Air bubbles in the LHM gives harsh ride?

steelcityuk · Unread post by **steelcityuk** » 25 Dec 2006, 20:23

I wondered if others had seen this before off the Yahoo XM groups -

http://f1.grp.yahoofs.com/v1/wC2QRRCQEQ ... ng_ecu.rar

Steve.

steelcityuk · Unread post by **steelcityuk** » 25 Dec 2006, 22:40

After trawling through loads of postings on XM forums, there's the suggestion that the hydraulic pressure needs to be high for the HA block valves to work, this could explain why the ride in my Xantia has been good with long regulator cycles and harsh with short cycles. Mine cycles at between 5 and 10 seconds even with a new sphere.

Steve.

RStacey · Unread post by **RStacey** » 26 Dec 2006, 18:29

I have often questioned the ride on my HA2 xantia which is mostly ok, but sometimes appalling especially within the first 2 miles after sitting overnight - I have followed the numerous threads on this subject, and have followed all the advice and done most of the checks, without finding anything not working as it should. I have however noticed repeatable changes in the hydraulic tank fluid level - if I check the level after a drive the indicator sits 3mm above the upper limit - if I lower the car to its stops and raise it to maximum again, the level will go down to spot on the upper limit - I can then raise and lower the car as many times as I like but always end up spot on the maximum level when the suspension is fully up. As far as I can see the hydraulics should be in exactly the same state and therefore the level should not be differant - has anybody seen this or have an explanation for it? - or is the differance in level the amount of air I'm expelling from the system when I first drop to suspension - If thats the case the suggestion in this thread that air can cause a harsh ride on an HA2 might be what I am seeing, and tank level may be a indicator - Am I barking up the wrong tree?

Regards

Richard

steelcityuk · Unread post by **steelcityuk** » 26 Dec 2006, 18:51

I could only put it down to the fact that after normal driving the suspension maybe slightly higher or lower than 'normal'. That could account for the small difference.

After further looking at XM threads I found this -

>>

With following pressure ranges your suspension
actually is very harsh, as centre spheres are opened
securely at ca. 150Bar:

cut-in: 138-140 bars
cut-out: 154-155 bars

I had once similar situation with pressure regulator -
it kept 130-150 Bar range, which was the reason of my
hard suspension. O-ring sealings you can order from
CIT dealer, those are cheap.

>>

This could be the cause of my problems.

Steve.

RStacey · Unread post by **RStacey** » 26 Dec 2006, 19:01

I wasn't checking the level at normal driving height - I always check it with the suspension right up as its meant to be done

Regards

Richard

steelcityuk · Unread post by **steelcityuk** » 26 Dec 2006, 19:06

Oh right sorry.

Steve.

Mandrake · Unread post by **Mandrake** » 26 Dec 2006, 19:16

Hmm,

Interesting idea about the main system pressure being too low. (sometimes)

It is something I had considered, but hadn't really explored.

By the way the cut in/out pressures quoted in that message are incorrect - they're 145 bars for cut in and 170 bars for cut out. (Or are they quoting the pressure they measured ? It's hard to tell without the full message context)

The way the shuttle valve in the Hydractive block works is that one end of the valve has the suspension pressure from the centre sphere pressing on it, and the other end of the valve has the pressure supplied from the electrovalve pressing on it - which should be either maximum system pressure of 145-170 bars, or zero.

As long as the pressure from the electrovalve is either zero or maximum the shuttle valve is kept securely at one end or the other. (Hard or soft mode) But if the pressure in the on mode (soft) was a lot less than maximum the valve could potentially move in respose to suspension bumps, which I suspect would indeed cause ride harshness.

(The reason being as you hit a bump the suspension begins to move softly and then suddenly the shuttle valve is pushed at least part way through its movement and the centre sphere is abruptly cut off mid bump)

For this to happen though, not only would the system pressure need to be a lot lower than normal, but the oil would have to be able to backflow from the shuttle valve, through the electrovalve and back up the main HP input supply line - and I'm not sure if there is a one way valve further upstream to prevent that. (I don't think so... I'll have to study the diagrams I have and see if I can find one)

If we assume for the moment that the functioning of the Hydractive unit is sensitive to low supply pressure, in theory inserting a small (ball type) one way valve in the high pressure supply line where it goes into the electrovalve should prevent even lower than normal supply pressure from affecting the ride - because it would be impossible for oil to flow back towards the supply in response to a bump (it has nowhere to go due to the one way valve) and therefore the shuttle valve remains locked in place - problem solved.

Does anyone know of a small one way spring loaded ball valve used anywhere else in a Citroen hydraulic system which could be adapted for the job ? (It would need to connect to a standard height corrector sized seal type hydraulic union where it goes into the Hydractive block)

By the way, I HAVE noticed that sometimes the brake pedal is quite a bit easier than normal to press to the floor - which is usually a sign of low supply pressure, but on the rare occasions that I have measured the main supply pressure it has been fine.

(The pedal has also been fine at the time I've measured it mind you)

Normally the pressure regulators are extremely reliable (at least in the sense that they don't give up and die) but I suppose its possible they could develop a fault where the regulated pressure varies around a bit at random (or is simply too high or too low) - and on a non-Hydractive model if the maximum pressure was 20 bars or so too low it would have no effect on the operation of the suspension at all - except if you were trying to carry maximum load in the car, then you might notice something...and you might notice the front brakes were not quite as sensitive as they should be...

Could this be related to the imfamous reseating of the ball valve in the regulator I wonder ? (In other words could that ball valve leaking be the actual cause of low/varying pressure from the pressure regulator....)

PS can you send me a link to that original XM message thread so I can read the whole lot ? Thanks.

Regards,
Simon

steelcityuk · Unread post by **steelcityuk** » 26 Dec 2006, 20:16

I link what I can but I've read hundreds of messages in the last couple of days.

Yes the pressures quoted were from a duff regulator. A pressure guage in line with the output of the regulator would be a good diagnostic tool.

I'll find out some links. In the meantime this is from another thread -

>>
Having followed over the years various threads on this topic I would
say that the two most important findings have been overheating of the
MOSFETS in the ECU in which the evidence seems overwhelmingly to point
to shot diodes on the coils, in my experience if you run the diagnostics
on the box you will get an error code indicating a problem with the
solenoid valve, if the valve resistance is good then the diode is the
most likely culprit. The second important item is system pressure, this
needs to be at or very close to 160 bar, if this pressure is low then
even though the EV's may be electricaly driven open, pressure pulses in
the suspension caused by normal operation can shut them, usually just at
the time when you really need them open, this is more a dynamic thing
and will not apply in most cases when the car is stationary. Based upon
my own experience early cars with an FDV may have problems getting to
this level if both the FDV and the brake valve are a bit "leaky".

>>

It does fit with my situation.

Steve.

steelcityuk · Unread post by **steelcityuk** » 26 Dec 2006, 20:19

First links -

http://autos.groups.yahoo.com/group/XM-L/message/45531

http://autos.groups.yahoo.com/group/XM-L/message/52485

Steve.

steelcityuk · Unread post by **steelcityuk** » 26 Dec 2006, 20:21

http://autos.groups.yahoo.com/group/XM-L/message/45576

Mandrake · Unread post by **Mandrake** » 26 Dec 2006, 20:24

Yes I've had the problem with the diodes too - I had one electrovalve with an intermitant diode and it caused random sticking in hard mode, and sometimes even cycling between hard and soft. I replaced the coil sleeve on the electrovalve (where the diode is located) and as an extra measure I also fitted a diode (for both front and back) in the wiring loom.

Interesting to see the comment about 160 bars - especially given that the cut-in pressure is only 145 bars, which means as you drive the pressure is gradually falling from 170 to 145 (depending on how much you use the brakes and a few other factors) then being topped up to 170 again.

Surely they thought of this when they designed the system ? On the other hand if it was getting down as low as 130 or less due to a fault I can definately see problems occuring in the functioning of the hydractive system...

Another thing that occurs to me is what effect would air in the line to the electrovalve have ? If there was enough air in that line perhaps it would prevent the shuttle valve being locked securely in place ?

Unlike most other lines which are flushed when the suspension is raised and lowered, that high pressure feed line to the electrovalve is not - it only consumes a tiny squirt of oil each time the valve switches on and off, so it could potentially take hundreds of on/off cycles of the electrovalve to purge that supply line of air...(it's quite a long pipe, travelling up around the side of the engine bay for the front one, and who knows how far from the rear one to the nearest junction)

Regards,
Simon

steelcityuk · Unread post by **steelcityuk** » 26 Dec 2006, 20:30

Hi Simon,

The link at the top should offer a save file to location, just tested it. You could try save target as.

Hmm. Maybe tolerances and age are what makes Hydractives fail as they get older. There's quite a lot of talk that the regulators get 'soft' as they get older, it's assumed it's down to fatigue of the spring. One member tested his and shimmed to bring it back into line. I suppose that if the valves get leakier and the regulator goes soft then it's a double whammy for the HA system and a bonus for the dealers.

Steve.

steelcityuk · Unread post by **steelcityuk** » 26 Dec 2006, 20:36

More links -

http://autos.groups.yahoo.com/group/XM-L/message/57311

http://autos.groups.yahoo.com/group/XM-L/message/57296

http://autos.groups.yahoo.com/group/XM-L/message/56115

It would be interesting to know if Bernie has had the pressure checked on his Xantia after having all the replacement parts fitted.

I need to trace the source of my leak!

Steve.

steelcityuk · Unread post by **steelcityuk** » 26 Dec 2006, 20:44

Is the security valve a one way design? Could that do the job you have in mind if you blank one end off?

Steve.

Mandrake · Unread post by **Mandrake** » 26 Dec 2006, 20:51

No the security valve isn't suitable. It's a pressure cutoff switch rather than a one way valve - if the pressure is below about 80-100 bars the output that goes to the suspension is cut off.... (leaving only the brakes supplied, for emergency braking)

Also its massive and wouldn't fit in the area needed

Thanks for the links, interesting reading, its got me looking along a different path now...

Regards,
Simon