Just like any other system a technician will face, when a fault surfaces within an electronic power steering assist system (EPS) it’s “sink or swim.” EPS came to be like many of the other technologies that surfaced on newer vehicles. By removing the load from the engine (created by the hydraulically assisted/power steering pump driven systems) fuel economy was improved and ultimately tailpipe emissions were notably reduced.
EPS allowed for steering assist via an electronically controlled, three-phase DC motor and electronic control unit (ECU) managing the operation of the system. The steering-column-integrated motor provides assistance in both directions of the steering wheel and follows the input from the driver (Figure 1).
These systems perform very well, are virtually silent and eliminated the dependency on the horsepower-robbing, hydraulically-assisted power steering systems of yesterday.
System Functionality
The computer-driven EPS is just that. A system that is not smart in the slightest. It’s simply programmed to respond to inputs so it can generate the proper output. Two of the main inputs to the ECU are the steering angle sensor and torque sensor(s) — depending on design.
The steering angle sensor is a resolver which allows for converting mechanical rotation into useful analog signals. The ECU uses these analog signals to determine the steering wheel angularity (Figure 2). The resolver signals that are output from the sensor correlate to a point within the 360-degree rotation in which the steering wheel is positioned. To clarify, this input serves as feedback to the ECU to know where the steering wheel is currently positioned. This is not only a factor in EPS strategy but also for other systems like stability control, to anticipate an oversteer or understeer condition.
Utilizing three circuits to function, the sensor has a primary winding (known as an exciter circuit) and a pair of secondary windings (known as sine and cosine). The two secondary winding circuits are offset from each other, and their associated signals vary in amplitude, sinusoidally, at any steering angle. The combination of the two secondary winding circuits’ signals provides accurate information that correlates with the actual steering angle.
The torque sensor is a dual hall-effect sensor. Its job is to report torque (steering effort/input) signals to the ECU and direction of rotation (Figure 3). There are two torque sensors so the ECU can distinguish between input from the steering wheel and input from the road wheels (which occurs when the vehicle contacts the curb, as an example). It’s the combination of these inputs that allows the EPS system to provide the right amount of assistance, in the correct direction and at the appropriate time.
Again, these are the main inputs that the ECU relies on to determine the amount of steering input effort, and the direction of that steering effort being applied to the steering wheel. The use of these signals and that of the vehicle speed will determine the amount of assistance required. The slower the vehicle is moving, the more power steering motor torque is required.
The EPS system is also used in newer advanced driver assist systems (ADAS) like intuitive parking assist. The electronically controlled EPS works in conjunction with the ultrasonic parking sensors/ADAS sub-systems. The EPS’ job in this instance is to negotiate steering wheel turning angle, vehicle trajectory and proximity to foreign objects/other vehicles so it can place the vehicle accurately and safely alongside a curb and in between vehicles in front and behind it, for instance.
EPS System Strategy
So how does this all come together? This scan tool capture of graphical EPS data tells a story. It displays action/reaction type testing to demonstrate how the EPS system strategizes to provide power steering assist (Figure 4).
In GREEN is the data from the torque sensor. Again, this is the data that represents the amount of force the driver is placing on the steering wheel. In BLUE is the data from the EPS assist motor current (amperage). Otherwise stated, “the electrical work being performed.” This represents how hard the motor is working to assist the driver in turning the steering wheel.
There are three visible events occurring on the scan tool capture (Left-to-right). The first event is derived from turning the steering wheel to the left. As can be seen, the GREEN trace indicates the driver provided input to the steering wheel. This is the input the ECU uses to initiate the EPS assist motor command. That command is being carried out as indicated by the BLUE amperage trace.
The second event is similar to the first. However, the steering wheel is now being turned to the right. This is indicated by an increase in amplitude from the torque sensor (GREEN trace). This time, the sensor signal is transitioning in the opposite direction, to match that of the steering wheel’s change in direction. As a result, the ECU commands the assist motor to output and provide assistance (BLUE trace).
The third event is another turn to the right. However, this event appears drastically different. During this event, the input was not at the steering wheel. Instead, I grabbed ahold of the tire and turned it to the right (input from the road wheel instead of from the driver, at the steering wheel). What is being displayed here is very minimal input from the torque sensor and as a result, very little assistance from the EPS motor (low current displayed in BLUE).
In fact, the only reason for the change in input torque being displayed is because of the steering wheel’s inertia as it is being driven by the rotating steering column. I hypothesize removal of the steering wheel from the column assembly would’ve eliminated most (if not all) of the torque signal input.
So, how does this all apply to the real world and how can it assist us diagnosticians in the work bays? Keep reading.
Testing a Lexus ES350
A good friend of mine was faced with a 2016 Lexus ES350 with a vague complaint from the driver of “Check power steering.” Like any driveability issue we face as diagnosticians, a road test (to not only duplicate the issue, but to also gather appropriate diagnostic data) should be conducted.
Equipped with a Toyota/Lexus GTS+ OE scan tool, the road test was conducted to gather data, and the fault was easily recreated. Alarmingly, the vehicle pulled to the right during a road test!
The vehicle was hoisted on a service lift to remove any load from the road wheels. Again, with the ignition switch in the “run” position, the steering spun to the extreme-right with no input from the driver.
It’s easy to assume that the vehicle’s power steering ECU has simply "gone bonkers" and that is where many technicians falter. But, if we take the time to address the concern and factor in the entire system (inputs versus outputs), we will ensure our ability of obtaining an accurate diagnosis.
With our knowledge and understanding of the individual components that make up EPS we can simply view the data that tells the entire story. This includes all the inputs and the outputs. Placing the data in graphical format will allow the story to be told over time.
Story Time
Since the fault was easily duplicated in the service bay, a road test was no longer required. In fact, a road test was quite dangerous, and the customer took a huge and unnecessary risk by not having the vehicle towed to the repair facility.
The information obtained from the scan tool capture was done right from the moment the ignition was turned to the “run” position, and the steering wheel began to turn on its own (Figure 5). So, what can be discerned from the capture?
BLUE = Wheel Speed
This represents vehicle speed and plays a factor in how much assistance to supplement the driver’s input. This indicates the vehicle is stationary/not moving and will require a lot of assistance to relieve the driver of any excessive effort.
LIGHT BROWN = Motor Current
This represents the mechanical work or effort the motor is outputting. This provides the necessary assistance (calculated by the ECU) to aid the driver in turning the steering wheel.
RED = Steering Angle Velocity
This represents how fast the steering wheel is rotated. It is reflected in “degrees per second.”
GREEN = Motor Rotation Angle
This is indicated in “degrees of rotation” and represents the change in angular position of the steering wheel.
PURPLE = Torque Sensor No. 1 + BROWN = Torque Sensor No. 2
Indicated in “voltage,” these inputs work in tandem to indicate either input from the steering wheel or input from the road wheels. Without having two different torque sensors, the ECU couldn’t distinguish between input from the road wheels or input from the steering wheel.
For instance, right-turn input from the road wheels will cause torque sensor voltages to oppose each other in one direction; Right-turn input from the steering wheel will cause torque sensor voltages to oppose each other in the other direction. Together, these inputs represent the driver’s input, meaning how hard the steering wheel is being turned, and in which direction.
So, the story to be told from left-to-right is as follows:
- The GREEN amperage trace shows about -80 amps. This is because the steering wheel is being held stationary as the ignition is switched to “run.”
- The RED steering angular velocity trace is at ”zero” because of the stationary steering wheel.
- The GREEN motor rotation angle trace indicates about 20 degrees (just to the left of the 12 o’clock position).
- The PINK+BROWN torque sensor traces indicate the left-turn force on the steering wheel. This input is the reason for the motor current being applied.
Just prior to frame No. 12 (according to the y-axis of the graphs), the steering wheel is released from hand and the torque sensor inputs go awry, indicating steering wheel input torque for a hard right-hand turn.
The ECU is programmed to respond to this input by increasing EPS motor current for assistance, as indicated by the LIGHT-BROWN trace. The RED trace and the BLUE trace show the angle and velocity of the steering wheel changing when the motor assistance is applied.
In the end a conclusive diagnosis can be drawn. I hope all of you realize that the torque sensors have failed and created a false input request for right-hand turn assistance. The component is non-serviceable and requires the replacement of the entire steering column assembly. After speaking with the customer, he admitted to striking a curb one cold, frozen winter morning and this vehicle exhibited the symptoms ever since.
Although the subject vehicle’s configuration required replacement of the steering column assembly, regardless of what failed (because it was all-inclusive), it doesn’t mean all vehicles will be configured the same way.
It is our job as diagnosticians to draw a diagnosis based on data acquisition and analysis. But none of that is possible without a thorough understanding of the system being analyzed, its configuration, and the functionality of the components that compise it.
The next time you are faced with a challenging symptom, step back and make sure you fully understand how it functions. Leverage the power of service information to determine how the system is configured. Refer to wiring diagrams to determine where and how to test the individual circuits. This will keep you out of trouble and force you to educate yourself.
By stepping away from the vehicle and focusing your efforts on developing a diagnostic game plan, your time will be better spent, and your accuracy (as well as efficiency) will increase many times over. And that is something you can’t hang a price tag on.
