Power steering is changing with new drive by wire and other technologies being implemented.
A large number of vehicles on the road today are still using some form of the first power steering systems developed. While a gear reduction or other design of simple gear box was used early on to aid in steering assist, the first real power steering systems used some form of hydraulics to multiply power sent from the driver to the steering components of the vehicle. Hydraulic power steering dates back as far as the early 1930s and has been used on nearly every make and model vehicle on the road. For this reason, most technicians are quite familiar with hydraulic power steering and the techniques used to diagnose it or make repairs.
The Hydraulic History
Hydraulic power steering systems come in many different designs but they relate back to some pretty basic principles. Hydraulic pressure is generated by a pump, normally of a gerotor or rotary vane style. The pumps are normally positive displacement, meaning that their flow rates are proportional to rotational speed. This can cause problems with a belt-driven pump, because higher engine speeds creates more fluid flow in the system, causing higher pressure. To help avoid characteristics like over steer at higher speeds and heavy steering at low speeds, restricting orifices and flow control valves are used to regulate pressure in the system. These help to maintain a more consistent pressure in the system by limiting flow or returning some of the fluid back to the system reservoir.Most systems use a pressure relief valve to prevent excessive pressure build up in situations such as the steering being held in the full lock position. Some systems, referred to as electro-hydraulic, incorporate an electric pump for hydraulic pressure. This helps to eliminate some of the problems incurred with a belt driven pump, like inconsistent speeds and pressures. A fluid reservoir is incorporated to hold the hydraulic fluid used by the system and to catch return fluid from the system. High pressure lines or hoses transport fluid from the pump to a steering gear box or rack and pinion assembly.
Hydraulic valves or cylinders are used in the gear boxes and rack assemblies to move the steering components. A variation of these systems uses an electronic pressure relief valve to reduce pressure in the system as speed increases. Commonly referred to as variable assist power steering, these systems help to control steering assist at a constant level based on vehicle speed. These systems have served the industry well for many years. But the need for lighter, smaller, more fuel efficient vehicles has forced the industry to rethink the design of all on board systems, steering being one of them.Moving Forward
As modern technology goes, so goes the automobile industry. One of the latest advents into steering controls has been the development of electric power steering, or as some refer to it, drive by wire. Granted, when the systems first began to appear in the markets, there were many of us who were very skeptical of the thought that some electrical system could have control over our steering. In actuality, nothing could be farther from the truth.
The electric power steering systems on the market today are merely just another form of power assist. The manual connection between driver and steering controls is still in place. The popularity of these systems has grown, and we are beginning to see more and more of them. Youdefinitely will see these systems coming into your shops for diagnosis and repair.Electric power steering systems have grown in popularity for many reasons. One of the first things you'll notice is the lack of any hydraulic components in the system. With no belt driven hydraulic pump present to constantly drain power from the engine, fuel efficiency is slightly increased, somewhere around one mile per gallon. This also eliminates another belt drive accessory from under the hood, saving space and weight.
Hydraulic lines, fluid reservoirs and hydraulic fluid are also eliminated. With one less fluid under the hood, there's less to maintain, one less fluid to create a leak and it even makes us a little greener, lessening our dependence on another product that can be hazardous to our environment.
The manufacturing process is more economical as well, due to a decrease in the size and number of parts. Even the steering gear itself can be made lighter and smaller without sacrificing safety or quality. Currently, electric power steering is being used in lighter, smaller vehicles, but as time goes by we'll certainly begin to see more of this technology used on different models across the board.
Limitations seem only to be in the area of motor design and current draw, as the motors in these systems tend to operate at fairly high loads. Another point of concern is in the area of "road feel," or how the driver interprets the feed back he receives from the steering as he drives the vehicle.While these systems vary somewhat based on manufacturer, they operate under the same basic principles. Most contain a variation of the same major components including an electric motor, a computer control module, a steering gear, a column and usually a steering torque sensor and/or a steering wheel position sensor. With that being said, I'll be relating the remainder of the information to a specific system.
GM'S EPS
While GM uses more than one electric power steering assist system, let's focus on GM's EPS . The Electronic Power Steering system (EPS) used in some GM models incorporates the basic components I have listed.
One model specifically using this system is the 2008 Chevrolet HHR. When driving this vehicle, the road feel or feedback received is comparable to hydraulic systems in use on other vehicles it's size. The steering is responsive and gives you a confident feeling of control while driving. This system incorporates a power steering control module, torque sensor, the body control module, EPS motor, a dedicated battery voltage supply circuit, a serial data bus for communication to other vehicle systems and the instrument panel cluster message center. The power steering control module uses inputs from the EPS and other vehicle systems to calculate the amount of steering assist needed. The EPS motor, torque sensor and power steering control module are not serviced separately. Any of these components requires replacement of the complete steering column assembly.The EPS system uses components that have unique operating characteristics. The EPS motor, for example, is a 12-volt DC reversible brushed type motor that has a current rating of 61 amps. Quite a bit of current flows through these systems, but the EPS motor is not designed to handle over 61 amps continuously.
Thermal overload protection is one feature built into the power steering control module, which is designed to limit the amount of current commanded to the EPS motor in the event of an overload situation. In thermal overload protection, steering assist will be at reduced levels until the system has had time to recover. The power steering control module itself contains eight different steering calibrations that it selects from at start up. This selection is made based on information it receives from the vehicles body control module over a serial data bus communication network. This allows the steering column assembly to be used in several different applications with out having to make any changes to the components.
Signal voltages are received by the power steering control module from the torque sensor to give an indication of the amount of torque being applied to the steering column by the driver. This helps the control module decide how much current will need to be applied to the EPS motor. Vehicle speed is also monitored and is sent via the serial data bus from the engine control module. Speed input is used to help determine the amount of assist to apply based on a particular driving situation. At higher speeds, assist is less to give a better feel and avoid over steer. More assist is needed at lower speeds, such as parking lot maneuvers and that can be detected by the control module as well.
The torque sensor is the power steering control module's main input for determining how much steering assist to apply. The torque sensor is located in a torsion bar section of the steering column between the input shaft, which comes from the steering wheel and the output shaft, which goes out to the steering shaft coupler. A series of detection coils and detection rings are internal to the torque sensor and are used as comparators to determine the amount of torque being applied to the steering column. Voltages signals relevant to the amount of torque applied are sent back to the control module for assist calculations. There is also a coil used in the torque sensor as a back up or to compensate for changes in voltage levels and changes due to ambient temperature fluctuations. This allows for more accurate torque detection.
Getting into Diagnostics
Diagnostics of the EPS system is pretty straight-forward. A battery positive feed is sent to the EPS assemble via a 60-amp maxi fuse located in the under hood fuse block and a single main chassis ground. The only other external power input is an ignition feed, which comes from a 2-amp fuse in the body control module fuse block assembly.
Four serial data communication lines connect from the power steering control module to the vehicles data communication network and are used to communicate information between the power steering control module, engine control module, body control module and instrument cluster. The power steering control module has the ability to detect malfunctions in the EPS system and any malfunction detected will cause the instrument panel to display a power steering warning message. There are 10 to 12 diagnostic trouble codes based on system configuration that can be read using a diagnostic scan tool. The diagnostic procedures in the code charts will direct you to multiple tests for the EPS system but, at this time, the components are replaced as an assembly.
While electric power steering is but one of the systems in use in today's vehicles, it is most definitely a step in the right direction. As modern technology increases, we are likely to see not only steering systems advance much farther but every other vehicle system as well. Look how far we've come in such a short time and even in these uncertain economic times we live in you can't stop the evolution of the machine we've grown so fond of.
Randy Wilson is an ASE Master Technician currently working for a Chevrolet and Cadillac dealership in Dothan, Ala. He is Advanced Engine Performance Specialist Certified with 25 years of experience under his belt. Wilson specializes in drivability, diagnostics and electrical systems.
