just keep getting smarter all the time. Sensors are being used to
monitor more and more functions, and to share information between
vehicle systems that formerly were mute or didn’t communicate with one
another. One such sensor is the steering position sensor. The sensor’s
basic function is to monitor the driver’s steering inputs. This
includes the angle of the steering wheel and/or the rate at which the
driver is turning the wheel.
the 1980s, automakers introduced the first generation of variable
assist power steering systems. The simplest of these systems used a
solenoid on the steering rack to reduce hydraulic power steering assist
when the vehicle was traveling faster than a certain speed (typically
above 20 to 25 mph). Reducing power assist as the speed increased
improved road feel and steering stability. The only sensor input
required for these systems was a vehicle speed sensor signal.
more sophisticated variable assist power steering systems added a
steering sensor to override the cutout solenoid when the driver made a
sudden steering maneuver. The Ford Variable Assist Power Steering
(VAPS) system on the 1988 Lincoln Continental and the Electronic
Variable Orifice (EVO) power steering system on the 1989 Thunderbird
and Mercury Cougar both used a steering sensor for this purpose.
these early Ford systems, the steering sensor functioned only as a
rotation sensor. It did not measure the angle of the steering wheel,
but only the rate at which the steering wheel was being turned by the
Most of these early generation steering sensors were optical
sensors with photo diodes inside that read evenly spaced slits in a
disc attached to the steering column. Turning the steering wheel to
either side generates a pulse signal that goes to a steering control
module. On the Ford VAPS system, the control module ignores the
steering inputs as long as the driver is making relatively slow
steering motions (less than 90 degrees of rotation per second). But if
the driver turns sharply or swerves, the control module reacts by
reducing current to the cutout solenoid to increase steering assist.
the event of a steering sensor failure, most the system was designed to
fail-safe with full power assist remaining on regardless of vehicle
speed, steering angle or rate of rotation.
you find yourself troubleshooting one of these older systems, most of
the diagnostic checks on the steering sensor will be made with an
ohmmeter. Measuring the resistance between the sensor’s connector pins
and comparing the values to specifications will tell you if the sensor
is good or bad. On some applications, an analog ohmmeter can be used to
detect pulses from the sensor as the steering wheel is rotated with the
More Sensitive Sensors
of the early steering position sensors are relatively low-resolution
sensors, and typically detect steering movements in 8-degree to
comparison, most of today’s steering position sensors are
high-resolution magnetic sensors that can detect movements of 1° or
less. Some have resolutions as low as one-tenth of a degree.
Why the change in sensitivity? Steering
angle sensors are used for a wider variety of purposes today, and for
fast-acting stability control systems to react instantly to changing
driving conditions, the system must be able to detect even small
changes in the position of the steering wheel. Engineers know
that the angle of the front wheels as well as the steering inputs by
the driver have a significant impact on vehicle dynamics, handling
stability and traction. Advances in onboard computing power in recent
years and system integration and information sharing via local area
networks (LAN) and controller area networks (CAN) now make it possible
for various systems to share information and sensor inputs.
Consequently, the inputs from a steering position sensor can be used to
vary power steering assist, to modify anti-lock braking and traction
control, to assist stability control, to modify the reaction of an
electronic suspension system, to modify torque delivery in an
electronic all-wheel-drive system, and to even monitor the driver
steering position sensor also will play a key role in future accident
avoidance systems that detect obstacles and steer the vehicle to avoid
the obstacle if the driver fails to react.
the road, the steering sensor may even be used to monitor the driver.
Engineers are now developing systems that detect subtle changes in the
driver’s steering inputs to determine if the driver is impaired or not.
The system “learns” the driver’s normal steering habits, and sounds an
alarm (or disables the vehicle) if the driver is steering erratically.
Who would have thought Big Brother might someday be lurking inside your
stability control is part of the ABS system, faults are self-diagnosed
and turn on one or more warning lights. Depending on the nature of the
fault and the application, a steering position sensor failure may
disable the stability control system and/or ABS system.
require the use of a scan tool on most vehicles, and the scan tool must
have the appropriate software that can access the stability control/ABS
system. If you find a steering position sensor code, you’ll have to
follow the diagnostic charts to isolate the fault as the problem may be
in the wiring or the sensor itself.
magnetic sensors, there are no moving parts so the units are fairly
reliable. Even so, faults may occur in the internal electronic
circuitry that processes and generates the sensor’s output signal. Loss
of voltage or ground can prevent the sensor from functioning properly,
so wiring faults should be ruled out before replacing the sensor. The
sensor itself is a sealed assembly and is not repairable or rebuildable.
steering sensors have their own built-in self-diagnostics and will
generate a warning signal to the stability control system or body
control module if it detects any internal faults. The stability control
system or body control module may also set a fault code if the signal
from the steering position sensor is lost or is out of range. The
system may also compare the steering angle reading against the yaw
sensor when the vehicle turns to see if the signals correspond.
on the vehicle application and scanner software, it may be possible to
read the steering angle through the scan tool. This would allow you to
rotate the steering wheel and look for a corresponding change in the
indicated steering angle.
Steering Sensor Precautions
sensors are located in the steering column typically behind the turn
signal assembly. The sensor may be integrated with the clockspring
assembly for the air bag system, but it is usually a separate
component. Access requires disabling the air bag system and removing
the steering wheel and turn signal assembly.
some vehicles, the steering sensor must be aligned or recalibrated if
it is removed or replaced. On some vehicles, this procedure is
relatively simple, but on others it requires the use of a scan tool
with the appropriate software.
some of the older Mercedes applications, the steering angle sensor is
initialized by turning the ignition to RUN, and rotating the steering
wheel lock to lock.
Cadillacs and other GM vehicles with Stabilitrak, some steering
position sensors have alignment marks that must be aligned before the
sensor is removed.
location of these marks will vary depending on the sensor, model year
and vehicle. Some replacement sensors have a locator pin that is used
for alignment. The details are covered in GM TSB 03-02-36-002.