Angle Sensor (SAS) measures absolute steering wheel position, rotational speed, and direction. It is typically mounted on the steering column behind the clockspring. The SAS communicates with the EPS and ESC modules via the Controller Area Network (CAN) bus. The SAS must be calibrated so that $0.0^\circ$ corresponds to the steering wheel being perfectly centered while the wheels are straight.

Zero-Point Calibration and Diagnostic Routines

A critical procedure is the zero-point calibration of the steering angle and torque sensors.

Calibration Procedure

This calibration must be performed after:

• Wheel alignment adjustments.
• Replacing the steering gear, EPS motor, or steering column.
• Disconnecting or replacing the vehicle battery (on some models).
• Removing or centering the steering wheel.
• Replacing steering or suspension components (e.g., tie rod ends, control arms, struts).

The calibration procedure using a scan tool is:

1. Position the vehicle on a level surface, ensuring the front wheels are straight and the steering wheel is mechanically centered.
2. Connect the scan tool to the OBD-II port, turn the ignition on (engine off), and retrieve any DTCs. Clear any existing DTCs before proceeding.
3. Select the EPS or ESC module and navigate to the Calibration routine.
4. Keep hands off the steering wheel (no torque input) and the vehicle stationary.
5. Initiate the calibration command. The ECU will reset the current sensor outputs to $0.0^\circ$ (for angle) and $0.0\text{ N}\cdot\text{m}$ (for torque).
6. Access live data and verify that the SAS reads $0.0^\circ$ (within a tolerance of $\pm 1.5^\circ$) and the torque sensor reads $0.0\text{ N}\cdot\text{m}$.
7. Road-test the vehicle to confirm normal returnability and verify no EPS warning lights or codes return.

If this calibration is not performed, the ESC and EPS modules may receive conflicting data, causing steering pull or setting DTC C1515 (Torque Sensor Zero Point Calibration Undone) or C1581.

Mechanical Diagnostics and Common Pitfalls

• Memory Steer: If the steering wheel does not return to center after a turn, the vehicle suffers from memory steer. This is caused by mechanical binding (seized upper strut mount bearings, binding ball joints, or over-tightened rack guide preload) rather than EPS failure.
• Steering Wander: The vehicle drifts side-to-side, requiring constant correction. Worn outer or inner tie rod ends allow the wheels to move independently. Insufficient caster angle also reduces self-centering force.
• EPS Current Draw: EPS motors draw up to $80\text{ to }100\text{ Amps}$ during stationary steering maneuvers. A weak battery, alternator, or corroded ground strap can cause system voltage to drop, causing the EPS module to temporarily shut down and default to manual steering. Always test charging system health when diagnosing intermittent steering heavy complaints.