Chances are you will never crack open a steering angle sensor (SAS) cluster to repair it. But, this sensor should not be a mystery. To be effective at ABS and stability control diagnostics, knowing how sensors measure angles and information is critical.
Why Measure Steering Angle?
The angle of the steering on its own can be used to determine where the front wheels are pointed. When combined with other pieces of information from the yaw, accelerometer and wheel speed sensors, it is possible to measure the dynamics of the vehicle.
With the collected information, the stability control system can be used to determine the driver’s intentions, how the vehicle is reacting and what corrections can be made with the ABS hydraulic control unit.
How Is the Angle Measured?
The steering angle can be measured with optical sensors, by assessing Hall effect and through other technologies. Theses sensors measure the movement of the steering wheel in degrees.
What is the Difference Between Analog and Digital?
Analog SASs are similar to throttle position sensors. SASs are wired with a 5-volt reference, chassis ground and signal outputs, and can be tested via a port under the steering column. As the steering wheel is turned, the SAS produces a signal that toggles between 0 and 5 volts as the wheel is turned 360º. It is possible to observe the 0- to 5-volt signal with meters connected to the two SAS sensors. When the wheels are straight, the sensors read 2.8V and 0.4V. If the readings are the same, the two sensors could be shorted together. Most vehicles produce a positive voltage turning right and a negative voltage turning left.
Digital steering angle sensors measure the angle and turn it into information that can be shared on a serial data bus or discrete connection with a module. Instead of changing voltage, these sensors produce a signal in code that indicates the steering angle. This data PID can be used for electric power steering, intelligent headlights and even stop/start systems.
Why Two Sensors?
In most cases, just one sensor would be fine. There is one crankshaft position sensor, one wheel speed sensor per wheel and only one output sensor on the transmission. But the steering angle sensor has two or three sensors packaged together. The only other sensors on a vehicle like this are the throttle angle sensor and gas pedal for vehicles with a throttle-by-wire setup.
The reason for multiple sensors is redundancy, accuracy and diagnostics. Multiple sensors are required since the steering angle is critical for the stability control system, and any discrepancy could mean the difference between making it around a corner or hitting another vehicle.
Most sensors are packaged together in a single unit. When combined, the two signals can provide a more accurate reading on the positions of the wheels and how fast the position is changing. The two signal outputs are checked against each other to ensure accuracy.
One of the more common problems being experienced on some high-end cars with power steering wheel adjustments stems from movement to make entry and exit easier. Unfortunately, this movement can damage the wiring harness for the steering angle sensor. When the wiring is damaged, it might cause intermittent problems.
Why Do They Need Calibration?
Many vehicles require the SAS to be reset or recalibrated after an alignment is performed or parts in the steering system are replaced. There are three types of reset procedures: systems that self-calibrate on their own, vehicles that require specific wires or buttons to be pressed and systems that require recalibration with a scan tool. If the SAS is out of calibration, most vehicles can tell if they are traveling in a straight line, but if the angle is too far off, the ESC could become disabled.
Some newer vehicles can auto-calibrate by having the wheel turned from lock to lock and then centered, and then cycling the key. There are many options for scan tools to reset SASs, some of which are integrated into an alignment system. But, most tools recommend that the calibration be performed on a level surface with a lock-to-lock turn to complete the calibration.