HYDRAULIC SYSTEM BASICS
1. Brakes, power-assisted steering, and suspension are all powered from a common hydraulic system - this operates at much higher pressures than the brake and power-steering hydraulics in 'ordinary' cars. The system uses a special mineral-based hydraulic fluid, Liquide Hydraulique Minerale - LHM for short. Ordinary brake fluids and water are death to a Xantia’s hydraulic system. In an emergency, ATF or engine oil can safely be used but the hydraulic system must be thoroughly flushed and refill with LHM as soon as possible.
2. The steering rack is similar to that in a conventional car with PAS.
3. The brakes on Xantias, unlike conventional cars, are powered rather than power-assisted. The brake pedal operates a valve (the doseur valve) that progressively admits very high-pressure hydraulic fluid into the brake system while providing some feel. The actual pedal movement is quite small. No hydraulic power equals NO brakes – not just loss of servo-assistance - apart from the admittedly powerful hand-brake.
4. Cars with the basic suspension have a total of 6 spheres:
- 1 for each wheel’s suspension,
- Rear ('Anti sink') accumulator,
- Main (front) accumulator.
5. Spheres are internally divided in two by a neoprene diaphragm that separates dry Nitrogen gas under pressure in the sealed ‘blind’ end from the other half, which is open to the hydraulic system.
6. The gas in ‘corner’ spheres takes the place of the springs in conventional cars. The hydraulic fluid in circuit between each strut (front) or ram (rear) transfers the load from suspension to gas spring. By varying the amount of fluid in each circuit, the system controls the ride height and provides the automatic self-levelling.
7. Various different hydraulic pumps are fitted to Xantias but none of them have sufficient capacity to meet the instantaneous demands. To meet momentary high demands, absorb shock loads and provide an emergency reserve for the brakes, the system has two accumulator spheres.
8. The Front (Master) Accumulator provides the pressure reserve for front brakes and front suspension.
9. The Rear (‘Anti-sink sphere’) accumulator provides the pressure reserve for rear brakes and rear suspension. It’s ‘anti-sink’ function is limited to providing a pressure reserve to help stop the back of the car sinking under load when the engine isn’t running and to ensure a prompt and positive closure of the rear anti-sink valve.
10. The front and rear accumulators are 'safety-critical' because they provide emergency hydraulic pressure for the brakes in the event of engine, belt or pump failure.
11. Early (93-94ish) 'non-anti-sink' Xantias have different plumbing and no anti-sink valve or sphere. My understanding is that they have a hydraulic system very similar to the BX.
12. Cars with Activa and Hydractive suspension have additional suspension sensors, suspension ECU, electrically-operated valves and extra spheres to provide more sophisticated suspension. My aim is to cover the majority of cars with
HOW IT SHOULD BEHAVE
Compared with a conventional car, a Xantia with its hydraulic system in good order will have:
- An amazingly supple ride that smothers corrugated and broken road surfaces – most potholes and the like should be heard more than felt,
- Yet corner as well as most modern saloon/estate cars.
- Suspension that is completely self-levelling regardless of load, but does take about 30 seconds to adjust when starting up.
- Suspension that can be raised to negotiate rough ground or floods/fords.
- Light, powerful 4-wheel disc brakes – many drivers new to a Xantia brake too sharply,
- A really effective handbrake (it operates on the FRONT discs).
SIMPLE DIAGNOSTICS
If there are a number of problems, symptoms can be very confusing.
Front Accumulator
The tick rate on the relief valve (with car warmed up, stationary and engine idling) should be at least 30 secs, double that for a hydraulic system in top condition. Anything less than 30s and the (front) accumulator needs replacing.
Rear Accumulator (aka Anti-sink Sphere)
Run the car for a couple of minutes at normal height, turn off the engine and sit in the boot. The back of the car should sink then rise again after 30 seconds or so. If the car doesn't rise, it could be either the rear height corrector needs lubricating (or the link replacing/re-fitting), or the rear accumulator (Anti-sink) sphere needs replacing. To check which it is: start the engine, wait for the suspension to settle and repeat the test. If the back doesn't rise - it's the height corrector.
Suspension Spheres on non-Activa cars
Bounce each corner of the car in turn, that corner should sink and rebound like soft conventional suspension. If it's hard and un-damped, (or doesn't move at all!) that sphere has 'blown'. The rears seem to fail earlier than front ones. As with anything to do with the brakes and suspension, suspension spheres must only be replaced as pairs (front or rear).
Momentary Brake ‘Failure’
If the brakes 'hesitate' (nothing happens for a fraction of a second after you press the pedal), assuming you haven’t just moved off with the dash STOP still light on, the brakes need bleeding before you have an entirely preventable collision!
The brakes always need bleeding if any of the spheres is changed; but otherwise, ‘air’ in the brakes lines is likely to be nitrogen gas from a failing sphere. 'Air' in rear offside brakes is likely to be down to a failing/failed anti-sink sphere.
Xantia hydraulic systems are no more complicated or difficult to work on than the equivalent in ordinary cars - just a bit different. Most mechanics are literally frightened silly by hydro-pneumatic Citroens. The experienced and very competent mechanic in our workshop being one: he happily provides me with advice and help for anything mechanical (he's worked on loads of Berlingos, Partners etc.) but at the mention of "hydraulics" or the sight of a can of LHM he vanishes into the tea-room!
For comprehensive practical overhaul instructions, please refer to Jim’s sticky “Spheres, Hydraflush and Brake Bleeding”
Last Edit 18/12/08
Guy
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