Electric Power Steering (EPS) has replaced hydraulic power steering in many new vehicles today. One of the advantages of electric power steering is that it eliminates the power steering pump, which can use as much as 8 to 10 horsepower under load. This improves fuel economy while also eliminating the weight and bulk of the power steering pump and hoses. Getting rid of the hydraulics also does away with fluid leaks and the need to check the power steering fluid. Electric power steering is also quieter than hydraulic systems because there is no pump noise and no fluid flowing through hoses and valves. But the most noticeable difference is in handling and steering refinement.
Electric power steering can be fine tuned with a precision that is hard to match with hydraulic controls. By monitoring the driver's steering inputs, vehicle speed, and other suspension dynamics, the system 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.
Better yet, because the system is software driven, it is possible to tap into the steering module and modify steering effort and feel. This can be done with a factory scan tool on some applications, and with aftermarket "tuner" scan tools and software.
Electric power steering can be found on a wide variety of vehicles including the Acura NSX (which was the first production car with this feature), Honda S2000, Toyota Cortolla, Prius and Toyota RAV4, numerous GM models including 2004 and up Chevrolet Malibu, Chevrolet Cobalt , Equinox, HHR, Pontiac G6 (except the Convertible, GTP and 2007 GT models), Pontiac G5, Torrent and Vibe, Saturn ION and VUE, 2005 and up Ford Mustang, and many many others.
Though some of the older electric power steering systems were actually "electro-hydraulic," and used an electric motor to drive a conventional hydraulic pump, the latest generation of EPS is all electric/electronic. The steering gear itself is a manual rack with an electric motor mounted on the steering column or the rack.
When the driver turns the wheel, a steering sensor detects the position and rate of rotation of the steering wheel. This information along with input from a steering torque sensor mounted in the steering shaft is fed to the power steering control module. Other inputs such as vehicle speed and inputs from the traction control or stability control systems are factored in to determine how much steering assist is required. The control module then commands the motor to rotate a certain amount, and a sensor on the motor provides feedback to the control module so it can monitor the motor's position.
The General Motors EPS system has several modes of operation:
* 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 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.
Turning the steering wheel all the way to one side will cause the Power Steering Control Module (PSCM) to command the maximum amount of current to the EPS motor. If the steering wheel is then held in this position for an extended period of time, the system will go into protection mode so the motor doesn't overheat. In this mode, the PSCM will limit the amount of current to the motor and reduce the level of power assist.
If the PSCM detects a high system temperature and the overload protection mode is enabled, 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 normal PSCM action (reduced steering assist) to prevent thermal damage to power steering system components. Consequently, there is nothing that needs to be fixed. But the driver needs to be educated about not cranking and holding the steering against either stop for a prolonged period of time.
If a sensor or other component in the EPS system fails, the self-diagnostics should detect the fault, set a code and disable power-assist. A warning light will illuminate to alert the driver, and the driver will notice a significant increase in steering effort when turning the vehicle. The vehicle will still be safe to drive, but it will require more steering effort to do so.
On the GM EPS applications, the electric motor is mounted on the steering column. The motor and PSCM module are both part of the steering column assembly, and are currently replaced as a unit if there is a problem with the system. On some other applications (Toyota, for example), the electric motor can be replaced separately.
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.
Electric power steering columns can be expensive to replace, costing anywhere from $600 to $1500 or more depending on the application. So accurate diagnosis is essential to avoid replacing parts unnecessarily.
After a PSCM or steering column replacement, it may be necessary to use a scan tool needs to calibrate the center position of the steering wheel position sensor, select the "steering tuning" for the vehicle, or calibrate the center position of the steering shaft torque sensor.
For customizers and street rod builders, Flaming River now sells an aftermarket electric power steering that can be adapted top fit various vehicles. But it is a very expensive item, costing $4000 to $6000 depending on the version. The benefits of using an electronic power steering unit in these kind of applications is that it simplifies installation and eliminates the need for a power steering pump and hoses.
The steering wheel position sensor determines the "on center" position. This is used to keep return assist from going over center once a turn is completed. The GM unit is 5 volt dual analog triangle signal device with a valid signal voltage range of 0 to 5 volts. The sensors signal 1 and signal 2 voltage values will increase and decrease within 2.5 to 2.8 volts of each other as the steering wheel is turned. This information can be very useful during diagnosis.
In GM EPS systems, the Power Steering Control Module (PSCM) must be set up with the correct "steering tuning", which are different in relation to the vehicles Powertrain configuration, sedan, coupe, tire and wheel size, etc.
GM has a technical service bulletin (07-02-35-004 Feb. 02, 2007) out on a noise problem with EPS steering on the Cobalt, Chevy HHR and Pontiac G5. The bulletin describes a steering column rattle or knocking noise that may be heard or felt at low speeds (5 to 15 mph). The noise is most noticeable when making a slow turn on a loose or rough surface. The EPS system can be temporarily disabled by removing the 60 amp EPS fuse in the under-hood fuse block. If the rattle persists, it is due to backlash within the steering column (assist motor gear mechanism). Replace the 60 amp EPS fuse and test drive again. If the steering column is identified as the source of the rattle/knocking noise, the steering column needs to be replaced.
General Motors is recalling 1.3 million 2005 to 2010 Chevrolet Cobalts, 2007 to 2010 Pontiac G5s, 2005 to 2006 Pontiac Pursuits sold in Canada, and 2005 to 2006 Pontiac G4s sold in Mexico to fix power steering motors that might fail.
GM says these cars are still safe to drive, but warns that a power steering motor failure can increase steering effort significantly, especially at speeds under 15 mph. If the electric power steering motor fails, you may see a warning light come on and hear a chime, or you may not. Eitehr way, brace yourself because the steering will suddenly feel much stiffer require much mor effort to turn the car.
GM says it will replace the electric motors on all 1.3 million vehicles, but that it will take some time to get the new power steering motors from their supplier, JTEKT Corp.. GM says it will notify car owners when the parts are available for installation.
The recall is in response to an investigation launched by the National Highway Traffic Safety Administration in response to over 1,100 complaints it has received about cars losing their power steering assist. The complaints included 14 crashes and one injury.
GM says it will fix the older models first because it typically takes about 20,000 to 30,000 miles of driving for the condition to develop. If the power steering assist fails, it usually comes back for a time after the car is shut off and restarted. But the problem won't go away. Fixing it requires replacing the electric motor that provides the power assist.
Isn't technology wonderful?