Most basic of systems need three sensor inputs of steering angle, wheel speed and vehicle speed. These basic systems come on many import and domestic vehicles that came out more than a decade ago. The systems had connections to the engine control module and maybe even the ABS system. But, these connections were one-way and helped the engine to compensate for loads on the alternator.
The most important sensor on some later-model systems is the torque sensor. Steering torque added a third dimension to the power steering system. A torque sensor measures how much force the driver is applying to the wheel and how much force is being sent through the rack to the driver. These sensors have become so sensitive that they can detect if the driver has his hands gently on the wheel, or if the vehicle is on a rough road.
The other part of the evolution of electric power steering is the computer processors that can take many inputs and quickly control the motor to control the steering.
Electric power assist does more than add force in either direction; it can push back to improve handling and driver comfort. These corrections could be as innocuous as adding a little assist for road crown, or as obvious as the vehicle steering on its own. But, a lot of the improvements in electric power steering have mostly gone unnoticed by the driver and even the technician.
The electric power steering system measures steering wheel torque being applied by the driver to keep the vehicle in the lane. When the software detects extra effort being used, the steering assist motor adds torque in the proper direction to prevent the driver from having to make corrections to keep the vehicle on the path he selected. This active assist reduces driver fatigue and effort and makes steering easier. The software automatically resets itself to compensate for changing road conditions or the vehicle turning on curves.
This software will compensate for a specific range of lead/pull up to its maximum limit within a set of parameters that can prevent a motor from overheating. This type of active compensation can be disabled with a scan tool so the cause of the pull can be remedied.
Other features can look at road feedback and vibration that might even be caused by an out-of-balance tire. If the imbalance is above a certain level, the power steering control module will disable the “smooth road shake compensation” mode. Most systems will set a code.
On some late-model trucks, the electric power steering can prevent the steering wheel from being ripped from the driver’s hands while off the road. This type of off-road mode can be engaged when the low range in the transfer case is selected. This rock-crawling mode can be found on trucks ranging from Jeep to Land Rover.
Early electric power steering systems liked to play it safe. Automakers programmed these systems to shut down the motor rather than let it burn out. At the earliest sign of trouble, the system would shut down, warning the driver via a dash light in the hope that the vehicle would be serviced.
This “protection” mode detects and estimates excessive amounts of current draw that could burn out the motor. Overheating can occur if the steering wheel is held at a locked position for a long time, or if the driver was compensating for a pull.
The early GM systems had rudimentary diagnostics and a malfunction indicator light in the dash. It performs a self-check when the vehicle starts to confirm signals and communicates with the ABS and ECM module.
This safe mode creates a diagnostic challenge for the technician because the vehicle will likely be brought to your shop in this shutdown condition, which makes observing the root of the problem difficult until more tests can be performed.
New systems are more diagnostic friendly and offer technician more insights into the modes and sensors. The modern electric power steering module communicates with at least the ABS and engine control module on a high-speed CAN bus. The power steering module can also look at other information through a gateway module. In some cases, the system will look at the ambient temperature to determine performance.
The most advanced electric steering application uses a bidirectional brushless motor, sensors and an electronic controller to provide steering assist. Like older systems, sensors located in the steering column measure two primary driver inputs — torque (steering effort) and steering wheel speed and position.
The controller processes the steering effort and wheel position through a series of algorithms to assist and return to produce the proper amount of polarity and current to the motor. What really sets more advanced systems apart from previous systems is the resolution of the torque and steering position sensors. The sensors can measure smaller changes in the driver inputs when compared to sensors on some 2005-‘11 GM systems.
There are three different types of electronic torque sensors, and they are classified as contact and non-contact types. A non-contact sensor uses a magnetic rotor with alternating pole pieces and is attached to the torsion bar. Hall-type sensors monitor the twist of the torsion bar by measuring the change in magnetic flux generated by its position to the vanes located on the sensor stator rings.
Torque and more advanced steering angle sensors are complicated to diagnose with a scope unless the manufacturer has test procedures and values. Often, the best way to start is with the data PIDs on a scan tool. If the sensor signals are erratic, the control module can rationalize that there is a problem.
The main problem with modern steering systems is the driver. The average driver will jerk on the wheel at the first signs of trouble. This can make a situation even worse. New steering systems work with the ABS/ESC systems to make small corrections to the steering to keep the vehicle pointed straight. These types of conditions can occur if a car is braking on surfaces with uneven levels of traction. The BMW system, for example, will make tiny corrections to the wheel angle (typically not noticed by the driver) to make the correction with the ABS modulator more effective. These system integrations would not be possible if the yaw and steering angle sensors were not accurate.
Adaptive or active steering changes the ratio between the driver’s actions at the steering wheel and the rack. In traditional vehicles, this is a fixed steering ratio. With this new technology, the steering ratio continually changes with vehicle speed, optimizing the steering response in all conditions.
BMW’s system puts an electronically controlled planetary gear between the rack and motor. The system can be locked if a problem is detected. Audi and Lexus place a gear reduction box between the steering wheel and rack with a locking mechanism included as a fail-safe.
Ford’s new variable-ratio system uses a precision-controlled actuator placed inside the steering wheel and requires no change to a vehicle’s traditional steering system. The electric motor actuator and gearing system essentially add to or subtract from a driver’s steering inputs.