Evolution: Xantia Hydractive Suspension Regulation - Info

[white exec]

C'est la vie...

[CitroJim]

It would explain the lack of soft suspension.

It surely would Pleased you discovered the reason! Had you not it would have troubled you for the rest of time; I know it would me

[Mandrake]

Whoops!

[blaueboot]

Time to raise up this thread again.

As a short introduction I will say that I'm a long time 'lurker' here, once in a while coming along when looking for technical answers.
And this time I do have a question big enough to break the silence.

A few years ago I was fed up with my Hydractive problems on my Xm (s2/2000) and stumbled upon this topic, got myself the mentioned hydractive regulators and did the modification to the car.
Very pleased with the result a driven ever since. But...

At this moment I'm struggling with quite some issue. And to be honest, I can't really recall when it really started, but I really started to notice after slightly over-inflating the hydractive-spheres. But I'm starting to suspect this issue started after doing the modification.

Under hard braking the tail of the car is rising. Rising so much that it slightly destabilises the ride of the car during higher speeds. Exactly the moment you do not wish for something like that to occur.
Now I know the first thought will be: "Bleed the brake-system, it has air in it."
Well.., I've been through that. Litres of LHM have left the brake callipers on multiple occasions. And there has been a bit of air and improvement, especially noticeable by the rear suspension becoming a bit more rigid / less floaty.
But the rear coming up remains...

Then there is a second curiosity.
When the tail has risen up this much, it doesn't go down immediately. Well, it does a little bit. But stops after a few millimetres with a little thud, until after a few seconds the height corrector starts to lower the height to normal ride height.
For me it feels as if after the tail has jumped up the anti-sinking valve closes and prevents the car from falling back to ride height. Then again, this implies that the rear going up is not only emptying the suspension spheres to fill the cylinders, but the cylinders suck in quite an amount of LHM from the main supply.
This behaviour was also happening after taking a speedbump fast enough to make the suspension really go through it's range of movement. Since I've properly bled the system from air, the suspension movement isn't excessive enough anymore to trigger the issue.

So I've got two questions. And I really hope the guys who did this modification as well are still around here to answer them.
1. Very obvious, but does anyone else who applied this modification experience(d) this issue as well. Or do they have a car that stays nicely level during hard braking.
2. Any suggestions / ideas what else could cause these issues? As:
- The modified connection of hydraulic tubing is done correctly and checked multiple times for errors.
- The main pressure regulator gives the right pressures, has been cleaned and doesn't leak internally.
- Same goes for the safety valve, brake valve/dosseur, height correctors, anti-sink valves and the electro-valves of the hydractive regulators).
- The hydraulic pump is cleaned and leakage free. And I'm quite sure it isn't sucking in air somehow.
- All brake callipers are clean and confirmed to work.
- All spheres are checked and maintained to be on their specified pressures. Exceptions are for the hydractive spheres. Those are over-sized and over-inflated (front 500cc/80bar, rear 450cc/57bar. Originally 450cc/75bar, 400cc/50bar) This modification as cause of the issues is ruled out as the issues still persist when disconnecting the hydractive ECU and thus isolating these spheres from the system.

The only components I didn't disassemble and clean thoroughly are the hydractive regulator blocks themselves. Then again, they are working perfectly. So I don't expect that any dirt in there (if there is) could cause this issue.

Looking at the schematic drawings of the new hydractive regulator, I'm wondering if there is an possibility of the suspension cylinders sucking in an amount of LHM and not supplying pressure to the rear brakes, making the rear come up. And somehow because of this for a moment forces the hydractive block to 'firm' state until the height corrector starts to do it's job.
I realise now I did not yet tried taking a speed bump with the raised rear to feel if it's only raised or 'firm' as well...

If my issue is also experienced by other users of this modification, I am toying with the idea to place back the deleted hydraulic feed to hydractive regulator. Connecting it to the orifices that have the bleed nipples on the new hydractive regulator blocks. Though this will become a bit of a hassle as the orrifices are not exactly on the same spot and same angle.

Well... That became a very a very extensive 'first post'. Hope you all could bear along my train of thoughts and maybe come up with some suggestions.
If I make any progress on my own I will definitely report it here.

[white exec]

Hello, blueb, and welcome!

As you say, a good few of us have done the revised regulator mod to our XMs now.
So far as I know, what you describe has not been experienced by others, and not on ours (a 2.5) here.

Rear end climbing up (and especially after the boot was loaded up, with heavy shopping) and then taking a while to get back down to normal height, sometimes with a downward thud, was something we had here with ours before the mod was done, but never afterwards. It always runs at correct (normal) height.

You've been very thorough in checking the system out, but how about the rear height corrector? Could it, or its linkage, be sticking? Maybe worth removing and fitting a re-furb kit to it.

You don't say that Soft/Firm is misbehaving, so it looks like the EV/solenoid and the new regulators are working ok.
I don't think your larger and slightly higher pressure centre spheres would cause any problem, but just provide some extra softness. I presume they are both without dampers in them.

(There is still a limit to how fast you can take a speed ramp. While the revised regulators prevent the system flipping into Firm by hydraulic shock/impulse and allow the system to remain in Soft, there can be a thud as a lightly-loaded rear end comes off a 'step' or ramp. Taken at moderate speed, this won't be felt anything like as much.)

Air in the rear suspension is (as you say) usually the cause of wandering rear end height. Perhaps look at the height corrector?

[xantia_v6]

The soft springing on a hydractive Xantia/XM means that it takes very little force to make the back end rise and fall, and when braking, the braking torque on the front axle tends to make the back end rise.

The rise of the back end is neutralised by the torque of the back brakes on the trailing arms.

The suspension ECU switches to firm mode under moderate braking, which stabilises the rear height by sharply increasing the spring rate. The point at which this switching occurs is a compromise between comfort and rear end stability.
Any increase in the pressure of the rear hydractive sphere will upset the compromise of the suspension switching point.

Assuming that you have reverted to stock sphere pressures, any excessive tendancy of the rear to rise under braking must be due to a front/rear brake imbalance. This is usually due to air in the rear brake circuit, but could be a fault in the brake proportioning.

I have had a car which exhibited rear end rising because the rear pads were replaced with "competition" parts with a lower friction coefficient.

Some of the symptoms you describe don't appear to be related to braking, but I would get the brake balance sorted first.

[white exec]

I had forgotten about the action of the rear brake trailing arms on ride height.
Rear brakes on XM do very little work in normal driving, and are quite modest in pad and disc size. The bulk of braking is done by the fronts.
As a result, there is a tendency for the rear brake calipers and pads to seize, and to become almost inoperative, except for very hard braking. Would be worth removing the pads, cleaning everything up, and making sure there is free movement.
The light braking at the rear is sometimes shown up by rear brake squeal, which can be got rid of by a few really hard applications of the brakes.

[Mandrake]

A few thoughts.

If the rear electrovalve is faulty, such as the internal diode or a dry joint inside the plastic casing which are the two failure modes I've experienced personally, when the ECU switches from hard->soft the car will actually only go into soft mode for half a second (during the full 12v DC burst) and then drop out back to the hard mode after half a second when the ECU is still commanding soft. (during the 4 volt PWM period)

This unexpectedly being in hard mode most of the time with short half second bursts of soft mode can lead to the height jumping or falling suddenly due to the pressure not getting much time to equalise.

This sort of electrovalve failure can be progressive and very intermittent and therefore very difficult to diagnose, although to be fair it seems to affect front valves more than rear ones. (Engine heat ?)

If you haven't done the diode mod yet I would definitely do it as diode failure is common in these, however be aware that I also had one valve that developed an intermittent (heat related!) high resistance internal joint that was not fixed by the diode mod - this valve would behave when the car was first started and during a manual bounce test, but after driving for 30+ minutes the valve presumably would heat up and then start misbehaving. In the end I replaced the coil sleeve from another valve that had a good coil assembly but a problem with the needle valve to make a good valve out of the two.

When you replaced the hydractive regulator block did you use the electrovalve which came with the replacement block or did you use your original one?

If you used the electrovalve which came with the block but you still have your old one maybe swap your original electrovalve back in in case the new electrovalve is faulty ? (just the small cylinder shaped solenoid with the connector - which can be removed and is compatible between the two block types)

As pointed out well by xantia_v6 there is a delicate balance between front and rear brakes. If the rear brakes are weak the rear will rise under braking, if the front brakes are weak the opposite will happen and the rear will crouch down under braking.

I would give the rear brakes a service first if you haven't - check the condition of the pads and discs etc, check for misalignment of the pads due to corrosion under the caliper etc.. all the usual stuff.

More often than not the issue is air in the rear brake lines - and I know you say you've bled them, however I had a similar problem on my Xantia V6 over the last year - the rear would hit the bump stops under braking, I would bleed the rear brakes, get a small amount of air out, then they would be OK but within a couple of weeks of use the problem would be back, and this would keep happening multiple times.

I eventually stumbled across the real cause of the problem while doing other work - on the Xantia V6 the hydraulic pump has a short "joiner" section of hose on the inlet side with two hose clamps. Unknown to me both of these hose clamps had rusted and split, (out of sight) but were still sitting in place embedded in the rubber of the hose, albeit not applying any clamping force.

So this joint was effectively working as a push fit joint. It never leaked any oil or showed any sign of trouble but it was letting air get sucked into the hydraulic system and for various reasons any air that goes into the hydraulic system will tend to accumulate in the rear brake circuits and get stuck there.

I replaced the two hose clamps, did another bleed of the rear brakes and they've been fine since - I've done a couple of thousand miles since then as my other car was off the road and the rear brakes have been spot on.

So any air leaks on the input side of the hydraulic pump (including right back to the joint on the tank) will cause air to accumulate in the rear brake circuit and mess up the rear brakes.

Another possible source of "air" is a failing sphere in the rear suspension. It's not uncommon for a failed (diaphragm popped) anti-sink sphere to slowly bubble nitrogen out into the rear brake/suspension circuit as well - the end result is much the same - no matter how often you bleed the rear brakes they will keep accumulating "air" in them affecting the brake performance. In that case the faulty sphere(s) need to be found and replaced.

The anti-sink sphere is the most likely culprit because it's the only sphere that can fail with no directly observable symptoms on the performance of the car, apart from weird issues like this.

Regarding anti-sink valves themselves - I think there is pretty close to zero chance that they could cause the symptoms described.

[blaueboot]

Thanks everyone for your extensive replies.

First of all I’ll say I forgot to add to the list that I have modified the hydractive ECU with the diodes
The currents sent out by the VN05N’s have been monitored with a scope and are confirmed to work properly over an extended period.
Besides that.., because of the higher sphere pressures there is a better noticeable feeling between the firm and soft state. So I’m quite confident the hydractive system works as it should.

The rear height corrector operators very smoothly and responds very well when being operated. I even considered if it might be to responsive. But it has about 3 seconds delay before it starts correcting after input.
Still.., it might be worth looking into that thing again if the raised rear after a speed bump problem remains.

So xantia_v6 brings up the brake force balancing.
I’ve already thought a bit about it, but for some reason I always assumed it should be something hydraulic. So the brake-valve, which is the brake force balancer, has been replaced with a cleaned, leak-free one, and comes out of a car I know. So I’m sure the brake balancer itself is okay.
From there I was trying to figure out if there is another way of loosing hydraulic pressure in the rear axle part as this could mean there is less brake-force applied on the rear brakes than intended
Never did I really consider that the brake-discs being a bit greasy, I just give a good wipe with some paper when spilling oil, could affect the braking to the point it would become a problem.
Then on top of that there is difference between quality brake pads and cheap ones in how much braking force they can generate.
I never replaced any brake-pads on this car, so I have no clue what’s on there at this moment. But it wouldn’t surprise me if the rear has some cheap, slightly greased brakes.

I made sure I ordered brake-pads for daily driving. So no ‘high performance’ / ’sports’ stuff which first need to be hot before it starts to perform.
Tomorrow they will be delivered and I can put them on the car right away (after applying brake cleaner to the discs).

By the way.., it’s good to know the rear is not supposed to ‘squat’ a little during breaking. If I end up with that I know now that I’ll have to replace the front pads for the same type as used in the rear.

Regarding the faulty sphere dissipating NO2 in the system or the pump sucking air in somehow.
Isn’t the system self-bleeding with exception for the brake lines? So since the brake lines are dead ends I do not really see how air could travel up in them.
And in a sense, even healthy spheres are constantly dissipating a small amount of NO2 in the system. That would mean bleeding the brakes is an annually job (At least, when using the IFHS spheres. Those fall below specs in 2 years Not sure how long originals keep within spec)

Okay, so for now I hope the changing of brake-pads is going to help. If not I’ll have to start looking in the hydraulic components again.
Will let you guys know the results tomorrow.

[white exec]

Thanks for posting the extra detail.
Let us know how you get on with the rear brakes.
Check the pistons move properly, and that the pads have movement inntye calipers.
Some copper grease on the back of the pads will deaden squeal, and a slight smear on the metal pad edges will help pad movement.
For good measure, bleed the rear brakes while you have the wheels off.

You're right about the system being largely self-bleeding, but the brake calipers being "dead legs". For the rest of the system, trapped air/gas will find its way back to the reservoir, and disappear (and go unseen).

[Mandrake]

So xantia_v6 brings up the brake force balancing.
I’ve already thought a bit about it, but for some reason I always assumed it should be something hydraulic. So the brake-valve, which is the brake force balancer, has been replaced with a cleaned, leak-free one, and comes out of a car I know. So I’m sure the brake balancer itself is okay.

The brake-valve is not really a "brake balancer" in the traditional sense as it only has a single shuttle valve which controls both front and rear on different ports. (And thus can't independently control their braking ratios)

AFAIK it's just a force feedback system which combines the effect of front and rear pressure at a fixed ratio - so when you apply force to the valve in one direction it opens both front and rear flow paths and the combined pressure from both front and rear brakes pushes back on the same shuttle valve to restore the position of the valve. (Thus making it almost completely pressure sensitive not travel sensitive)

Instead, the maximum rear braking pressure is capped by the available rear suspension pressure. So the front/rear braking force ratio is the same with a heavily loaded car or a lightly loaded car - up until you brake hard enough that the rear brakes reach the capped suspension pressure and from that point onwards as you brake harder only the front brakes receive additional pressure.

The anti-dive of the rear suspension is nothing hydraulic it's purely passive - the front brake torque lifts the rear suspension and the rear brake torque pulls it down - as long as they are in balance as designed the car will stay level. If you brake really hard the rear will still rise a little because the front/rear brake pressure ratio is not constant once you hit the maximum rear brake pressure for the load in the rear of the car.

From there I was trying to figure out if there is another way of loosing hydraulic pressure in the rear axle part as this could mean there is less brake-force applied on the rear brakes than intended
Never did I really consider that the brake-discs being a bit greasy, I just give a good wipe with some paper when spilling oil, could affect the braking to the point it would become a problem.
Then on top of that there is difference between quality brake pads and cheap ones in how much braking force they can generate.
I never replaced any brake-pads on this car, so I have no clue what’s on there at this moment. But it wouldn’t surprise me if the rear has some cheap, slightly greased brakes.

Anything mechanical that causes the rear brakes to work less effectively will affect the front/rear balance and cause the rear to rise more. Grease on the discs/pads, corrosion, hard pad material, disc runout, (warped or not seated properly) pad frames seizing in the pad carrier etc...

Regarding the faulty sphere dissipating NO2 in the system or the pump sucking air in somehow.
Isn’t the system self-bleeding with exception for the brake lines? So since the brake lines are dead ends I do not really see how air could travel up in them.

The hydraulic system apart from the brake lines is self bleeding - but it self bleeds at a fairly low rate. If there is a lot of air being sucked in it will still accumulate in the lines as it won't bleed as fast as additional air is sucked in. It can have quite a big effect on ride quality too and will tend to cause a crashy ride.

Brake lines are dead ends yes, but that doesn't mean oil doesn't flow in and out of them when you press and release the brake. The inside diameter of the pipes is extremely small so even the small amount of pad movement (half a millimetre or so) results in the oil in those small pipes travelling a significant distance. Once aerated oil goes through the brake valve into the rear brake line it has the chance for bubbles to get trapped (by buoyancy) in the various up and down sections of pipe as it runs to the back of the car.

This happens over time but it definitely happens. On my car air was building up in the rear brake lines to the point where the rear brakes weren't working properly within a couple of months just from a slight air leak on the hydraulic pump input.

And in a sense, even healthy spheres are constantly dissipating a small amount of NO2 in the system. That would mean bleeding the brakes is an annually job (At least, when using the IFHS spheres. Those fall below specs in 2 years Not sure how long originals keep within spec)

The nitrogen leakage of a healthy sphere is infinitesimally small, especially the multi-layer spheres that can last 10-20 years, totally different to residual nitrogen actually passing through a crack in a split diaphragm. Orders of magnitude difference in leakage rate.

[xantia_v6]

So xantia_v6 brings up the brake force balancing.
I’ve already thought a bit about it, but for some reason I always assumed it should be something hydraulic. So the brake-valve, which is the brake force balancer, has been replaced with a cleaned, leak-free one, and comes out of a car I know. So I’m sure the brake balancer itself is okay.

The brake-valve is not really a "brake balancer" in the traditional sense as it only has a single shuttle valve which controls both front and rear on different ports. (And thus can't independently control their braking ratios)

AFAIK it's just a force feedback system which combines the effect of front and rear pressure at a fixed ratio - so when you apply force to the valve in one direction it opens both front and rear flow paths and the combined pressure from both front and rear brakes pushes back on the same shuttle valve to restore the position of the valve. (Thus making it almost completely pressure sensitive not travel sensitive)

Instead, the maximum rear braking pressure is capped by the available rear suspension pressure. So the front/rear braking force ratio is the same with a heavily loaded car or a lightly loaded car - up until you brake hard enough that the rear brakes reach the capped suspension pressure and from that point onwards as you brake harder only the front brakes receive additional pressure.

I am sure that the statement above is wrong, the Xantia/XM brake valve is a proportioning valve where the actual (not just the peak) rear brake pressure is proportional to the suspension pressure.

Having driven a vehicle (not Citroen) with a faulty brake proportioning valve, I am sure that a proportioning valve limiting only the peak pressure would be a very bad idea.

looking at the diagram from https://www.citroen-klub.si/CitDocs/XM/doseur.pdf



and the diagram that is visible through most of the video:


The shuttle (4) that activates the rear brakes has the rear suspension pressure acting on it so as to turn on the rear brakes, but this is counteracted by the compensator spring (12) which is turning the rear brakes off. This balance between the spring and hydraulic pressure provides the proportioning effect.

But more practically, if you have non-standard rear corner spheres fitted, that will also cause instability of the rear height under braking, as the design of the system relies on switching to firm mode with a high spring rate and damping when the brakes are operated. I have seen that the rear height is unstable with wrong spheres fitted. I have driven Xantia V6s with either non-hydractive spheres or estate spheres fitted and both exhibited height instability.

[Mandrake]

Are you sure that is the brake control valve for a Xantia Hatchback ?

If you look at the valve in the engine bay it is very short and squat similar in size to a height corrector (at least the visible part protruding in the engine bay) and does not look like the one pictured above - the one above may be a version of the valve for Estates ?

For at least some older models of Citroen the Hatchback/Sedan versions of the car don't use the same brake control valve as the Estates.

[white exec]

Parts show the same doseur for Xan I and II, Saloon and Estate - 4860 11.

[blaueboot]

How the brake valve exactly works is still beyond my ability to comprehend. Even though I've watched the Citroën Hydractive Training multiple times, at the moment the explain the compensator part my mind just blanks out. What I do do know for sure is that the weight on the back of the car determines the pressure in the spheres and thus determines the max available pressure for the rear brakes.
I think that also means that when the rear for some reason starts to rise during braking the available pressure for the rear brakes drops. I have no clue though if the compensator in the brake valve is there to, well.., 'compensate' for this.

Anywayy.... Thanks a lot guys for thinking along with me and doing some valuable suggestions. The car is behaving well and ready just in time for holiday travel.

The culprit was an accumulation of three things.
- The first was air in the brake system, being sucked in by the pump at a slow rate. This was not the main problem, but masked the underlying problems being;
- The rear brake pads / discs. I've spent about 2 hours of cleaning with brake cleaner before the wiping papers didn't show dirt anymore. The pads themselves went to the bin as they were of some unknown brand. And I can't blame the former owner for choosing those. Before my reasoning would also choose cheap pads as the rear breaks are rarely really used. So why expensive parts. Well, thanks to FCF I know now.
- The rear height corrector. Even after the new clean brakes and another bleeding of them, the rear still came up a bit, although not consistently all the time. It was a lot slower now and it needed a bit aggressive or long slow braking for it, but still it rose sometimes. But hitting the brakes at low speed would even drag the rear down and make it 'pop' up.
By know I also started to notice the rear changing height a lot when accelerating/decelerating. Like I mentioned before, as if it is too smooth.
And then I remembered, the corrector on the rear not being from a Xm! It's from an older type of car as the damper-valve is still serviceable contrary to the plastic ones found in the correctors on the Xm.
Now I don't know yet if this older design also has different behavioural properties, but after exchanging it for a spare cleaned one which originates from a Xm the car finally behaves as it should.

Only under real hard braking (hitting ABS) the rear will rise few millimetres. This I can blame on the Hydractive sphere and enlarged damper orifices found in the Hydractive Regulator, as when the suspension is in Sports/Firm state the car will remain level, no matter how hard I hit the brakes.

So again.., thank you all very much. Very happy with the results/ride right now!