Keeping on Track with Steering Position Sensors

Keeping on Track with Steering Position Sensors

Cars 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 information may be used to vary hydraulic pressure in a variable assist power steering system, or by a stability control system to improve handling, braking and traction under changing driving conditions.

Early Applications
In 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.

The 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.

On 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 driver.

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° 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. On later versions of the Ford VAPS system that appeared on the Ford Probe, the steering sensor also measured the actual angle of the steering wheel. At speeds below 6 mph, or when the steering wheel was turned more than 45° off-center, the VAPS system delivered full power assist. But at higher speeds or when the steering wheel was within 45° of center, power assist was reduced.

In 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.

Diagnostics on these early systems did not require a scan tool because the information from the steering sensor was not shared with the powertrain control module, antilock brake system or anything else. On the Lincoln, the system could be diagnosed by plugging an analog voltmeter into the steering diagnostic connector located near the master brake cylinder. This put the system into a self-diagnostic mode that would cause the voltmeter needle to sweep back and forth to indicate fault codes. On the T-Bird and Cougar, a test light could be used for the same purpose, and to verify a signal from the steering angle sensor. Rotating the steering wheel at least 220° in one direction would cause the light to come on for about three seconds. If the light did not come on, it meant there was a problem in the steering sensor or wiring.

If 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 key on.

More Sensitive Sensors
Most of the early steering position sensors are relatively low-resolution sensors, and typically detect steering movements in 8° to 9° increments. By 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 antilock 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 himself.

The 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 steering sensor also provides input for the new “parking assist” systems that are now starting to appear on certain luxury vehicles. The parking assist system uses a rear-mounted video camera to survey a parking space, then calculates the steering maneuvers needed to parallel park the vehicle without driver assistance. The steering sensor provides feedback to the system as the steering maneuvers are being made.

Down 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 steering column?

Electronic Stability Control
Electronic stability control (ESC) systems are being added to more and more new vehicles. According to the Insurance Institute for Highway Safety, for the 2008 model year, ESC was standard on 63% of new passenger vehicle models and optional on 15%. ESC was standard on 64% of cars, 95% of SUVs and 12% of pickups. Stability control requires monitoring the steering angle and driver’s steering inputs via a steering angle sensor on the steering column. It also requires monitoring the motions of the vehicle itself with a lateral acceleration sensor and yaw sensor (which are combined into a single sensor on many applications).

The steering angle sensor tells the ABS control module where the driver is steering the vehicle while the body motion sensors tell it how the body is responding. At the same time, the ABS wheel speed sensors are monitoring tire traction and slippage on the road itself. The control module takes all of this information into account and compares the sensor inputs to its programming to determine overall vehicle dynamics. If the vehicle is losing traction, understeering or oversteering, the ABS/TCS/ESC system kicks into action and applies individual wheel braking as needed to keep the vehicle under control.

Almost a Rollover
A test drive at the Bosch proving grounds in Farmington Hills, MI, a year ago illustrated to me what a dramatic difference stability control can have on vehicle handling and safety. The vehicle I was in was a Chevy Tahoe SUV equipped with a Bosch ESC system that could be deactivated by the driver. The first run was made with the ESC system off.

At 35 mph, the driver made a sudden swerve to the left and then back to the right (the “J” maneuver) to simulate avoiding an obstacle in the road. With no help from the ESC system, the vehicle’s high center of gravity caused it roll so much that it lifted the two right wheels over a foot off the ground. The SUV would have flipped had it not been equipped with special anti-roll I-beams mounted on the front and rear bumpers. The beams extended outward about 3’ on either side and acted like giant training wheels to keep the vehicle upright.

When the same steering maneuver was repeated with the ESC system engaged at 45 mph, all four wheels remained on the ground and the body remained flat. There was no risk of a rollover. By sensing the driver’s steering inputs and change in body yaw and momentum, the ESC system was able to effectively counterbrake the individual wheels to keep the vehicle under control. It certainly illustrated how easily ESC can improve vehicle safety.

Stability Control Applications
The first vehicles to be factory-equipped with electronic stability control were the 1995 BMW 750iL and 850Ci models with a 5.4L V12 engine. These were followed in 1996 by a more sophisticated Electronic Stability Program (ESP) system on V12-powered Mercedes S600 models.

The first domestic cars to get this technology were 1997 Cadillac Seville STS, DeVille Concours and Eldorado ETC models. The GM system was called StabiliTrak, and is now found on a growing number of GM cars and trucks.

Diagnostics
Because 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.

Diagnostics 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.

In 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. Some 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.

Depending 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
Steering 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.

On 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.

On 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.

On Cadillacs and other GM vehicles with Stabilitrak, some steering position sensors have alignment marks that must be aligned before the sensor is removed. The 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.

On 2004 Toyota RAV4, Tacoma and Tundra, and 2005 Scion xA models, the steering angle sensor initialization procedure goes as follows if the sensor has been removed or replaced. For more info, see Toyota TSB BR001-04.

With the Toyota scan tool, you choose Diagnosis, then 1 – OBD/MOBD, then enter the model year and model. Then you scroll down to 4 – ABS/VSC, choose 6 – Reset Memory and press Yes. After the memory reset is completed, choose 7 – Signal Check. The VSC/TRAC and ABS should begin to flash at 0.13-second intervals. Then press Exit to quit the signal check.

If you don’t have a Toyota scan tool, the following also works:

1. Put the transmission in Park, and turn on the ignition.

2. Using a jumper wire, short terminals Ts and CG on the DLC3 diagnostic connector four times or more within eight seconds. This will erase the recorded steering sensor zero point.

3. Turn off the ignition switch and remove the jumper wire.

4. Make sure the steering wheel is centered, and turn the ignition switch back on.

5. The stability control warning light should come on, then go off after about 15 seconds.

6. Wait at least two seconds after the light goes off, then turn the ignition back off.

7. Use the jumper wire to reconnect terminals Ts and CG on the DLC3 connector, and turn the ignition back on.

8. The traction control warning light should come back on and start to flash after about four seconds. The zero point has been reset. The ignition switch can now be turned off and the jumper wire removed.

On Audi and Volkswagen models with electronic stability control, the G85 steering angle sensor requires a “zero position” calibration if the sensor is removed or replaced:

1. Start the vehicle and turn the steering wheel one turn to the right and one turn to the left.

2. Drive in a short distance in a straight line on a level surface at a speed not higher than 20 km/h.

3. If the steering wheel is straight during the test drive, stop the vehicle with the wheels pointed straight ahead. Make sure the steering wheel does not move.

4. Keep the engine running and do not switch off the ignition.

5. Using the VW scan tool, select mode 03 – ABS Brakes, then Login – 11, then Enter 40168, then Do It!, then Basic Settings – 04, then Group 060, then Go! This will set the zero (centered) position of the steering sensor and display a message that reads: “Steer. angle sender compens OK.” If you get an error, it may mean the Login was not successfully performed.

6. Click Done/Go Back and you’re finished.

Federated Wants You to ‘Get Dirty with Kenny’
Staunton, VA — Federated Auto Parts is offering its Car Care Center members a chance to race NASCAR driver and dirt track legend Ken Schrader at Schrader’s I-55 Raceway in Pevely, MO.

“Ken was generous enough to let us have the run of his track for a couple of days and allow some of our Car Care Center members to get into a car and learn the ropes of dirt track racing along with a behind-the-scenes look at Ken’s racetrack,” said Phil Moore, vice president of marketing for Federated Auto Parts. “This is an exclusive promotion for our Car Care Center members and Ken has guaranteed that the winners will be getting dirty from eating his dust on the track.”

To be eligible to win a chance to “Get Dirty with Kenny,” a shop must be fully enrolled as a Federated Car Care Center member by July 15, 2008. All members in good standing are automatically registered to earn a chance to win the all-expense paid trip to I-55 Raceway.

ALLDATA Celebrates 70,000 Customers
ALLDATA held a media day event at its headquarters in Elk Grove, CA, April 3, to celebrate its latest accomplishment — 70,000 customers — and to spotlight how ALLDATA is helping automotive service professionals boost their business success.

ALLDATA President Jeff Lagges and other company executives discussed how ALLDATA is reinventing itself for a changing market. Their presentations addressed ALLDATA’s commitment to providing high-quality OEM repair information and business solutions for automotive service professionals to help them increase their competitiveness in the marketplace, elevate their productivity and profitability, and forge stronger relationships with their customers.

ALLDATA executives also discussed today’s market challenges, how ALLDATA’s product suite can turn those challenges into opportunities as well as the company’s vision for the future.

Lagges opened the event with a video presentation he produced that took the audience on a walk down ALLDATA memory lane, from its humble beginnings in 1986 as the pioneer of auto repair data, to a full-fledged provider of service and diagnostic information, shop management software and customer relations tools for the automotive repair and collision industries. It included both customer and employee testimonials.

Lagges pointed out that one of the major components in the company’s success, and an impressive 90% customer retention rate, has been to always maintain direct contact with customers, primarily through a direct salesforce. “We measure our success by our customers’ satisfaction,” he said.

As the company looks forward, its goals are to continue to build more awareness in the marketplace, focus on the health of the industry in general (ALLDATA is growing, while its market is shrinking), continue to help improve productivity at the fender and the counter so that shops are growing and not just surviving, and to continue its mantra of focusing on customer service and support.

Steve Gill, vice president of program development, echoed similar remarks in talking about industry issues and the need for best practices for shops to remain competitive and survive. He emphasized that the most pressing issues facing shops are vehicle reliability (which can be offset with proactive maintenance), diagnostics representing an increasing portion of repairs, the continued need for tech experience and training, and new vehicle technologies where integrated system repairs will require OEM data.

Gill offered “best practices” advice that encompassed profit margins and ways to increase net profit, shop work mix, productivity, aggressive marketing programs and insurance relationships.

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