A beginner's guide to Xantia hydraulics.

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Old-Guy
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A beginner's guide to Xantia hydraulics.

Post by Old-Guy »

Ever since I bought our 1995 Xantia 1.9 TD Sx in April 2006, I've been trying to understand how its hydraulic system works. I've compiled the following by shamelessly cribbing nuggets of information posted on this forum by various contributors – my thanks and acknowledgements to you all.

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’ :shock:
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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Last edited by Old-Guy on 18 Dec 2008, 18:02, edited 2 times in total.
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Re: A beginner's guide to Xantia hydraulics.

Post by xantia_v6 »

Old-Guy wrote: I'm sure I've got some of this wrong, so please put me right!
I think that you have got the gist of the basic system, but to avoid confusing people you need to mention some of the important variations and changes to the design.

Early cars (93-94ish) had significantly different plumbing, with no anti-sink (hence one less sphere), and a Flow Diverter Valve for sharing the hydraulic pump between the steering and other functions.

The steering circuit (on cars with antisink) has no connection with any accumulator, the only components shared between steering and the rest of the system are the oil tank, the feed pipe to the pump and the LHM itself.

You lumped the Hydroactive 2 system, fitted to VSX, Exclusive in with the Activa system, wheras they are functionally and hydraulically separate from each other (and 90% of cars fitted with Hydroactive do not have Activa).

I am doubtful that your test for the anti-sink shpere is valid. My understanding is that in the sit-in-the-boot test, the main accumulator is being tested.
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Thanks xantia_v6

Post by Old-Guy »

Oops - I should be forever embarrassed if someone with an early car spent ages looking for an 'anti-sink' sphere! :oops:

I might just simplify the section on Activa and Hydractive as I only have experience of the basic anti-sink system, which I believe is by far the most common?

Thanks for clarifying how the steering works - I had occurred to me that the idea of front brakes and steering 'sharing' emergency pressure from the front accumulator wasn't clever - but that's what I'd gleaned from somewhere. If I'd thought more about it, I'd have known that with the engine off there's no PAS - doh! :roll:

My Xantia failed both the accumulator tests given, and had a blown n/s/r sphere. As all spheres were original (late 1995), I changed them all, flushed the system and bled the brakes. Now passes all tests!

I'll admit to a little confusion on the sit-in-the-boot test, but it seemed logical to me that it was the rear 'anti-sink' accumulator sphere providing the pressure to raise the rear. Can anyone provide a definitive answer and/or other tests for the rear 'anti-sink' accumulator sphere, please?

Guy
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Post by deian »

Hi Guy,

I pm'd you some more specific details, no expert myself, but it may help, I do have a habit of posting big blocks of info, so I didn't want to gatecrash ur project.

Good write up, although remember to do the corrections, new guys would likely read the first post of the thread than go through pages looking for the corrections.

Fair play to you.

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Post by red_dwarfers »

This is a great idea Guy!

The anti sink sphere tests do have some faults, I recently replaced my main accumulator and have done the self leveling test on the front of the car. When I sit on the slam panel the car will rise back up but will not sink back down again. This isn't due to my accumulator but I have a dicky front height corrector :lol:

Also, when I sit in the boot of my Xant when the engine is switched off, it will rise and then fall when I get out, yet I've no history of the rear accumulator ever being replaced (something I must do). I think its a case of waiting for the anti sink valve to snap shut and pressing the brake pedal, if the back of the car drops suddenly your rear accumulator needs replacing.

Another few points
- if you have an anti sink model, and one end sinks (or both!) then your anti sink value is a bit dicky.

- It would be worth checking the forum for the posts regarding moving the suspension up and down and finding faults with the height corrector linkage.

Thats all I can think of at the moment

Please correct me if needs be!

I think we need our own Wikipedia! :lol:
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Post by Xaccers »

A sinking A/S model can mean there is a LHM leak somewhere.
Dex used to sink, and that was due to the pipe on one of the rear calipers needing tightening.
Cassy sinks as her N/S/F caliper is leaking.
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Post by deian »

Xac wrote:A sinking A/S model can mean there is a LHM leak somewhere.
Dex used to sink, and that was due to the pipe on one of the rear calipers needing tightening.
Cassy sinks as her N/S/F caliper is leaking.
I assume Dex was a car that you had before? You're a bit whacky aren't you Xac.

... :idea:
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Post by Xaccers »

deian wrote:
Xac wrote:A sinking A/S model can mean there is a LHM leak somewhere.
Dex used to sink, and that was due to the pipe on one of the rear calipers needing tightening.
Cassy sinks as her N/S/F caliper is leaking.
I assume Dex was a car that you had before? You're a bit whacky aren't you Xac.

... :idea:
I've owned 4 Xantias, you kind of have to be whacky :)

Dex now lives with Jim (his 2.1TD), whom he's much better suited.
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Post by Pleiades »

Dont want to be a nit picky type thing, but ATF will not harm an LHM system in any way. Infact the Americans use it instead of LHM as LHM is not easaly available over the pond.

Although personally I wouldnt use it unless it was an emergency as it is a bit thicker than LHM, but I would rather use ATF than engine oil.

What realy needs to be stressed is that standard brake fluid is death for the hydraulics, as is water! Why?? Read below.

A little warning about water.

Quite a few years ago, whilst the BX was still a fairly modern car, we had a chap come in to our workshop with his BX blowing return pipes off the front struts, quite normall strut failure (we thought)
When the struts were removed, we noticed that the LHM was very milky looking, left to settle over lunch revealed that water was seperating out.
Well, we contacted the chap, who came in to have a look at the problem, by this time we had removed the reservoir and found it to have quite a large water to LHM mix.
We asked if he knew how it got there and he said, "yes, I put it in"
Apparently because he had a small leak, he asked his mate what to top up with as he did not have any LHM (HERE COMES THE PUNCH LINE)
His mate who he says was a sort of mechanic said "well it's a mineral system so you can top it up with mineral water"
Off he pops down to the shop and buys a few bottles of perrier and fills up his LHM reservoir.

He did say that it seemed to work fine, well it would, untill the rust started to take its effect.

As the car was only about 3 years old and far to young to scrap, it was a big bill for lots of hydraulic parts and about 6 flushes to get the water out.

Here endeth the lesson.....

Regards.
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Post by _kid_ »

Pleiades wrote: His mate who he says was a sort of mechanic said "well it's a mineral system so you can top it up with mineral water"
Off he pops down to the shop and buys a few bottles of perrier and fills up his LHM reservoir.
:lol:
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Post by myglaren »

Pleiades wrote: His mate who he says was a sort of mechanic said "well it's a mineral system so you can top it up with mineral water"
Wonderfull tale and an object lesson in dumnity©.

Should have been Vichy water, everone knows that :!: :D
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Post by deian »

Oh and just a quick note for old-guy and everyone else who may not know Pleaides...

(He or) They ARE the aftermarket Citroen Hydraulic specialists. They have a site somewhere which is http://www.citroen-hydraulics.com/.
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