Electrical Power Steering Repair
Electrical Power Steering Repair
EPS has been around for more than 10 years. Understanding EPS system operation will help speed up your diagnostic times.
Dave Macholz/Motor Age — 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.
Can you remember driving your first manual steering car? You really remember it if you had one of these cars and had to park it. Why was that, you ask? Well, because steering effort is highest at low vehicle speeds. I think that’s why half the pictures of hot-rodder’s from the 1950s feature men and women who look like they have been spending a great deal of time at the gym lifting weights. Steering with these manual steering gearboxes was quite the workout! On the other hand, steering effort decreases as vehicle speed increases. That is why it was so much easier to drive an old manual 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.
Source: Motor Age