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EPS Steering Assist

Modern electric power steering (EPS) steering gears are inextricably linked with body and chassis control electronics, which is a very good reason for an alignment technician to develop a working relationship with a modern, enhanced scan tool.

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Following the Straight and Narrow: Why the Scan Tool is the Alignment Technician’s Best Friend

Modern electric power steering (EPS) steering gears are inextricably linked with body and chassis control electronics, which is a very good reason for an alignment technician to develop a working relationship with a modern, enhanced scan tool. 

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As just one example, the degree of steering assist to the driver is electronically linked to vehicle speed. To control steering assist, the EPS module needs a vehicle speed sensor (VSS) input that likely originates from the anti-lock braking system (ABS).  

In many cases, we might need an enhanced scan tool to locate the source of a missing VSS signal. Let’s also not forget that, with the new advanced driving assist systems (ADAS), features like “lane keeping assist” require electronic data from many different operating systems, including electric steering, to keep the vehicle centered in its driving lane. In a nutshell, the enhanced scan tool will provide most of the data needed to successfully diagnose problems related to the EPS.

ELECTRIC STEERING COMPONENTS 

The use of an electric motor like that found in Nissan’s Electro-Hydraulic steering to power a conventional hydraulic steering pump does not qualify as an EPS steering system. In contrast, an EPS steering system must be connected to the steering shaft on a conventional steering gear or be integral to a rack-and-pinion steering gear to provide direct steering assist to the front wheels.  

EPS is composed of a torsional input sensor, a steering wheel angle sensor, a 12-volt EPS assist motor and a control module. The control module processes steering wheel input data and relays that data to other safety-related systems like vehicle stability controls (VSC) and lane-keeping assist modules. All of these components might be incorporated into a single non-serviceable unit or each might be individually serviceable, depending upon vehicle application.

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TORSIONAL INPUTS

Conventional hydraulic and electric steering incorporate a torsion bar-type steering torque sensor that indicates steering wheel torque by deflecting or twisting a few degrees in response to right or left steering wheel torque, which is electronically translated into degrees of steering shaft deflection. 

This torsional input data is relayed to the EPS steering module, which reacts by pulsing the steering assist motor with battery voltage. More torsional input from the steering wheel increases the pulse width on the steering assist motor, further increasing steering assist. When the steering control module senses an irrationality in steering wheel torque input, it should store a corresponding diagnostic trouble code (DTC) in the steering control module, which we can retrieve with an enhanced scan tool. 

STEERING WHEEL INPUTS 

Steering wheel angle position sensors are an integral part of electric steering because they indicate the relationship of the steering wheel to the centerline of the vehicle and to the driving lane. Steering angle input is vital to many safety functions including vehicles equipped with electric vehicle stability controls and lane keeping assist. The most important thing for an alignment technician to know about electric power steering is that an incorrect steering angle input can produce a steering quality complaint, which is why the steering angle must be reset after each wheel alignment or EPS service.

CENTERLINE VS. THRUST LINE 

Let’s differentiate between thrust angle and the vehicle centerline. The thrust angle is the angle in degrees at which the rear drive axle is pushing the vehicle in relation to the vehicle’s geometric centerline. In contrast, vehicle centerline is the actual, longitudinal center of the vehicle’s chassis.  

In theory, the thrust angle should align with the vehicle centerline. In reality, non-adjustable rear suspensions like those found in solid rear drive axles and some independent rear suspension systems seldom align perfectly with the vehicle centerline. The result will be that when front wheels are aligned with the thrust angle of the rear wheels, a “dog-tracking” effect will occur as the vehicle travels along its driving lane. That in itself does not create a problem with ADAS as long as the steering angle sensor is corrected for an off-center thrust angle.

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The rear wheel thrust angle is adjustable on most current vehicle models equipped with independent rear suspension. When the rear wheels are aligned exactly parallel to the vehicle centerline, we have zero degrees thrust angle. When we align the rear wheels first and the front wheels second, all four wheels should be parallel with the vehicle centerline. In any case, the wheels and the steering angle sensor should align with the vehicle centerline, as should the radar transmitter providing data to the active cruise control and automatic emergency braking. All three angles should ultimately be aligned with the geometric center of the driving lane. 

AAA AND ADAS: The American Automobile Association (AAA) separates ADAS-related driving systems into five categories:

1) Automated Driving tasks, which include adaptive cruise control and “dynamic driving assistance.” 

2) Collision Alerts, which include forward collision and lane departure warnings. 

3) Collision Mitigation, which includes automatic emergency steering, forward automatic emergency braking, and reverse automatic emergency braking.  

4) Parking Assistance, which includes fully-automated, semi-automated and remote parking assistance. 

5) Miscellaneous Driving Aids, which include automatic high beams, driver monitoring, night vision, and the like. 

NETWORKED ELECTRONICS

According to the latest list of “automated driving tasks” produced by the AAA, “dynamic driving assistance” controls vehicle acceleration, braking and steering (The Society of Automotive Engineers (SAE) standard definition of L2 Autonomous Systems outlines this functionality). In other words, we might see situations in which a vehicle stability control module might take control of the EPS steering gear during emergency driving situations.

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While most 2018 vehicles won’t feature autonomous driving systems, almost 14% will feature Lane Keeping Assist, which is directly controlled by the EPS steering. Many vehicles will also include driver monitoring, which warns the driver by “buzzing” the steering wheel when the vehicle begins to wander from its driving lane.

USING ENHANCED SCAN TOOLS 

The definition of an “enhanced” scan tool is an aftermarket unit that contains many original equipment manufacturer (OEM) scan tool features and diagnostic capabilities. Remember that, while the aftermarket scan tool might not have full OEM capability, it also might contain diagnostic features not found in the OEM tool. 

As for using an aftermarket scan tool, I begin by storing the vehicle identification number (VIN), all diagnostic trouble codes and individual data stream captures in the scan tool memory for future reference. Most modern vehicles will automatically poll all modules for communications issues and trouble codes. If that feature isn’t available, you should manually poll all modules and store that data in your scan tool.  

I have solved some no-code EPS problems by using the scan tool to graph the torque and steering angle sensor voltage outputs to detect any voltage variations or drop-outs when steering from lock-to-lock. Regarding pulling complaints, the rear and front wheel alignment must be correct before the steering angle sensor can be relearned. If your alignment machine isn’t capable of electronically performing torque and steering angle sensor relearns, your currently updated scan tool usually has that capability.  

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When diagnosing no-code problems, learn to analyze serial data with your enhanced scan tool. This often requires jumping from one window to another to analyze data inputs and outputs. To simplify this problem, relevant data from most scan tools can be customized to fit a single window. Select the most important data for your diagnostic purposes.

Never just use your aftermarket scan tool as a mere code reader because it has so many more diagnostic features like voltage graphing, code tips, and data storage capabilities. The scan tool learning curve can be a little lengthy, but it’s time well spent and will make you a far more productive alignment diagnostic technician. 

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