Electric Power Steering (EPS): Operation, Types, Sensors, Calibrations and Diagnosis

1. Operating principle of electric power steering

Electric power steering, or EPS — Electric Power Steering, uses an electronically controlled electric motor to assist the effort the driver applies to the steering wheel.

In a conventional EPS, the mechanical link between steering wheel and wheels still exists. The motor provides additional torque to the column, the pinion or the rack, depending on the design.

Unlike conventional hydraulic steering, EPS does not need a hydraulic pump, reservoir or hoses to provide assistance.

1.1 What happens when the driver turns the wheel?

1. The driver applies torque to the steering wheel.
2. The torque sensor tells the module the size and direction of the request.
3. The module considers other information, such as vehicle speed, steering wheel position and electrical conditions, depending on the design.
4. The controller calculates the desired assist torque.
5. The power electronics circuit drives the electric motor.
6. The motor transmits assistance to the steering mechanism.
7. The system monitors the response and corrects the command continuously.

1.2 Assistance on demand

EPS can reduce energy consumption compared with systems that continuously drive a hydraulic pump from the combustion engine. However, it is not correct to say that the electric motor is always completely off when driving straight.

The controller may apply small corrections, compensate for friction, damp oscillations or keep assistance functions active according to the vehicle’s strategy.

1.3 Driver torque and assist torque

In simplified form, the effort transmitted to the mechanism can be represented as:

The torques must be considered on the same reference axis, with the corresponding gear ratios and mechanical efficiencies.

This expression illustrates the principle; it does not replace the control model and engineering calculations of a real EPS.

2. Difference between manual, hydraulic, EHPS and EPS steering

3. Main electric power steering designs

The most common classification considers where the motor applies the assistance. Do not confuse the motor position with the location of the electronic module, which may be integrated or separate.

3.1 C-EPS — Column assist

In Column Electric Power Steering, the motor and reduction gear apply torque to the steering column. It is a common solution in compact cars, especially when steering loads suit this design.

Advantages: compact assembly, possible installation inside the cabin and relatively simple integration on some platforms.

Points to watch: column, reduction gear, couplings, torque sensors, mountings and integrity of the electronic circuit.

3.2 P-EPS — Pinion assist

In Pinion Electric Power Steering, the motor transmits assistance to the pinion mechanism, directly or through a reduction system.

There are variants, including designs with a second pinion dedicated to assistance.

Points to watch: gear mesh, reduction gear, bearings, gearbox mounting, sensors and electronics.

3.3 R-EPS — Rack assist

In Rack Electric Power Steering, the motor acts on the rack through a transmission mechanism, which may use a belt, a ball screw or another solution.

This design can meet high force demands and is used in different vehicle categories, depending on the design.

Points to watch: force transmission, rack, guides, environmental seals, motor, sensors and modules.

3.4 Which design is best?

No design is universally superior. The choice depends on vehicle mass, axle load, available space, cost, desired behavior, electronic integration and safety requirements.

Final precision depends on the whole system: sensors, control, motor, mechanical transmission, suspension and tires.

4. Electronic and mechanical components

5. Torque sensor: how the EPS senses effort

The torque sensor is a central element of the system. In many designs, it uses the small elastic deformation of a torsion bar to measure the driver’s effort.

The sensor may use magnetic technologies or other measurement solutions. Depending on the design, there are redundant channels and plausibility checks to detect faults.

5.1 Torque is not angle

Torque: the twisting effort applied to the steering wheel.

Angle: the angular position of the steering wheel relative to a reference.

Angular velocity: how fast the steering wheel is being turned.

This information may be measured by different sensors or by integrated modules. It must not be confused during diagnosis.

5.2 Torque reference fault

If the sensor reports a nonexistent or inconsistent effort, assistance may become asymmetric, be limited or be disabled by the safety strategy.

Diagnosis requires reading parameters, codes and specific procedures. Do not improvise a mechanical or electronic adjustment of the zero point.

6. Electric motor and electronic control

Many modern EPS units use brushless motors, often with electronic current and position control. The exact design depends on the supplier and the vehicle.

The module controls motor current to produce the desired assist torque and supervises temperature, power supply and sensor plausibility.

6.1 Relationship between current and torque

As an approximation for certain motors, the torque produced is proportional to the current component responsible for generating torque:

K_t: motor torque constant; I: corresponding current, according to the model used.

This relationship is for teaching. Real control may involve currents on specific electrical axes, thermal limits and nonlinear characteristics.

6.2 Why does EPS draw high current?

During maneuvers with high resistance at the wheels, the motor may need to produce significant torque. This increases electrical demand and makes the integrity of the power supply essential.

Voltage drops, connections with excessive resistance and charging system problems can cause limited or temporarily lost assistance.

6.3 Thermal protection

The controller can reduce assistance when the estimated or measured temperature exceeds protection limits.

Repeated and prolonged maneuvers under high load can contribute to this heating. The strategy and limits depend on the manufacturer.

7. Variable assistance and dynamic behavior

EPS can vary the assist torque according to speed, effort, driving conditions and control strategies.

7.1 When parking

The system usually provides high assistance to reduce the effort needed in low-speed maneuvers.

7.2 On the highway

Control may reduce assistance and apply damping and on-center feel strategies, favoring more stable responses.

7.3 Return to center

Vehicle geometry and tire forces remain fundamental for return. Some EPS add electronic friction compensation or return-assist functions.

A return fault must not automatically be blamed on the module. Caster, tires, joints, column, gearbox and electronic strategy must be evaluated.

8. Integration with other vehicle systems

8.1 Electronic stability control — ESC

The steering angle can be used by electronic stability control to interpret the path the driver wants. The source and sharing of this signal depend on the design.

8.2 Lane keeping assist

In vehicles equipped with this function, the EPS can apply limited torque commands to help the driver stay on course, according to the system’s strategy.

8.3 Park assist

Some systems use the EPS to carry out steering movements during automated or partially automated maneuvers.

8.4 CAN network and communication

The EPS can receive information from other modules over automotive communication networks. Message or configuration faults can compromise assistance even when the motor and local sensors are intact.

9. Preventive inspection of electric power steering

1. Check tire pressure, sizes and condition.
2. Inspect tie-rod ends, rods, ball joints and joints.
3. Examine boots, mountings and signs of impact.
4. Check the battery, charging system and electrical connections.
5. Inspect connectors, harnesses and ground points.
6. Look for moisture or corrosion.
7. Check dashboard warnings and stored codes.
8. Compare assistance in both directions.
9. Watch for return, noise and tight spots.
10. Record conditions before any disassembly.

10. Disassembly and assembly: workshop principles

EPS is a safety-critical system. Some units can be repaired with specialized procedures; others are supplied as assemblies that cannot be repaired in a conventional workshop.

10.1 Preparation

1. Identify the vehicle, EPS variant and part number.
2. Consult OEM procedures, torques and calibration requirements.
3. Record trouble codes and relevant parameters before erasing any information.
4. Center the steering using the specified method.
5. Follow the SRS safety procedure when working on the column.
6. De-energize the circuits according to the manufacturer’s instructions.
7. Use suitable tools and supports.

10.2 Removing a C-EPS

1. Remove trim and access components without damaging connectors.
2. Identify and protect electrical connections.
3. Disconnect the intermediate shaft according to the manual.
4. Prevent improper rotation of the steering wheel and clock spring.
5. Remove the column fasteners in the specified sequence.
6. Remove the assembly without straining sensors, motor or harnesses.

10.3 Removing a P-EPS or R-EPS

1. Check whether the subframe and nearby components need support.
2. Disconnect power and communication in a safe condition.
3. Remove tie-rod ends with a suitable puller.
4. Disconnect the column without changing the steering center.
5. Remove fasteners and the gearbox according to the procedure.
6. Protect connectors and mechanical parts from contamination.

10.4 Assembly and validation

1. Confirm compatibility of the replacement assembly.
2. Center the gearbox and column according to the manufacturer.
3. Install new fasteners when required.
4. Apply the correct torques and sequence.
5. Reconnect power, grounds and communication.
6. Check travel, freedom from interference and mountings.
7. Perform programming, coding or calibration when prescribed.
8. Check geometry and steering wheel centering.
9. Carry out a static test and a controlled road test.

11. Calibrations after maintenance: when are they needed?

A mechanical or electronic job may require calibration, initialization, coding or learning. These procedures are not synonyms and are not required after every repair.

The technician must identify exactly what the manufacturer requires for the vehicle and the replaced component.

11.1 Steering angle sensor (SAS) calibration

The angle sensor reports the steering wheel’s angular position, when present in the design. Its signal may be used by the EPS, stability control and other functions.

On some vehicles, calibration or initialization is required after replacing components, alignment, loss of reference or electronic work.

On others, the system performs automatic learning or requires no additional procedure for that job.

11.2 Torque sensor calibration or learning

Some systems require neutral-point learning, torque sensor calibration or initialization after replacing the column, gearbox or module.

This procedure may differ from steering angle sensor calibration. Do not assume one routine replaces the other.

11.3 Programming and coding

A replacement module may need software programming, variant coding, parameterization or pairing with the vehicle.

The physical compatibility of an EPS gearbox does not guarantee it will work correctly without the necessary electronic procedures.

11.4 Clearing codes is not calibrating

Clearing trouble codes removes the records the system allows, but does not fix the cause or automatically perform calibration.

Some learning functions are started by specific routines. Others occur during driving conditions defined by the manufacturer.

12. Diagnosis with an automotive scan tool

The scan tool is important for checking trouble codes, parameters and service routines, but it must be used together with electrical measurements and mechanical inspection.

12.1 Information that may be available

• Torque requested by the driver.
• Calculated or applied assist torque.
• Steering wheel angle and angular velocity.
• Motor current or command.
• Supply voltage.
• Measured or estimated temperature.
• Assistance status and limitations.
• Communication messages.
• Current, historical or intermittent trouble codes.

The availability and exact meaning of each parameter vary with the manufacturer and the diagnostic equipment.

12.2 How to interpret the data

An apparently inconsistent reading must be compared with the test conditions and the technical documentation. There is no universal torque sensor, current or angle value that applies to every EPS.

The technician must look for consistency between the driver’s command, the system’s response and the mechanical behavior.

13. Power supply: battery, fuses and grounds

EPS can demand high currents during maneuvers. That is why the quality of the power supply directly affects its operation.

13.1 Battery

A battery with reduced capacity or poor connections can contribute to voltage drops when assistance is requested.

13.2 Charging system

In vehicles with an alternator, charging faults can compromise the energy supply. In electrified vehicles, the supply may depend on other designs, such as DC-DC converters.

13.3 Voltage drop under load

A connection may show electrical continuity at rest and still produce an excessive voltage drop when the EPS motor draws current.

The evaluation must be done under load and at the measurement points indicated in the manual.

For the same unwanted resistance in a connector, higher current increases the voltage drop. This explains why contact faults may appear mainly during maneuvers.

14. Main faults and symptoms

15. EPS diagnostic flowchart

15.1 Why not start by replacing the module?

A power supply fault, a sensor with an inconsistent signal or a mechanical problem can produce symptoms similar to an ECU fault.

Replacing modules without confirming the cause increases costs and can introduce new coding and compatibility problems.

15.2 Intermittent fault

When the problem appears only in certain conditions, recording voltage, temperature, steering wheel rotation and DTC history can be decisive.

It is important not to clear records before collecting useful information.

16. Care with impacts, water and end of travel

16.1 Impacts with potholes and curbs

Impacts can damage wheels, tires, tie-rod ends, suspension arms and steering components. After hitting obstacles, check play, deformation, geometry and assistance behavior.

An impact does not automatically prove that the EPS motor’s internal gears are broken.

16.2 Washing and moisture

Although many components have environmental protection, high-pressure jets aimed at connectors, modules or seals can promote water ingress.

After signs of moisture, inspect connectors and harnesses according to the appropriate procedure.

16.3 End of travel

Holding the steering wheel forced against the stop can place high loads on the motor and mechanical transmission. Some systems limit assistance for protection, but that does not justify continuously forcing the mechanism.

16.4 Low voltage

Battery, charging system or connection faults can reduce assistance capacity. The vehicle must be evaluated before continuing to drive with abnormally heavy steering.

17. Safety: column, airbag and unexpected movement

17.1 Essential precautions

• Do not disconnect energized components outside the service procedure.
• Do not improvise by applying voltage directly to the EPS motor.
• Do not measure SRS pyrotechnic circuits with unauthorized instruments.
• Do not freely rotate the clock spring out of its correct position.
• Do not alter sensors or mechanical limiters without technical instructions.
• Do not run active tests with people near moving parts.
• Do not release a vehicle with unstable assistance or binding steering.

18. Workshop case studies

19. Professional inspection and delivery checklist

20. Technical references and safety criteria

For professional training, it is important to distinguish terminology standards, safety requirements and manufacturer-specific procedures.

Sources for further study:

• ISO 26262-1:2018 — Road vehicles — Functional safety.
• ISO 8855 — Vehicle dynamics and road-holding ability.
• SAE J670 — Vehicle Dynamics Terminology.
• Bosch Mobility — Electric steering with a second pinion.

These references do not provide universal disassembly procedures, torques or calibrations for all vehicles. Always consult the service documentation for the application.

21. Technical quiz — 10 questions

Choose one answer for each question. The commented answer key is in the next section.

22. Commented answer key

23. Technical summary for review