Tell the world! Electric Power Steering is here! 

In an era where more and more automotive systems are being overrun by electronics, you can now add the power steering system to the list.  The system uses the Body Control Module (BCM), Power Steering Control Module (PSCM), torque sensor, discrete battery voltage supply circuit, EPS motor, class 2 serial data circuit, and the Instrument Panel Cluster (IPC) message center to perform the system functions. Let's take a closer look at how these components work together.

                                                                                                            

The PSCM uses a torque sensor as it's main input for determining the amount of steering assists. The steering column has an input shaft, from the steering wheel to the torque sensor, and an output shaft, from the torque sensor to the steering shaft coupler. The input and output shafts are separated by a torsion bar, where the torque sensor is located. The sensor consists of a compensation coil, detecting coil, and 3 detecting rings. These detecting rings have toothed edges that face each other. Detecting ring 1 is fixed to the output shaft, detecting rings 2 and 3 are fixed to the input shaft. The detecting coil is positioned around the toothed edges of detecting rings 1 and 2. As torque is applied to the steering column shaft the alignment of the teeth between detecting rings 1 and 2 changes, which causes the detecting coil signal voltage to change. The PSCM recognizes this change in signal voltage as steering column shaft torque. The compensation coil is used to compensate for changes in electrical circuit impedance due to circuit temperature changes from electrical current and voltage levels as well as ambient temperatures for accurate torque detection.

The sensor is a 5 volt dual analog inverse signal device with a valid signal voltage range of 0.25-4.75 volts. This is a fancy way of saying that when the voltage on one signal line goes up, the other signal will go down the same amount, giving an almost mirror effect (see the snapshot to the left). When applying torque to the steering column shaft during a right turn, the sensor's signal 1 voltage increases, while the signal 2 voltage decreases within the valid signal voltage range

 

 

 

 

 

With the advent of the new Electronic Power Steering Systems,
   Power Steering fluid, pumps and drive belts may go the way of the timing light !

The PSCM uses the steering position sensor to determine the steering system on center position. Since the power steering motor provides a slight amount of return to center assist, the PSCM will command the power steering motor to the steering system center position and not beyond. The sensor is a 5 volt dual analog triangle signal device with a valid signal voltage range of 0-5 volts. The sensors signal 1 and signal 2 voltage values will increase and decrease within the valid voltage range, and stay within 2.5-2.8 volts of each other as the steering wheel is turned.

 

 

 

 

 

 

 

The EPS motor is a 12 volt brushed DC reversible motor with a 58 amp rating. The motor assists steering through a worm shaft and reduction gear located in the steering column housing. The motor is serviced separately from the column. It  also has to be able shed a lot of heat under the dash so putting in any aftermarket material that would get in the way would lead to overheat and failure of the system. Notice the large heat sinks on the photo to the right.   

 

 

 

Let's face it, nothing gets done on today's vehicles without a module and this system is no different. The PSCM uses a combination of torque sensor inputs, vehicle speed, calculated system temperature and the steering calibration to determine the amount of steering assist. When the steering wheel is turned, the PSCM uses signal voltage from the torque sensor to detect the amount of torque being applied to the steering column shaft and the amount of current to command to the EPS motor. The PSCM receives a class 2 vehicle speed message from the Engine Control Module (ECM) to determine vehicle speed. At low speeds more assist is provided for easy turning during parking maneuvers. At high speeds, less assist is provided for improved road feel and directional stability. The PSCM nor the EPS motor are designed to handle 65 amps continuously. The PSCM will go into overload protection mode to avoid system thermal damage. In this mode the PSCM will limit the amount of current commanded to the EPS motor which reduces steering assist levels. The PSCM also chooses which steering calibration to use when the ignition is turned ON, based on the VIN. The BCM contains all 8 of the steering calibrations which are different in relation to the vehicles powertrain configuration, sedan, coupe, etc. The PSCM has the ability to detect malfunctions within the EPS system. Any malfunction detected will cause the IPC message center to display the PWR STR warning message.                                                                                                                                                       

 

All right, I know what you are thinking, when and where will I as a tech see this? Well, the Saturn ION and VUE and Equinox are using a version of it 
 right now that does not use a Torque Sensor. The Malibu starting in  2004 (see photo above) and the new Pontiac G6. Eventually you will see it spread to other divisions and models making Power Steering fluid, pumps and drive belts may go the way of the timing light !

 

Mik Stubing
Lead Instructor GMTCNY
[ Up ] [ Abbreviations ] [ EPS ] [ Intermittent Problems ] [ Lighting Systems Super Feature ] [ Obstacles ] [ Lean Diagnostics ] [ DMM Basics ] [ Micro 64 Systems ] [ Strategy Diagnosis ] [ SerialData Superfeature ] [ Using SI ] [ Using Tech  Assist ] [ Wide Band O2 ]