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
VARIABLE ASSIST POWER STEERING
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 VARIABLE ASSIST POWER STEERING (VAPS)
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
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.
LINCOLN & T-BIRD STEERING SENSOR
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.
FORD PROBE VAPS STEERING SENSOR
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
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.
SYSTEM & SENSOR DIAGNOSIS
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
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
The diagnostic procedures for the three Ford variable assist
power steering systems are all different, so here's a summary of
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
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
* One needle sweep followed by a 8 to 9 second pause
followed by a second needle sweep means the module and solenoid
* 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).