Hydractive 1

The Hydractive I suspension system appeared with the XM. Unlike the simpler hydropneumatic suspension used on the DS, GS/GSA, CX, BX and some XMs, this one has two modes of operation, soft and hard. The suspension functions in soft mode but it will be switched to the hard mode when the computer deems this necessary for the sake of roadholding and safety.

To achieve this, the first hydractive system adds two spheres (one for each axle) and an electric valve to the struts and spheres of the standard hydropneumatic setup.

During normal driving, the computer keeps the suspension in soft mode most of the time but—based on the input provided by many sensors (steering wheel, accelerator pedal, body movement, road speed and brake), including the Sport/Comfort switch on the dashboard—the suspension ECU decides when to switch to hard mode; in other words, when to deactivate the additional spheres for extra roadholding and safety.

When the driver selects the Sport setting, the suspension is switched to hard mode constantly. This setting is not what any Citroën driver would call comfortable… The successor system, Hydractive II overcomes this limitation.

The illustration only depicts the differences to the standard hydropneumatic layout already presented in the previous section:

1 A standard Citroën sphere base which fits a sphere without a damper block. The sphere volume and pressure differ for the front and rear, as well as according to the model of the car;

2 A hydraulically controlled isolation valve that connects or isolates the sphere from the rest of the suspension, modifying the string constant of the suspension;

3 A ball and piston valve arrangement that limits fluid cross-flow between the left and right suspension struts in case of body roll. This valve is disabled for suspension height corrections, in order to guarantee that the fluid pressure in the corner struts remains equalized;

4 Two damping elements similar to those used on the corner spheres, acting as dampers for the center one;

5 An electrically controlled valve driven by the suspension ECU. In order to reduce heat build-up, the computer uses pulse width modulation to achieve a constant current through the coil. The initial voltage is higher to make the valve react quicker but it is reduced to a smaller value once the inductive effects have been overcome, should the valve stay on for a long enough time. The valve is capable of being on indefinitely when driven with this sustained current.

The front and rear suspension circuits are identical and the same electrovalve serves both subsystems.

Soft, hard, soft, hard…

The default electrical mode of the suspension, when the electro-valve 5 is not energized, is hard.

While the computer keeps the suspension in soft mode, the electro-valve 5 is energized by the ECU, opening the feed pressure onto the isolation valve piston 2 and by moving it, connecting the center sphere 1 to the rest of the suspension. The fluid in the suspension has to pass through two damping elements 4 (one for each strut connection). When both struts move in unison, the center sphere behaves as a standard sphere with a damper hole twice as large as a single damper element, but when the car starts to roll, the fluid has to move from one strut to the other, passing through both damper elements consecutively. In addition to this double damping, the sphere 1 itself acts as a damping string, absorbing quick changes in pressure between the two dampers. This dampens the body roll to some extent even in soft mode.

Whenever the computer feels it necessary to switch to hard mode, it closes the electro-valve 5, not allowing the main feed pressure to move the isolation piston 2. The pressure inside the center sphere 1, always higher than that of the return path under normal operating conditions, will move the control piston into a position which closes off the center sphere completely. The remaining pressure in this sphere remains unknown but as the main circuit pressure might change while the suspension is in hard mode (due to either the dynamics of the suspension—acceleration, braking, movement due to uneven surface—or the vehicle height altered by the driver), the computer equalizes the pressure periodically by enabling the control block to assume the soft position for a short period of time.

Hard mode serves three reasons. First, it provides higher resistance to body roll. The cross-flow of LHM from one strut to the other has to pass through both damper blocks as in soft mode, but it is additionally limited using the piston and ball valve 3, now switched into the hydraulic circuit between the damper elements instead of the center sphere. The ball is positioned in the fluid so that any cross-flow moves the ball and thus limits the flow, dampening the body roll as well.

Second, it limits dive and squat by helping out the height correctors. A stiffer suspension damps the vertical motion and therefore reduces the amount of correction required.

Third, hard mode not only limits the suspension travel between the body to the road but between the suspension elements and the body. Its aim is to reduce suspension movement at the cost of comfort but to gain safety, limiting the influence of the body movement to steering, very important in extreme situations like a flat tire.

At the same time when the body roll is present, the car might need to change the ground clearance as well: for instance, when braking in a curve. The valve 3 therefore has an additional piston which lets the LHM flow between the circuits of the struts and of the height corrector. If the body has to be raised, the pressure in the height correctors will be higher than that in the suspension. This higher pressure pushes the piston, which in turn dislodges the ball and the pressure will raise equally in both struts (without dislodging the ball, only one of the struts would receive the fluid, resulting in incorrect operation).

When the vehicle is making a sharp left turn, tending to roll to the right, the right strut will be compressed and the left one expanded. The fluid is then forced from the compressed strut to the expanded one, moving the ball in the valve towards the outlet of the left strut; as soon as it reaches and covers the outlet orifice, it closes off any further cross-flow. The corner spheres are now isolated and has to provide all the damping themselves.

If the body has to be lowered, the higher pressure in the struts will dislodge the ball again, opening the piston towards the return line ad the fluid will escape from both struts, lowering the vehicle.

Sensory perceptions

The computer of the suspension system takes its input signals from the various sensors and based on a set of rules, dynamically activates the electric valve.

There are eleven inputs to the ECU. First, the Comfort/Sport switch on the dashboard, enabling the driver to choose between the two settings. The status light on the instrument panel informs about the setting selected (it does not indicate the mode the suspension is currently in).

The second input comes from a vehicle speed sensor. This inductive magnet tachogenerator generates four pulses per rotation, that is approximately five pulses per meter traveled (although this depends somewhat on tire size). It is located on the gearbox where the speedometer cable attaches, or in some versions, on the cable itself. The ECU determines the acceleration of the car by evaluating changes in vehicle speed for the duration of one second.

Another input arrives from the steering wheel angle and speed sensor, an optoelectronic device consisting of two infrared light beams, interrupted by a rotating disc with 28 holes. The ECU senses the quadrature signal changes of both sensors to effectively increase the resolution of the sensor (28 pulses per steering wheel revolution) by a factor of four. This produces one edge change every 3.214 degrees of steering wheel rotation. The direction of turning can be determined by the sequence of the edge changes.

To make decisions, the computer needs to know the straight ahead position of the steering wheel. The sensor does not have a built-in zero position (as it would not always work, due to misalignment and wear in the mechanical components). The computer uses heuristics instead:

First, the straight line position is assumed if the vehicle speed is above 30 km/h and the steering wheel position was not changed (an error margin of up to 4 pulses is allowed) for the last 90 seconds. Second, we know the maximum number of pulses in both directions from the center (lock to lock angle divided by two). If the steering wheel is found to turn more than this value (an error of up to 4 pulses is accepted here, too), this is a clear indication of an incorrect center reference: in this case the center position will be adjusted by the surplus.

The rotational speed of the steering wheel is determined by measuring the time elapsed between the individual pulse edges coming from the sensors.

A similar sensor informs the computer about the movement of the car body. Two infrared beams, the disc having 45 notches, similarly quadrupled  by the ECU. Excessively long intervals are considered coming from slow height changes resulting from the driver selecting a different height setting, and are consequently discarded.

The sensor is connected to the front anti-roll bar, to the right of the height corrector linkage. Due to its location, it is capable of detecting both squat and dive, and to some extent, body roll. But as the sensor is mounted off-center, its sensitivity to roll is about three times less than the sensitivity to squat and dive. In all directions, it can measure both movement amplitude and speed of movement, using the same process as the steering wheel sensor does.

The throttle pedal position sensor is located below the dashboard, right next to the pedal mechanism, where the pedal can operate its sprung lever as it moves. The sensor is a  potentiometer with an integrated serial resistor in the wiper’s circuit.

The entire travel of the potentiometer is quantized into 256 steps by the analog-digital converter inside the ECU. The 5 V reference is supplied by the ECU itself. Due to the gas pedal initial position and maximum travel, about 130 to 220 steps out of 256 are being actually used.

The brake pressure sensor is a simple pressure activated switch located on a hydraulic conduit connector block, right next to the ABS block, at the bottom of the left front wing, in front of the wheelarch, under the battery. The switch makes contact at 35 bars of braking pressure.

The door/tailgate open switches are located on the door frame and in the boot latch. The door switches are all wired together in parallel and connected to one input line (and routed to the interior light dimmer and timer as well). The tailgate switch is connected to the other input line (and routed to the boot light and the tailgate opened detection input for the status display on the dashboard, too; the door open and bonnet open signals for the status display are generated by a separate set of switches, independent of the ones used for the suspension).

The usual ignition switch provides a power-on signal, triggering and internal reset and self diagnostic run in the ECU. Turning the ignition on and off also triggers internal events that guarantee proper pressure equalization between the center and corner spheres.

The brain behind the suspension

The ECU is a small microcomputer sensing the input signals coming from the various sensors. A very interesting and important aspect of the system is that it uses the driver of the car as a major part of its intelligence, making the operation very simple but effective. To achieve this, most of the sensors read the controls the driver operates.

The software contains the description of various conditions (status of the input lines and internal timers) governing when to activate-deactivate the electrovalve switching the suspension to either hard or soft mode. These conditions can be formulated as rules.

Every main input sensor has an associated rule: when the value collected from the sensor exceeds a specific threshold, the suspension is put into hard mode and the computer starts a timeout counter. For the suspension to return to soft mode at the end of the timeout period, the threshold must not be exceeded again during this time. If it was exceeded, the suspension stays in hard mode and the timeout starts all over again.

There are four additional rules overriding the normal operation—even if the sensor inputs call for a generic rule to be applied, these four conditions are checked first:

  • the computer puts the suspension into soft mode when the ignition is turned on or off. This setting prevails until 30 seconds elapse or the vehicle speed exceeds 30 km/h, whichever comes first;
  • if the computer determines any problem with its own operation or any of the input or output devices (including inconsistent values like no body movement but a vehicle speed above 30 km/h), the suspension will be switched to hard mode and stay there until the ignition is turned off or the doors are opened with the vehicle speed below 30 km/h. The ECU does run a self-diagnostic routine when the ignition is turned on but some sensors cannot be tested at this time, only during normal use;
  • whenever the suspension stays in hard mode for more than one minute, the computer switches to soft mode momentarily to assure the equalization of pressures in the corner and center spheres. If the circumstances still call for hard mode, the suspension will revert within 50 ms and restart the one-minute timeout period;
  • below 30 km/h opening the doors or tailgate overrides any other rules and puts the suspension into soft mode to equalize the pressures in the spheres.

As already mentioned, the steering wheel sensor is used to derive two inputs values: steering wheel speed and angle. These values are treated separately with the purpose of calculating the lateral acceleration of the vehicle (vehicle speed, steering angle) and the potential change in this acceleration (vehicle speed, steering wheel speed). It is seemingly done this way to save memory which would otherwise be required for a full three-parameter lookup (based on vehicle speed, steering wheel angle, steering wheel speed). The steering wheel sensor rules actually give a measure of potential body roll. Body roll is significantly reduced in hard mode, consequently, the rules were set up to ensure that the body roll is minimized when there is potential for it, still the suspension stays soft to absorb bumps when there is no body roll caused by the vehicle changing direction.

If the acceleration or deceleration (braking) of the vehicle exceeds 0,3 g (approximately 3 m/s2) while the actual speed is above 30 km/h, the suspension will be switched to hard mode and a timeout of 1.2 seconds begin.

The table below shows the thresholds of steering wheel angle and rotating speed. If any of these values exceed the threshold for the actual vehicle speed, the suspension will switch to hard mode; it will revert to soft when the corresponding value drops below the threshold for at least 1 second if the switching was triggered by the steering wheel angle and 2 seconds if triggered by the rotational speed:

Vehicle speed (km/h)

Steering wheel angle (deg)

< 30

always soft













141 >


Vehicle speed (km/h)

Steering wheel speed (deg/s)

< 30

always soft







121 >


The body movement amplitude and speed is derived from the output of the body movement sensor, although the two values are used in a different way.

The values delivered by the throttle pedal sensor are used with reference to the vehicle speed in order to anticipate the vehicle dynamics as a result of acceleration or deceleration. The rules for this sensor represent a reaction to probable vehicle squat (on acceleration) or dive (on deceleration). Both are significantly reduced when the suspension is in hard mode.

The suspension ECU quantizes the pedal position into five discrete steps: 0, 30, 40, 50 and 60 percent of the complete pedal travel. The computer measures the time elapsed as the pedal travels from one step to the next in either direction. If this time is inside the intervals shown in the table, the suspension will switch to hard mode. It will revert to soft if the pedal movement becomes slower for at least the duration of the timeout specified:

Pedal press speed (ms) Tiemout (s)
< 100 1
101–150 2
Pedal release speed (ms) Tiemout (s)
< 100 1
101–200 2

The brake pressure sensor detects the pressure in the front brake hydraulic circuit. Since this is a fixed threshold sensor, the suspension setting rule is simple: if the vehicle speed exceeds 30 km/h and the pressure is above 35 bar in the brake circuit, the suspension switches to hard mode. The system stays so to prevent excessive dive when brakes are applied while any of these two conditions are met (the timeout value is one second).

Without ignition and electrical feed to the suspension computer, the electro-valve would immediately return to hard mode. Loading or unloading the car, people getting in or out would induce pressure differences in the hydraulic system. These differences would equalize abruptly when the system is started again, causing the car to jump or sink vehemently. In order to avoid this, the computer allows an additional 30 seconds of timeout starting when any of the doors is opened or closed (as communicated by the door and tailgate open sensors) , leaving the electro-valve energized for the duration of the timeout.

It is important to note that the suspension will switch to soft mode even with the ignition switch turned off. Early cars did not have this feature built directly into the computer but used an additional relay and circuits. On those models, the constantly energized electro-valve can drain the battery if the doors remain open for a long time. Starting with the H2 suspension computer (from ORGA 4860, February 28, 1990) the door sensors are observed by the ECU itself and the operation is enhanced with a 10-minute timeout period. After this interval, the electro-valves will always return to the hard, non-energized state.

Changing the state of the ignition switch provokes a transition to soft mode for a maximum of 30 seconds; reaching a vehicle speed of 30 km/h will cancel this mode prematurely. When the ignition is turned on, the ECU also runs a self-test diagnostic sequence lasting three seconds.

When the suspension selector switch is set to the Sport setting, all sensor inputs except for the vehicle speed sensor are ignored. Below 30 km/h the car stays in soft mode and switches to permanent hard mode above this speed. The suspension status light in the instrument panel has two functions:

  • when the ignition switch is turned on and the suspension set to Comfort, it will light up for the duration of the ECU self test. If the computer detects any malfunction in the course of this test, the light will flicker one or more times during this period;
  • when the suspension is set to Sport, the status light will remain lit to inform the driver of the setting chosen. The status light actually lights up or extinguishes only when the suspension rules have been changed in response to the mode select switch. This takes a short while because the internal timeouts are reset and some of the sensors are recalibrated. Because of this the light changing state is slightly delayed from the mode switch changing state.