problem with speed sensitive steering.

[hotrod_8]

i have the typical stiff steering at lower speeds (i like), but at hwy speeds my car will pull hard to the left, and when i have to hold a turn for a while i can feel it cycle between stiff and easy steering.

its not a alignment issue, i can do 50mph and its laser straight. can i unplug a sensor to stop this? or am i stuck with it?

 

[philly0420]

1st gen. {at least for a 96)

Section 11-02: Steering System, Power 1996 Mark VIII Workshop Manual

DESCRIPTION AND OPERATION

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System Components
The electronic variable orifice (EVO) system consists of the following:

EVO power steering control valve actuator (3783) attached to the CIII power steering pump (3A674)
shock absorber electronic steering sensor (18B015) attached to lower steering column
vehicle speed sensor (VSS) (9E731) located on transmission
data link connector located under the hood, by the passenger side shock absorber tower
control module located in passenger compartment on the side wall below the evaporator. Refer to Section 04-05 .
EVO Actuator Assembly

The EVO power steering control valve actuator power steering control valve actuator generates a differential pressure, dependent on flow and current input.

This differential pressure controls the spool valve in the power steering pump.
The EVO power steering control valve actuator regulates the hydraulic flow and, when combined with a current controlling device, yields a variable assist power steering system.
At low speeds of 16 km/h (10 mph) or less, the power steering control valve actuator is controlled to provide full power steering pump flow for full power steering assist.
At higher vehicle speeds, the power steering control valve actuator is controlled to reduce power steering pump flow and to reduce the power steering assist.
For evasive maneuvering at high speeds (steering wheel rotation rate greater than 22 rpm), the power steering control valve actuator is controlled to increase the power steering pump flow, thereby increasing power steering assist.


Steering Wheel Rotation Sensor

A shock absorber electronic steering sensor, mounted on the steering column, is used to establish the rate of the steering wheel rotation.

Once the steering wheel rotation rate reaches 22 rpm and greater, the output current to the power steering control valve actuator is reduced and greater power steering assist is provided.
After the steering wheel rotation rate drops below 22 rpm, the output current returns to the amount of current regulated by the speed sensor input within one second.

Vehicle Speed Sensor

The vehicle speed sensor is located on the transmission and is used to establish the vehicle speed.

The output current to the power steering control valve actuator is determined by the vehicle speed and the steering wheel rotation rate.
When the vehicle speed is 10 mph (16 km/h) or less, the power steering control valve actuator output current is close to 0 mA, and full power steering assist is provided.
As the vehicle speed continues to increase, the current will increase linearly.
Conversely, as vehicle speeds decrease, the current decreases.
Data Link Connector

The data link connector is used for diagnostics. Refer to Section 11-00 .

Control Module

The control module contains a microprocessor which analyzes the vehicle speed sensor input and the electronic steering sensor input continuously. With this information, the microprocessor controls the EVO power steering control valve actuator, thereby regulating the fluid flow from the power steering pump.........


Seems like 2nd gens might be a bit more sophisticated. Here's what I found...

Electronic Variable Orifice (EVO) Power Steering

The electronic variable orifice (EVO) steering system controls the level of power steering assistance based on vehicle speed and the rate of change in the steering position. Greater power assistance is provided at lower speeds to lessen steering effort and increase maneuverability. Less power assistance is provided at higher speeds to raise the steering effort for increase directional stability and greater "road feel."

Steering effort increases with vehicle speed in a smooth and continuous manner in the mid-range speed, reaching a constant level at high rates of speed. A high rate of change in the steering wheel position, which might indicate an evasive maneuver, will result in an increase in the power assistance.

The EVO communications are integral with the Air Suspension Module.

 

[NoLimit95]

I had the same issue in my 94 when I was taking an off ramp and going around 40 on a long curve, the steering would go back and forth, like it was assisting me or something. Changed out racks with a 93 rack and it never done it again. I never had the pulling and stiff steering you're talking about. Mine was just a rack gone bad.

 

[hotrod_8]

hey i popped my power steering cap and it smells like power steering fluid, i thought these cars need auto trans fluid for the power steering? what ever is in it is coal black and needs changed.

 

[Roadboss]

Use Mercon V for your fluid.

 

[unity]

You can just disconnect the power steering thingy on the back of the pump. Two wires. People often mistake how it works. I think Philly pretty much covered it, but I did not read. The way it works is that the system by default has FULL assist. Then when it wants to back off assist, such as on the highway, it "interferes" with the flow of fluid making steering stiff. If you disable it, you will have full assist at all times.

Been there, done that and covered this thing more times that I care! My former Mark had a pump without the variable assist. Was never an issue.