First off, what is a variable assist unit? On some vehicles, the amount of power assist the steering system receives is dependent on the speed of the vehicle (i.e. Speed Sensitive steering). A variable assist unit, often referred to as a EVO (Electronically Variable Orifice) or "Servotronic" steering module, is a small electrical solenoid (motor) normally mounted on a steering rack or gear box. The solenoid opens or closes depending on the speed of the vehicle, restricting the power steering fluid flow travelling into the rack or gear. Drive faster and you need less assist, so the solenoid closes and restricts the fluid flow. Make a turn in a parking lot and you need lots of assist, so the solenoid opens and allows more fluid to enter. This is controlled by the car's computer after it has received readings from the car's speed sensor.
If a vehicle came from the factory with EVO or Servotronic steering, such as the Lexus LS400, then you need to put back on a steering rack with the EVO on it. Most late model high end vehicles, such as many Mercedes Benz models, have these units built in. Since the EVO is tied to the computer of the vehicle, you cannot really add an variable assist setup to a car that did not come equipped with it.
One more thing to keep in mind. The Servotronic unit can be removed from the rack, see below. So often you can buy a rack "supplied without EVO" and you can swap your old EVO unit over. Since the EVO’s are expensive, this could save you lots of money if you just have a leak or something else not related to the EVO.
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