# Electric Power Steering

## Overview

### Component Location

1. Steering wheel

2. Steering column assembly

3. Steering rack assembly

### General

The Electric Power Steering (EPS) system comprises a steering wheel, a collapsible steering column assembly that is bolted to the car cross beam, a collapsible intermediate shaft comprising upper and lower links, and an electrically operated rack and pinion steering gear. The column connects to the steering gear via the intermediate shaft. The steering tie rod ball joints are screwed onto the ends of the rack bar, and connect to forward facing steering arms that are integral with the aluminum suspension knuckles.

## Component Descriptions

### Steering Column Assembly

1. Intermediate link - lower

2. Intermediate link - upper

3. Sliding carrier

4. Steering column

5. Steering column control module

6. Steering wheel

The steering column assembly comprises an aluminum base plate, a sliding carrier, pivot brackets, and a collapsible shaft assembly. The sliding carrier is connected to the base plate using an M-shaped wire, which is permanently deformed by the sliding carrier when the column is subjected to a severe impact. The upper column mounting bolts pass through the column base plate and are secured into weld nuts in the car cross beam.

The three-spoke steering wheel is manufactured from molded rubber, with a leather trimmed rim. It is secured to the steering column using 66 non-indexed splines and a flathead internal hexagon bolt.

Switches for turn signals, cruise control, steering column position, gear selector, and wiper/washer controls are housed in an assembly that is a press fit onto the steering column assembly.

The steering column is adjustable for reach and rake. Two electric motors turn plastic drive nuts that are attached to drive screws, which in turn are connected to the upper pivot bracket and shaft assembly.

### EPS Electronic Control Unit (ECU)

1. EPS ECU

The EPS ECU is a sealed unit mounted on the motor. It houses the microprocessor and the output stage for the motor. The ECU is pre-programmed with vehicle data and operating parameters with which to calculate the motor drive current, based on signals sent by the torque sensor and CAN data signals from other vehicle systems.

The ECU can be interrogated for Diagnostic Trouble Codes (DTCs) and live data parameters using Toolbox.

### Steering Gear

1. Steering rack motor

2. Steering rack

3. Gaiter

4. Inner tie rod

5. Outer tie rod

The steering gear is a rack and pinion unit comprising a cast aluminum housing, torque sensor/pinion assembly, rack bar, electric motor, and EPS ECU assembly and drive nut. Steering tie rod ball joints are screwed onto the ends of the rack bar.

NOTE: Only the outer tie rods are serviceable parts.

### Motor

1. Steering rack motor

The electronically commutated brushless motor is mounted on the steering rack. The motor is positioned parallel to the rack bar and is connected to the rack bar via the drive nut and a toothed rubber belt. The rotation of the motor turns the drive nut via the rubber belt and is translated into linear motion of the rack bar, using a series of ball bearings housed in a precision-ground helical groove machined into the rack bar and drive nut.

### Torque Sensor

1. Torque sensor

The torque sensor mounted on the steering gear input shaft measures the torque applied by the driver at the steering wheel. The sensor contains resistive elements and a pole wheel with magnets. It is connected to the steering pinion with a torsion bar.

## Operation

### Steering Operation

When the driver turns the steering wheel, the torsion bar is rotated, causing the resistance in the torque sensor element to change. This change in resistance is received by the ECU, which analyzes the information along with inputs from other vehicle sensors, such as speed, angle of steering turn, etc. The ECU then generates a current signal to provide the correct amount of assistance for the situation.

The motor receives the current signal from the ECU and converts it into rotary motion. The rotation of the motor is transferred to the drive nut via the rubber belt and is changed into linear motion of the rack bar via a series of ball bearings housed in a helical groove machined into the rack bar and drive nut. Linear movement of the rack is transferred to the tie rod ball joints, steering arms and road wheels.

Motor current and force on the rack bar are directly proportional to the torque input from the driver. The electric steering is speed-sensitive, so steering receives more assistance at low speed and less assistance at high speed.

The EPS has 3 different modes: Comfort, Sport, and Standard. Comfort provides the most steering assistance. Sport provides the least assistance, to give the driver maximum steering feedback. Standard provides a balance between the two opposites.

### System Failure

If the vehicle power supply fails, the driver can continue to steer the vehicle due to the mechanical connection between the steering wheel and the wheels. However, far greater effort than normal is required to turn the steering wheel.