Sinking rear end problem

[noz]

Hi all,

It must be the season for hydraulic faults. I have an issue with my daily runabout 2.5TD. When I press the footbrake, about 10 seconds or so later the back end slumps down to the bumpstops before coming back up as normal. On the return to normal height it simply takes the requisite time for the pump to push fluid back into the two(three including the hydractive sphere)rear spheres. This depends on the engine revs at the time. If it happens at tickover the recovery can take 2 min or so. If it happens whilst cruising it can take 10-15seconds.

When the rear end sinks it sinks very quickly i.e. far quicker than it would if it was being lowered via the height corrector. It is particularly bad when I'm towing because the additional weight on the towbar makes the fall very dramatic with the rear mudflaps making a horrible noise on the road, very disconcerting.

Thinking through the logic of what must be happening we can state the following:

• It is not a burst pipe becuse there's no loss of fluid from the system.
• The height correctors work properly because they control the height of the car as they should.
• Therefore, the fluid which was in the two (possibly 3 ) wheel spheres must be permitted to leave the circuit very quickly. This would either be back to the reservoir or to another part of the circuit at a lower pressure.
• Say, for example, the hydractive sphere was completely empty for some reason but the two wheel spheres were at their normal level (ignore how that could be possible for the moment). Then, by pressing the brake, somehow the hydractive sphere opened up and the fluid contained in the two wheel spheres was shared equally between all three spheres. If this was the case then each sphere containing 2/3 the amount of fluid as normal would not allow the rear end to slump all the way down to the bump stops. It would come to rest at a lower position but not all the way down.
• Whilst in cruise, if the rear end slumps then immediately before that the suspension was not in hard mode. In which case, the hydractive sphere must have been in circuit already, ruling out the scenario just described above.
• When the rear end slumps it does so very quickly indicating that the fluid is being permitted to leave the two wheel spheres and the hydractive sphere virtually unimpeded. It is unlikely that any of the return pipes leading back to the reservoir could conduct that quantity of fluid in such a short time and almost certainly could not sustain the temporary spike in pressure which would be needed to make it possible. So, by deduction, the fluid must still be contained in steel/CuNi HP piping but travelling from the suspension spheres to another part of the system.

That brings me to the first level of speculation on the way to discovering the fault. The one element of the system which volunteers itself is the anti-sink sphere which in conjunction with the sc/mac (anti-sink) valve presents a link with the press of the brake pedal in conjunction with the fall in the rear end a short time later. If the anti-sink sphere was somehow empty but isolated from the three suspension spheres and was then suddenly connected then the fluid from the suspension spheres would rush into the anti-sink sphere. The A/S sphere juslt like the regulator has no orifice with which to slow down the transfer of fluid.

If this is indeed the correct explanation then the next task is to figure out how the anti-sink can empty and why pressing the brake should open the anti-sink valve.

Any thoughts appreciated.

to be continued......................

[lez]

dropping when foot on pedal or raising when foot on pedal, brake valve, dropping is a leak to the return, lifting is an internal leak from front to rear feeds inside the valve.

slow to rise at tickover, main pump is leaky / worn pistons internally.

[citroenxm]

The "Anti Sink" Sphere, is just NOT That! Its not a sink prevention sphere but a Rear Brake Reserve Sphere...

I dont think the sinking is related to the Sphere being Flat Noz. I reckon this because I have a Anti Sink Xantia with 240k on the clock and a rather original "Anti Sink Sphere" The car still does not sink...

Does the car sink after the key is switched off?? Does it sink while the electrovalve is active then stop once it clicks off??

Having said all that, it sounds like its loosing pressure between the brake valve and the rear brakes... Strange one. Ive probably made things and thoughts worse now...



Paul

[kenhall1202]

Hi Noz,

I have had a very similar problem on my anti-sink 2.1TD for years. Luckily it only occurs on very rare occasions, usually randomly only within the first few miles of a journey and it is not related to brake operation.
The fault has persisted through at least two complete changes of all four rear spheres so I do not think the spheres themselves are to blame. A prolonged flush with Total Hydraurincage made no difference. I have not investigated further but my next thought was to try swapping the rear hydractive block. As you say the symptoms suggest a rapid transfer of LHM between unequal pressure spheres being the reason for the rapid rear end drop.

You might find this thread started by Alphadave relevant - he found out that the rapid rear end drop problem was related to a damaged pressure regulator!

http://club-xm.com/forum/index.php?showtop...rear+suspension

Ken

[nightmare]

Hi NOZ,

Just seen this on another thread.

"Hi shin

I have just come across a "feature" of Hydractive 2 rear pipework with anti-sink that I did not know about.
When trying to reuse some recovered 3.5mm pipe that had a good flare and nut I cut it to length only to find a long thin stiff steel wire running along the centre of the pipe.
I put it down to a manufacturing fault with the pipe and used another spare bit.
That was some while ago.
Recently while making up pipes I found another pipe with the same centre wire present.
So I made some enquiries and was advised that these special pipes are part of the anti sink system to reduce flow rates on valve opening.
It seems that if you replace these pipes with standard ones the rear end of a XM fitted with anti-sink will occasionally drop like a stone onto the rear bump stops.

This may be nothing to do with your problem but I though it was worth recording somewhere on the site and your post seem to be a good place to put it.

Researching this is still ongoing so as I find out more I will add the information.

John"


HTH

[robertmnorton]

Hi Noz, i'm you must have seen this before, but as reminder.......
robertm

[Jan-hendrik]

Hey Noz,

I think your problem might be due to a malfunctioning of the rear stiffness regulator block. Hydraulic pressure required for proper operation is not maintained because of a weak pump, or / rather a faulty pressure regulator.
Without a set up to test pressures the easy thing is to replace the PR and see what happens.
Make sure your main accumulator and SMAC sphere are correctly pressurized.
But who am I to tell you what to do . . .

This post has been edited by Jan-hendrik on October 26, 2012 03:29 pm

[noz]

Hi Robert,
Thanks very much for that file. I hadn't seen it before. It looks like it describes my problem reasonably well. The main difference is that on mine the rear sinks very rapidly whilst driving and then slowly makes its way back up to the normal height, the speed of which is determined by the engine revs (and hence pump speed) at the time. The diagnosis file suggests the sinking takes place when parked. Anyway, its an easy test so very worthwhile doing. It'll probably be over the weekend before I get a chance to try it. I'll let you know how I get on. Thanks.
p.s. It would be useful if you could put that file in the "Self-Help" section which acts as a kind of repository for all sorts of useful files.

Hi Jan,
thanks for the suggestions. I have recently changed the regulator with one kindly supplied by citroenxm. I took the opportunity to recharge the regulator sphere when I had it off the car. I have also had the SMAC sphere of and recharged that too. Unfortunately, none of these efforts so far have resulted in any improvement. No help offered is ever refused, thanks. After all, the fact that I'm asking for help means that I don't know how to fix this one.

Hi Nightmare,
I did read that post but until you pointed it out its significance never really dawned on me. I have indeed changed a few pipes as they have burst. I'll now go back and check to see if one of the changed pipes matches the one with the internal wire. If I'm lucky, I might even still have the pipe(s) I took off.


It dawned on me that one other possibility might even be that it is self-inflicted. I have changed a few pipes for cupro-nickel in the last couple of years. On one occasion I did two pipes at the same time. I have been wondering what would happen if I'd got two of them mixed up. In order to determine that for sure I'd need to trace each pipe against an accurate piping diagramme. However, I don't have a piping diagramme for an XM with Hydractive 2, Anti-Sink, ABS and Dirass. I have just about every other permutation but not that particular one. I'd be obliged if anyone has such a diagramme they could share with me.


cheers

noz

[Jan-hendrik]

QUOTE

Hi Jan,
thanks for the suggestions. I have recently changed the regulator with one kindly supplied by citroenxm. I took the opportunity to recharge the regulator sphere when I had it off the car. I have also had the SMAC sphere of and recharged that too. Unfortunately, none of these efforts so far have resulted in any improvement. No help offered is ever refused, thanks. After all, the fact that I'm asking for help means that I don't know how to fix this one.

Hi Noz,

Alphadave, as reported in this topic, found out his problems with the rear suspension were due to a faulty pressure regulator. Citroen call this a circuit breaker. After I replaced it on my ES9J4 my rear sus problems disappeared. The problems thought to be low hydraulic system pressure. BUT, I did not experience sudden drop, but stiffening of the rear suspension as the rear stiffness regulator lacked sufficient pressure for proper operation.
An expert on a different forum suggested that low pressure of circuit breaker sphere (main accumulator sphere) and SMAC sphere are suspect.

[robertmnorton]

Hi Noz, have trawled through all i have, but as you nothing with that combination. Having just recently replaced all 4 rear over axle pipes - one by one - from memory, the 3 x 3.5mm pipes go to 1: the h/c, from the priority valve 2: the anti-sink valve from the priority valve. 3: the left side 3 way union from the brake control valve. The 4.5mm from the ABS block to the r/h 4 way union, but is the only 4.5mm so not likely you confused this one. Again this is from memory so if any doubts let me know and i'll do a physical check, it'll only take a half hour or so (i deleted the exhaust transverse box when i had the s/s system tailored in so access is easy).
robertrm