11.3 Electronic Power Steering (EPS) Diagnostics
Key Takeaways
- An electric power steering motor can draw 60-100 amperes at full assist, so it is fed by a dedicated high-current fusible link straight from the battery.
- A torque sensor uses two independent signal tracks whose values must correlate, and a divergence beyond the calibrated window sets a correlation fault and removes assist.
- Low system voltage from a weak battery or failing alternator is a frequent cause of reduced or intermittent assist, so charging system health must be verified first.
- The steering angle sensor is shared with stability control and normally requires a calibration routine after any steering, suspension or battery-disconnection work.
- Losing electric assist increases steering effort dramatically but the steering stays mechanically connected, so the vehicle remains steerable.
What the Competency Standard Requires
Repair Electronic Power Steering (EPS) requires you to diagnose faults with a scanner and clear codes, check and replace a faulty fuse, relay and control module, check the power steering motor and replace faulty parts, and check the wiring harness for cuts or damage and repair or replace as required. The listed underpinning knowledge includes the basics of electronic power steering and types of sensors and their use.
Why EPS Replaced Hydraulic Steering
A hydraulic system drives a pump from the engine continuously, consuming power even when the vehicle is travelling straight. Electric power steering draws current only while assist is actually being applied, which improves fuel consumption, removes the pump, the hoses and the fluid, and allows software features such as speed-sensitive assist, lane keeping and self-parking.
Three EPS Layouts
| Layout | Motor location | Typical application | Service note |
|---|---|---|---|
| Column assist (C-EPS) | On the steering column inside the cabin | Small and medium cars | Motor and control unit are often a single serviceable assembly; noise transmits into the cabin |
| Pinion assist (P-EPS) | On the steering gear pinion in the engine bay | Medium cars and light commercials | Exposed to heat and water; connector corrosion is common |
| Rack assist (R-EPS) | A belt-driven ball-nut on the rack itself | Larger and heavier vehicles | Highest current draw; usually a complete rack replacement |
In all three the steering column remains mechanically connected to the road wheels. When assist is lost the vehicle is still steerable, but effort rises sharply, especially when parking. That is an important point for customer advice: loss of assist is a serious drivability complaint but not a loss of steering control. Steer-by-wire systems, in which the mechanical link is replaced by redundant electronics, are the exception and are found on only a small number of models.
The Torque Sensor
The torque sensor sits in the steering column and measures how hard the driver is twisting the steering wheel, usually by measuring the small angular deflection of a torsion bar.
- It is a dual-track device. Two independent signals are produced, typically arranged so that as one rises the other falls, or so that the two sum to a constant value such as 5 V.
- The module continuously checks the two tracks against each other. A divergence beyond the calibrated window sets a correlation fault, and the module removes assist immediately because it can no longer trust the driver's input.
- Torque sensors are not usually serviced separately; they form part of the column or gear assembly.
- In live data you should see the torque value sitting near its centre value with the wheel released, and swinging cleanly and symmetrically to each side as the wheel is turned against resistance.
The Steering Angle Sensor
The steering angle sensor (SAS) reports absolute steering wheel position and rate of rotation. It is shared with the stability control system, so its faults appear in both places.
- It is often integrated into the clockspring assembly, which means any airbag or clockspring work can disturb it.
- It requires calibration or zeroing after a wheel alignment, a steering or suspension repair, a clockspring replacement, or on some vehicles after a battery disconnection.
- An uncalibrated sensor typically causes uneven assist between left and right turns, an illuminated stability control lamp, or a straight-ahead position that the module believes is off-centre.
Power Supply: Where Most Real Faults Live
This is the single most useful diagnostic insight in the section. An electric power steering motor can draw 60-100 amperes at full assist, which is more than most other loads on the vehicle apart from the starter. It is therefore supplied by:
- A dedicated high-current fusible link or mega-fuse taken directly from the battery or the main distribution point.
- A heavy earth bolted straight to the body or the subframe.
- A separate low-current ignition feed for the module's logic circuits.
The consequences are predictable:
| Fault | Symptom |
|---|---|
| High resistance in the high-current feed | Assist fades or cuts out during parking manoeuvres when current demand peaks, then returns |
| Corroded or loose earth | Intermittent assist loss, often worse in wet weather |
| Weak battery or failing alternator | Reduced or pulsing assist, because system voltage sags under load; the module reduces assist deliberately to protect system voltage |
| Blown mega-fuse | Complete loss of assist with a stored supply-voltage code |
| Logic feed lost but power feed present | No assist and no communication with the scan tool |
Therefore: test the battery and charging system before condemning an EPS module. A vehicle with a tired battery can set genuine EPS undervoltage codes while every steering component is perfect, and replacing the module will not fix it. Carry out the voltage-drop tests from section 3.2 on both the high-current feed and the earth while an assistant turns the wheel against resistance with the vehicle stationary, because that is when demand peaks.
Thermal Protection
EPS modules include a thermal derating strategy. Sustained low-speed manoeuvring, such as repeated full-lock parking or a long period of kerb-side shunting, heats the motor and the driver stage. The module then reduces assist progressively and may set a temperature-related code.
This is designed behaviour, not a fault. Customers describe it as "the steering went heavy after parking for a while and then came back". Recognising it prevents an unnecessary repair. A genuine fault would not recover after a cooling period.
Diagnostic Sequence
- Scan all modules. EPS codes often accompany stability control, ABS or engine codes, and the pattern shows whether the root cause is shared, such as a network or supply fault.
- Check battery state of charge and charging system output before anything else.
- Read live data: system voltage, motor current, torque sensor tracks, steering angle, assist level and module temperature.
- Inspect the high-current fuse or fusible link and its connections.
- Voltage-drop the power feed and the earth while assist is being demanded.
- Inspect the harness at the column boot and along the steering gear for chafe, heat damage and water ingress, which the competency specifically calls out as checking the wiring harness for cuts or damage.
- Check connector pin tension and corrosion with the pin drag method from section 2.3.
- Only then consider the motor or module. Test the motor per the manufacturer's method; most modern EPS motors are brushless three-phase units driven by an inverter inside the module, so a simple resistance test across two pins is not a valid assessment.
- Repair, clear codes, calibrate and road test.
Calibration and Verification
After almost any EPS-related repair the following may be required, and the manufacturer's service information is the authority:
| Procedure | When it is needed |
|---|---|
| Steering angle sensor zero point calibration | After alignment, steering or suspension work, clockspring replacement, module replacement or on some vehicles a battery disconnection |
| Torque sensor zero point / neutral learning | After column, gear or module replacement |
| Module configuration or coding | After module replacement, to match the vehicle variant and tyre size |
| End stop learning | On some systems, to teach the module the rack travel limits |
Verification road test. Confirm on a road test that assist is present and symmetrical left and right, that the steering wheel returns to centre naturally after a turn, that the wheel sits straight when travelling straight, that no warning lamps illuminate and that no codes return.
Faults the Practical Assessment Likes to Present
- Assist lost after parking manoeuvres, restored after a rest - thermal derating, which is normal.
- Assist lost intermittently in the wet - corroded earth or connector at the steering gear.
- Heavier steering to one side only - uncalibrated steering angle sensor.
- No assist with an undervoltage code and a two-year-old battery - the battery, not the EPS system.
- No assist and no scan tool communication - the logic supply or the network connection, not the motor.
Safety
- Disconnect the battery and observe the manufacturer's stand-down period before working near the steering column, because the airbag and clockspring are in the same assembly. Never probe airbag circuits, as covered in section 12.1.
- Lock the steering wheel before removing a clockspring, and never rotate the road wheels or the column with the clockspring disconnected, since this de-centres the ribbon and destroys it.
- Never apply power directly to an EPS motor on the bench. Full assist torque at the column can cause a serious crush injury, and the motor is designed to be driven only by its inverter.
- Re-torque steering fasteners to specification with new fixings where the manufacturer specifies single-use bolts.
- Always road test in a safe area after steering work, starting at low speed.
A customer reports that the power steering assist on their vehicle fades away during tight parking manoeuvres and then returns after a short drive. The steering system stores an undervoltage code, and the battery is four years old. What should the technician verify before considering the electronic power steering module?
On a vehicle with column-assist electronic power steering, the driver reports that the steering suddenly became very heavy while driving and a warning lamp illuminated. What is the correct description of the vehicle's condition?
A vehicle has had a wheel alignment and a clockspring replacement. Afterwards the driver reports that the steering feels lighter turning left than turning right and the stability control lamp is illuminated. What is the most likely cause?