1934 Ford 40 1947_ford_v8_deluxe 1926 Ford Model T


Electronics and Sensors STEERING SENSORS

Steering sensors are used on some vehicles that "variable assist" power steering. The sensor monitors the rate at which the steering wheel is being turned so that power assist can be increased or decreased as needed. The amount of change that takes place will depend on vehicle speed and how the system is calibrated.

The feature that makes variable rate power steering different from ordinary power steering is the ability to vary the level of power assist according to the vehicle's speed. With ordinary power steering, the amount of power assist remains constant. A system that provides a high amount of assist for easy parking maneuvers and low speed driving may feel vague or twitchy at highway speeds. So variable rate power steering changes the amount of power assist according to speed and steering inputs so the driver can enjoy the best of both worlds: easy steering for parking and low speed maneuvering, and reduced assist at higher speeds for good road feel and stability.

It's important to note that not all variable assist power steering systems use a steering input sensor. Some are simply speed dependent and change the amount of assist based on input from the vehicle speed sensor only. Others, such as Honda, are purely mechanical and do not reply on electronic inputs.

Ford introduced Variable Assist Power Steering (VAPS) in 1988 on the Lincoln Continental. The system is an electro-hydraulic speed-sensitive control system that varies the level of power assist according to both the vehicle's speed and rate of steering wheel rotation. VAPS provides maximum assist at low speed to minimize steering effort while progressively reducing the amount of assist as the vehicle's speed increases. The minimum level of power assist is approximately half the maximum.

Ford also offers "Electronic Variable Orifice" or EVO power steering on the '89 Thunderbird and Mercury Cougar, and a somewhat different type of VAPS system on the GT version of the Ford Probe.

Both the Lincoln VAPS and T-Bird EVO systems include two input sensors (a transmission mounted speed sensor and a steering column mounted steering wheel rotation sensor), a trunk mounted control module (which is integrated with the chassis computer on the T-Bird Super Coupe and Cougar XR7) and an "actuator valve" to vary the amount of hydraulic pressure that flows from the power steering pump to the steering gear. The actuator valve in the Lincoln and T-Bird is a stepper motor that opens and closes an orifice in small steps to restrict the pump's pressure output. The actuator valve on the Lincoln is mounted on the steering gear while the one on the T-Bird replaces the outlet valve on the power steering pump.

At speeds up to 5 mph the control module's output to the actuator valve is zero milliamps (mA) and the actuator valve is wide open allowing full assist as required. As the vehicle's speed increases above 5 mph, the module starts to close the actuator valve to reduce power assist in direct proportion to speed. At 20 mph, module output to the actuator valve is 225 mA and the actuator is partly closed to reduce steering assist 20 to 25%. At 80 mph, the module's output reaches its maximum of 550 mA closing the actuator valve to its minimum orifice opening. Steering assist is now reduced 50% from its maximum level. As the vehicle's speed drops back below 80 mph, the actuator valve starts to open again to gradually increase power assist in proportion to speed.

The steering wheel rotation sensor also influences the amount of assist that's provided in the Lincoln and T-Bird. The sensor consists of a photo diode that reads evenly spaced holes in the column mounted disc. As long as the wheel is turned at a rate slower than 15 rpm, steering input has no effect on power assist. But if the rate exceeds 15 rpm, the output current to the actuator valve is reduced proportionately to provide more assist. And if the wheel is jerked to one side or the other during a sudden evasive maneuver (a rate that exceeds 100 rpm) the actuator control current is cut allowing full power assist to return until the maneuver is completed.

The VAPS system in the Probe is similar to the Lincoln and T-Bird except that it offers only two different levels of steering assist instead of a range of levels that change in proportion to speed. The Probe system uses a two-position solenoid to regulate pressure to the steering gear instead of an actuator valve, and a steering wheel "angle sensor" rather than a rotation sensor.

At speeds below 10 km/h (6.2 mph). or when the steering wheel is turned more than 45 degrees to the right or left, the solenoid on the pressure line is open and the system provides full assist. At higher speeds or when the steering wheel is at an angle of less than 45 degrees off-center, the solenoid is closed to reduce the amount of assist.

The Probe control module is located under the driver's seat rather than in the trunk, and it is "adjustable" in that it offers three different settings. A three-position slide switch on the side of the module allows for a 10% harder than normal steering effort, or a 10% lighter than normal setting so the system can be fine-tuned to the driver's preference.

All three variable assist power steering systems are designed to fail safe, meaning if power is lost to the actuator valve or solenoid valve, the valve goes the full open position allowing full power assist under all driving conditions. The only indication of trouble, therefore, might be a loss of road feel at highway speeds.

It's important to remember that variable rate power steering only affects the amount of pressure that reaches the steering gear at higher road speeds. The only way it could reduce power assist at low speed would be in the unlikely event the actuator or solenoid valve jammed shut. More likely causes of reduced power assist would be a loose pump drive belt, low fluid level, bad pump or internal leaks in the steering gear. Likewise, a case of "morning sickness" where the steering feels sluggish when the car is first started on a cold morning then improves as the car warms up is probably due to wear in the steering gear spool valve housing.

The diagnostic procedures for the three Ford variable assist power steering systems are all different, so here's a summary of the basics:

On the Lincoln, you check the system out by plugging an analog voltmeter into the diagnostic connector located under the hood near the master brake cylinder. The connector is a two receptacle socket. The white/black wire is ground and should be connected to the voltmeter's negative lead. The white wire goes to the module and should be connected to the voltmeter's positive lead.

Once the voltmeter is connected, turn the key on. It takes about a minute before the self-test begins. If the voltmeter needle makes four sweeps, the module is okay and you can proceed through the accompanying diagnostic chart. Six sweeps or no sweeps indicates trouble.

The diagnostic connector is located in the upper left glove compartment on the T-Bird LX and Cougar LS, and under the hood near the strut tower on the T-Bird Super Coupe and Cougar XR7. Rather than using a voltmeter to count needle sweeps on this system, Ford recommends using a test lamp that can be plugged into the diagnostic connector (which you can fabricate using connector 14489-EA, socket 13728-A and a number 194 or 161 bulb. To begin the diagnostic check, plug in the test light and start the engine. If the lamp comes on for one second, the module is okay and you can proceed to the next checks. If the lamp only flickers, the module is bad. If it doesn't come on and there's nothing wrong with the bulb, turn the ignition off and check the 14-pin wiring harness connector at the control module in the trunk. If there's battery voltage (12 volts) between pins #7 (hot) and #5 (ground) with the ignition on, the module is defective. No voltage means a short or open in the power circuit to the module.

Assuming the test light flashed on for one second when the engine started, the next self-test checks out the actuator output circuit. If you don't see any flashes after a two second delay the actuator output circuit is okay. But if you get six flashes, there's a short or open in the circuit. The module will stop the self-test at this point if a problem has been detected and won't come back on until the ignition has been cycled off and on. The next self-test checks the steering wheel rotation sensor. Rotate the steering wheel from lock to lock. As soon as the steering wheel is turned at least 220 degrees in one direction, the test light should come on for three seconds indicating the steering wheel sensor is working. No light means a problem at the sensor or wiring circuit.

You can check the sensor by disconnecting the 14-pin connector at the module, using a jumper to connect pin #12 to pin #5, and using an ohmmeter on the 1000 ohm scale to check between pins #1 and #5, then between pins #5 and #6. With the key on, rotating the wheel should cause the ohmmeter needle to jump every nine degrees of rotation if the sensor and harness are good. To check the vehicle speed sensor, the car must be run with the wheels off the ground (or test driven) at a speed greater than 15 mph without turning the steering wheel. The test lamp should come on and remain on as long as the speed is above 15 mph. Now drop the speed below 10 mph. The lamp should go out if the system is functioning properly.

The VAPS diagnostic connector on the Probe is located under the hood near the windshield wiper motor. The connector has three terminals (two vertical, one horizontal). Attach the positive lead on an analog voltmeter to the left vertical terminal (yellow/black wire) and the negative lead to the horizontal ground terminal (black wire). Set the scale to read from 0 to 15 volts.

The "Quick Test" on this system is begun by starting the engine with the steering wheel in the straight ahead position. Once the engine starts, the control module will produce one of several diagnostic codes which you'll read as sweeps of the voltmeter needle.
* One needle sweep followed by a 8 to 9 second pause followed by a second needle sweep means the module and solenoid are okay.
* Five needle sweeps followed by a pause followed by one sweep in a total time span of 10 seconds indicates a problem with the solenoid circuit. The solenoid itself can be checked by disconnecting it from its wiring harness and using jumper wires to feed it 12 volts directly. It should click if good. The solenoid's resistance, which can be checked with an ohmmeter, should be between 3.4 and 6.9 ohms.
* Three needle sweeps followed by a 4 or 5 second pause then another sweep indicates a problem with the VAPS ignition sensing circuit, which feeds off the ignition coil.
* Two needle sweeps followed by a 6 to 7 second pause and another sweep indicates trouble steering angle sensor circuit. Resistance between the sensor terminals should be as follows with the wheel in the straight ahead position: 20-30K ohms between terminals H and F, and E and F, and 40 to 60K ohms between terminals H and E. Turning the wheel 180 degrees to the right should cause the resistance reading between terminals F and H to jump to about 50K ohms.
* No needle sweeps indicates a fault in the VAPS control module power or ground circuit. There should be at least 10 volts at the black/green wire terminal at the module connector. Resistance should be less than 5 ohms at the black wire ground terminal at the module connector (which you can also verify through the diagnostic connector).

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