If a '97 Crown Vic pulls into your shop with the rear bumper dragging on the ground, it's helpful to know that the dome light must work properly before the compressor will run. Semi-active suspension Load leveling requires a change in spring rate or position. The control unit receives information about vehicle behavior, interprets that information as road surface condition and driver intent, then adjusts damping to a programmed level of firmness. Control decisions can be calculated instantaneously, and today's actuators work so fast, effective adjustments can be made in mid-bump. Damping adjustment is usually accomplished with a shock absorber that has several different size orifice valves to control the flow of oil, with a solenoid or stepper motor used to control valve selection.
Some systems also adjust spring rate, using air springs and fast acting solenoid valves.
While this all sounds quite sophisticated, most of the hardware is not new. Bilstein has made manually adjustable shock absorbers since the '60s, but those on the Ferrari have 17 different settings that are selected automatically by the control unit. Up front are two solenoid-equipped adjustable struts and two height sensors. Low side voltage is pulse-width modulated at a frequency of two kilohertz, effectively controlling the current supplied to each individual solenoid valve. As pulse width is increased, damping is also increased.