It’s no secret that vehicle manufacturers are under the gun these days to make their vehicles more fuel efficient. Federal mandates and consumer demand for more fuel-efficient vehicles are changing the way automotive engineers look at vehicle design. One way they are achieving higher miles per gallon is by eliminating accessory loads from the engine. A major contributor to this accessory load is the hydraulic power steering pump. Electrical power steering (EPS) is a result of eliminating the power steering pump and has drastically changed the way we diagnose and repair steering systems. These systems first came about on production hybrid vehicles in the early 2001 Model Year Toyota Prius and have since made their way into many other production vehicles across all manufacturers. Although these systems have been around for more than 10 years it is important that we know how the system works in order to fix them right the first time.
gear box car on the highway. When you add power steering to the equation, the effort to turn the vehicle while parking or at low vehicle speeds is greatly reduced. However, the steering pump that was attached to the engine via a drive belt was spinning the pump regardless of the speed of the wheels. In regards to fuel economy, this is not an efficient way of achieving power steering.
On an EPS system, we eliminate the hydraulic pump and, in turn, the parasitic load on the engine. The rack and pinion that we all are familiar with remains somewhat the same as in previous designs, but the power assist is now provided by the means of either a two wire type brushed DC motor, or a three-wire type brushless DC motor. These motors are mounted either externally to the rack or can be integrated within the steering rack itself. Some manufacturers also use motor placement on the steering column. A DC motor is a great solution for assist purposes, but in terms of serviceability can be quite costly. Problems with the DC motor typically result in replacing major components such as the column or the steering rack, although some DC assist motors are serviceable on their own.
In order to control the amount of assist that the DC motor provides, most of these systems require a power steering control ECU that will vary both the direction of rotation and the amount of amperage, or current flow to the motor based on demands from the system. As with any ECU there will be inputs and outputs. The motor is typically the output here. The inputs to the ECU can be easily understood if you think back to the old manual steering car. Steering effort is greater at low
vehicle speeds and is less at higher vehicle speeds. The steering ECU receives a speed signal via the CAN bus to determine how much assist should be provided to the motor. The ECU also will need to know which direction to turn the motor. This is typically provided by an input such as a steering angle sensor and or torque sensor. Steering effort also can vary based on road conditions, such as if you are driving on gravel, hot asphalt or on ice. In order to compensate for these conditions, a torque sensor measures the effort required to turn the wheel. The harder it is to turn the wheel, the more assist is needed. An input to the ECU to measure motor performance also might be necessary depending on the type of motor utilized. In particular, brushless DC motors will need a sensor to determine the position of the motor for motor control. Most ECU’s also monitor the motor current draw to prevent motor overheating when the steering wheel is tuned against the locks.
Overall the EPS system is complex but relatively easy to understand. Thanks to advances in engineering there is also quite a bit of monitoring of the system by the ECU that will save the technician diagnostic time in the long run.
Getting to Work
Now that we know the basics of system operation and are familiar with major components (the rack, ECU and sensors) let’s take a look at some issues you will have to address in diagnosing, servicing and repairing these units.
Diagnosing EPS faults- The beauty of advanced technology is that most manufacturers have designed self-diagnostics into the systems for system monitoring and repair purposes. Diagnostic trouble codes are typically stored within the EPS ECU and can be retrieved with a scan tool to aid in diagnosis. As always, DTCs are not a magic bullet, but they are a start on your path to a diagnosis. If a DTC sets it is typically accompanied by a warning light on the dash in the shape of a steering wheel or other symbol, depending on manufacturer. Once you have tracked down the DTC it will be up to you to put your electrical diagnostic skills to work. Knowledge of scan tool usage, meter usage, and electrical fundamentals are essential.
Failsafe Strategies – EPS steering complaints are often similar to loss of power steering complaints in hydraulic assist vehicles in that the customer may complain of increased steering effort and/or no power assist. Many manufacturers have implemented fail-safe strategies for the EPS system that can result in this type of complaint. A fail-safe strategy is designed to default the system to a mode of operation that will protect the driver and passengers as well as the EPS system itself. In the case of the EPS system, a DTC within the system may trigger the fail–safe. There are several fail safe modes available including no assist, and assist based on a set mile per hour strategy. Be sure to refer to DTC descriptions within the service information. This is an advanced system, and a short term subscription to factory information may save you some headaches.
Steering angle / torque sensor calibration – The most common maintenance scenario for the EPS system is the zero-point calibration for the steering angle and or torque sensor. Many a technician has learned this by accident. In order for the EPS system to function properly, the EPS ECU needs to know when the steering wheel is in the “0” position, or the position in which no assist is needed. This would be when the wheels are pointed straight ahead, parallel to the rear wheels. This calibration needs to be performed when carrying out various procedures or replacement of parts including, wheel alignments, replacement of the steering gear assembly, steering column, EPS ECU, and other various sensors. Be sure to consult service information for the year, make and model you are working on to verify if this procedure needs to be completed.
Assist Map writing – On Toyota vehicles with EPS it is sometimes necessary to write the assist map into the EPS ECU if DTC 1581 sets AFTER performing the zero point calibration. In order to do this you will need to use the Techstream or a scan tool capable of accessing this feature. To perform this function with the Techstream, enter the following menus: Chassis / EMPS / Utility / Signal Check. When performing the signal check, the assist map is written to the EPS ECU.
Communication – The CAN communication system is put into use by the EPS system to quickly transmit data between ECUs. For example, vehicle speed and steering wheel position. The EPS ECU needs to see information about vehicle speed and stability control. Why stability control? Some manufacturers have engineered software that utilizes the steering system to help overcome an out of control or skidding condition and other stability control issues. Understanding how the CAN system operates and what to do when a CAN related problem occurs is another piece of the fundamental knowledge that today’s technicians need to be ready for. If you suspect a problem with communication, look for diagnostic codes that begin with the letter “U”. U codes typically come about as a result of communication faults.
Now that you have scratched the surface of EPS by reading this article it is time to get busy doing your homework. I always tell my college students that what they learn today is just a start on the path of lifelong learning. Seek out as much information as you can on every subject and you will see the fruits of your labor in your service bay.
