Blue Print Tech: Electric Power Steering continued

EPS communication, safety features, diagnostic data and trouble codes

Blue Print Tech: Electric Power Steering continued
EPS is fitted to the majority of all small and medium sized cars.

Continuing from Blue Print’s last technical article, this week Blue Print is looking at the communication between the EPS unit and the rest of the vehicle, the safety features built in to the system to protect the driver in the event of an electrical failure and the diagnostic data and trouble codes (DTCs) generated when a fault occurs.


Electric power steering opens up all sorts of possibilities from speed sensitivity to self-parking and driving. The Hyundai has three features which use information from the CAN Bus system:

1. Speed Related Assistance: As the vehicle speed increases, the amount of motor assistance is reduced.
2. Active Return: Below 25mph the system will assist the return to the Actual Steering Position (ASP).
3. Yaw Dampening: Information about excessive understeer or oversteer will affect the amount of assistance if the steering is turned in a direction which will exacerbate excessive yaw.

The G-Scan and G-Scan2 are OEM level diagnostic equipment for both Hyundai and Kia with, mostly, self explanatory live data.

Calibrated steering angle offset

This is the difference between the set Actual Steering Position and the centre position, as calculated by comparing wheel speed. Offsets up to 5° are acceptable; larger offsets indicate the need for recalibration.

Percentage of de-rating

If the temperature of the ECU rises above 85°C, the motor current is de-rated, which actually indicates normal function at 100 per cent de-rating and will give a maximum current of 78A.

Maximum de-rating is 40% which reduces maximum current to 31A, thus limiting the assistance and load on the MDPS unit.


If the steering fails the steering can only be turned with great effort, so a failure of the system would be very disconcerting for the driver.

Total failures can be caused by a failure of the motor, its sensors or a loss of battery power.


The failsafe relay and motor relay are energised when the ignition is turned on, giving motor assist until all three following conditions are met:

1. The ignition feed is off.
2. The CAN system indicates the vehicle speed is less than 3Kmh.
3. The CAN system indicates the engine speed is less than 500Rpm.

These conditions indicate that the vehicle has stopped and the ignition has been turned off so it is safe to shut-down.

The system will continue to operate if there is a loss of CAN signal whilst driving, loss of ignition signal whilst driving or both.

The EPS warning light will not illuminate for safety reasons until the start of the next drive cycle.

If the CAN system fails whilst driving then the system will continue to operate and the EPS light will illuminate.

The system will continue to assist for 1 minute when the ignition is switched off, and if the ignition is switched on and no CAN signal is available, the system will operate with the engine running or not.

What goes wrong?

The system can generate 32 different DTCs, which can be grouped into types.

Transient faults can occur during operation such as:
• C1261 – ASP not calibrated: This can happen if the battery voltage drops too low during operation or if the battery is disconnected while the ignition is on.
• C1603 – Derating (EPS Thermal Protection): Commonly reported by drivers as derating can cause an alarming reduction in assistance.
• C1653/4 – Voltage too high/too low: Can be caused by low voltage in the battery or jump starting from another vehicle.

Faults of a more terminal nature include:
• C2400 – Motor failure: The system will have detected open or short circuits in the motor windings or a failure of the speed and position sensors.
• C1290 – Torque sensor failure: This fault could be either an electrical failure or a failure of the optical system.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

The reCAPTCHA verification period has expired. Please reload the page.

Have your say!

0 0

Lost Password

Please enter your username or email address. You will receive a link to create a new password via email.