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Suspension

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Any work on the hydraulic parts below the car should be carried out with the car safely supported on axle stands or standing over an inspection pit. Never work or allow anybody to work on the car (not even for a small adjustment or even just inspection) when it's only the high setting of the hydraulic system that keeps the car up. Even a slight adjustment of the height control linkages or any other failure can cause the car body to drop many inches and easily crush anybody causing death or very serious injury. This is not a theoretical warning, it has already happened. Even with the car safely supported, always watch out for the possible drop in height and allow sufficient room for it to happen.

Anti-sink system

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The only purpose of the anti-sink system is to keep the car from lowering when not used, it does not interfere with the normal functioning while in use. It attempts to minimize leaks inside the system by having only one element that can leak, the anti-sink valve itself.

Changing the LHM

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Changing the LHM and rinsing the system is a part of normal service and care for all hydraulic Citroëns. Don't neglect this if you want reliable and trouble-free operation from you car. Dirt in the system eventually wears out some of the components if it's not dealt with in time. Although the system is extremely robust in itself, this does not mean that you should tempt fate. If the flushing does not cure an eventual problem, then something has worn out and is not sealing properly, so a lot of the flow of LHM goes from the high pressure parts straight back into the returns, which leaves less for its intended purpose.

ECU pinout

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Finding the current suspension mode

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Failures in the Hydractive system (generally problems with sensors or their wiring) can force the suspension to be stuck in hard mode. To diagnose the fault, it would be convenient to know which mode the car is currently in. As H II systems use a 1 kHz chopper to form the current operating the valves, this leads to an interesting way to learn the current status of the suspension: this chopped current actually disturbs the radio reception. Tune your radio to a spot somewhere near 700 kHz where there is no incoming broadcast. When the solenoid valves are energized (that is, the suspension is in soft mode), you will hear a characteristic buzzing sound from the radio. As you drive around, you can observe what triggers the hard mode erroneously.

Flushing the hydraulic system

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There is a product called Hydraurinçage (sometimes also called HydraFlush, the French name is pronounced as \id-ro-ra[n]-sa'zh\ or, more precisely, using the IPA phonetic symbols: ɪdrɔrɛŋ'sɑ:ʒ). It's a special cleaning liquid manufactured by Total, orange in color; your nearest Citroën dealer is sure to have it on stock. The price is more or less the same as that of the regular LHM.

General troubleshooting

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First of all, take a peek into the LHM reservoir. If the fluid is yellowish, it is old, damp or even diluted with engine oil. If it is dirty, it was probably never ever changed. Proceed to flush the system and renew the LHM.

Hard suspension

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If you came from other Citroëns, most notably CX or DS, you might be disappointed. Citroën—to save room in the engine compartment but also for economical reasons, to share some components with Peugeots—redesigned the front suspension. The traditional layout had a triangle trailing arm (the suspension cylinders and the wheels attached at opposite corners of the triangle) while the new setup uses McPherson struts (the cylinders and the wheels attached at the same point). Later models (XMs and Xantiae) use a more sophisticated, computer controlled suspension system instead to ensure both comfort and stability. No matter what you try, the BX will remain inferior to these other Citroëns; sad but true.

Hydractive ECU problems

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The ECU operates three electrovalves to switch the suspension between hard (default) and soft modes. Thanks to the solid-state relay, this setup is much more reliable than a traditional relay could have been. The SSR also has thermal self-protection, so in case of overheating, it simply shuts down temporarily and as soon as it returns to normal operating temperatures, it resumes operation. Still, there is a possible avenue for failure: the protecting diode. While working, this diodes does its job. However, when it breaks, it will start to hinder the correct operation of the Hydractive system.

Power steering

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The XM came equipped with one of two possible power steering systems. The conventional one is practically equivalent to the BX one, with sizes of cylinders adjusted for proper assistance force. This is called DIRASS from the French Direction Asistée. The other is the self-centering, speed sensitive, fully amplified system to be found on the SM and the CX as well, called DIRAVI (Direction Appel Rasservi) or, sometimes, Varipower. This system is completely different from the conventional one, even the supporting hydraulic components are different. Its peculiarity is that it automatically centers the steering even if the car is standing still, and is less servo assisted at higher speeds and tighter curves.

Rebuilding the flow distributor

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Put the distributor unit flat on a surface, so that the side with the two conduit sockets points upwards and the pressure adjustment screw (between two plugs on the side of the unit) is to the right, the side with the three conduit sockets will point downwards then.

Rebuilding the front struts

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You will need a seal kit (there are two such kits, part number 95 650 941 only contains the main piston seals while part number 95 597 221 has all necessary seals) and a slide bearing (part number 96 004 372) to rebuild each front strut. Apart from some common tools, you will also need a 36 mm socket wrench or spanner; a 18 inch (45 cm) "Stilson" pipe wrench; a bench vice (or the pipe wrench at least); a 6 mm (¼ inch) diameter steel bar or pin punch of about 8 inches long; a propane-butane gas blowtorch (not an oxy-acetylene one) or another heat gun; and some Loctite or similar thread lock.

Rebuilding the pressure regulator

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Release the hydraulic pressure pressure and undo all the conduits leading to the pressure regulator (have a rag handy, there is going to be a lot of LHM leaking out).

Recharging or renewing?

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A sphere is only as good as the neoprene membrane that contains the pressurized gas. Occasionally, the interface between the membrane and the retaining plate inside the sphere is to blame for leakages, but usually any problem is associated with the membrane itself. Like anything that's under constant flexing, it'll break down over time.

Removing a suspension sphere

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The proper Citroën workshop manual method—in contrast to what the Haynes manual says—is to loosen the suspension sphere with a chain wrench ¼ turn while the system is still under pressure. Be very careful and absolutely sure that it is really only by a quarter of a turn. The pressure in the system is huge (the force behind the sphere can reach more than one ton, that is several thousand pounds), so opening it up with all that high pressure liquid behind it can be extremely dangerous.

Replacement spheres

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To start with, these are the spheres originally fitted... 

Spheres and comfort

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With all those various spheres and damper diameter holes, for any given Citroën model or even accross models, it is difficult to see at first how and why all these physical factors influence the suspension comfort.

Suspension Mathematics

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The mathematical model...

Various suspension systems

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The differences between the various Hydractive systems are subtle in principle but great in execution.

XP Suspension problems

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  • Noise from the front
  • Front struts
  • Trailing arm bearings