Electronic Power Steering
The first versions of electric power steering were actually "electro-hydraulic" which used an electric motor to drive a
conventional hydraulic pump. However, EPS has come a long way
since then. By most accounts, the first production vehicle to
feature electronic steering was an Acura NSX. The NSX steering
gear utilized a manual rack and pinion with an electric motor
mounted on the steering column. Soon after the EPS system was
introduced on the NSX every manufacturer had their own version
of electronic power steering.
One of the first things for the engineers to overcome was the feedback the driver needed when turning the steering wheel. This feedback was minimal at first because the electric motor driving the steering was so smooth that one finger could glide the steering from left to right. So a certain amount of resistance had to be built into the system in order for the feedback from the steering wheel to have the right feel. Then, things like how to make the electric unit return to a center position after a turn and other aspects of the steering system had to be worked out.
Through a lot of research as well as trial and error a lot more was learned about steering than just turning left and right. There seemed to be a lot of things that for the most part, we take for granted. When all of the various factors were figured into the operation of the electronic steering it was obvious that some programming and software was needed and with that the PSCM (Power Steering Control Module) became a part of modern electronic steering system.
How it works
When the driver turns the wheel, a steering angle sensor, detects the rotational position and the speed of rotation, all the while more information is gathered from a steering torque sensor mounted in the steering shaft. That information is then fed to the power steering control module. Other outside information, such as the vehicle speed, ABS functions, cruise control, traction control, lane assist, and stability control systems all factor into determining how much steering effort is required. The control module then commands the steering assist motor to rotate while a sensor on the motor provides a feedback signal to monitor the motor's actual position.
Nearly every conceivable issue you can think of regarding the operation of the electronic steering has been thought out and dealt with. Even if you have an operator who has a habit of turning all the way right or left and then tends to hold the wheel in that position. The PSCM detects a high system temperature or current draw and switches the overload protection mode on. A DTC C0176 "System Thermal Error" code may be set. On some models, DTC C0476 "Electric Steering Motor Circuit Range/Performance" may also be set. These DTCs indicate that the PSCM has gone into reduced steering assist mode to prevent thermal damage to power steering components. Consequently, there’s nothing that needs to be done as far as recalibrating or any type of mechanical repair, but the driver needs to be educated about not cranking and holding the steering against either stop for a prolonged period of time.
Advantages of E powered steering
Electric power steering can not only refine the steering during different modes such as parking lot turning or highway maneuvering. It’s also quieter. No more squealing power steering belts, growling power steering pumps or fluid leaking on the garage floor. Besides, there’s about a 10 horsepower savings too, let alone the weight difference between an electronic system and a hydraulic system. The electronic steering systems not only improved steering quality but improved the overall fuel economy without any modifications to the engine.
Electric power steering can also be modified with the software which is virtually impossible with hydraulic controls. This is accomplished by monitoring the driver's steering inputs, vehicle speed, and other suspension dynamics. With today’s electronic systems, they can provide just the right amount of steering feel and effort to match rapidly changing driving conditions. EPS can deliver extra effort when you need it, and reduce steering effort when you do not need it. It can even provide steering assist when the engine is off.
More importantly for some older drivers and or handicapped drivers, because the system is software driven, it is possible to tap into the steering module and modify the steering effort and feel. This can be done with a factory scan tool on some applications, or with specially designed aftermarket "tuner" scan tools and software.
Replacement
In most applications, the motor and PSCM module are both part of the steering column assembly, and are currently replaced as a unit. On some applications (Toyota, for example), the electric motor can be replaced separately. On a few other applications the electric steering assist motors can either be located at the base of the steering column, as with GM, or integral with the steering rack, as with Honda. The GM type uses a separately serviced motor assembly, while the Honda type requires rack replacement if the motor is defective.
Re-calibrating
After replacing the assembly, some programming will be needed. Generally, it involves turning the wheel from the center position to the right and left and back to center so that the sensor range can be established for the PSCM. This re-calibration of the steering angle is not just for the driver but for the other systems that use the electronic steering such as a lane assist/departure control system.
The steering wheel position sensor determines the "on center" position. This is used to keep the return assist from going over center once a turn is completed. GM uses a 5 volt reference signal sensor that triangulates the exact position. The system utilizes two separate return signals that vary from 0 to 5 volts. The voltage values will increase and decrease within a 2.5 to 2.8 volt range of each other as the steering wheel is turned.
Diagnosing
The General Motors EPS system has several modes of operation, which is typical for almost all the other manufacturers.
Normal mode -- Left and right assist is provided in response to inputs and vehicle speed.
Return mode -- Used to assist steering return after completing a turn. Feedback from the steering position sensor prevents the EPS system from "overshooting" the center position.
Damper control mode -- Used to improve road feel and dampen kickback. This mode typically kicks in at higher vehicle speeds.
Protection (self-protect) mode -- Protects electrical components from thermal damage and excessive current flow if the steering is held all the way to one side in the lock position too long.
In most instances you’ll find “C” and “U” codes in regards to the steering system. “U” codes are data line, or communication codes, while the “C” codes pertain to the actual system failures. If a catastrophic failure does occur the odds of losing steering entirely is minimal. More than likely the steering will be very stiff and hard to turn.
A good scan tool with the capabilities to read the “C” codes as well as performing any needed BIAS functions is essential. Another valuable tool is a scope or digital multi-meter to measure pulse width and provide any diagnostic information and component test information found in the service manual.
There’s a good chance if you’re seeing a failure in an electronic steering system that you’re not alone. Check for any TSB’s out there that describes the condition and repair for the vehicle. Take advantage of all your diagnostic options before starting the repair and follow any updated information for the latest calibrations and programming.
Electronic steering is here to stay. At first I was skeptical of the systems and had the mindset that they were doomed to failure. But, as time has passed, I’d have to say I’m pretty impressed with electronic assisted steering systems. Sure beats variable pressure displacement steering or the frustration of trying to pull those power steering pulleys off. Sometimes technology gets it right, and I believe this is one time they did.
Time to head down the road to the next repair. Keep em’ between the ditches, and off the tow trucks.
