Measuring Cut-in/cut-off Pressure On Xm

[jorgy9]

Hi all, well I thought I should add my little stone in reaching 5000 messages asap, so, there you go:

I finally got hold of a Citroen kit for measuring those, but I have
only the BX official schemata as to how to do that.

Looks a very simple task, but in the BX schemata there are two versions:

-BX 14, you plug the pressure gauge in the Pressure Regulator outlet
-BX 16, 19, plug into security valve inlet.

Any ideas about what do I do on a 3lt XM with DIRAVI, and what's the
difference -is it because the BX 16 and 19 have assisted streering- ?

George

[jorgy9]

Well, I measured today the cut-in and cut-out pressures on the pressure regulator outlet, with 2 different gauges, results:

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

This means the lower one in marginally on the limit (145+-5bars), and the high quite lower than required (170+-5 bars). So the system never attains its highest pressure.

Next step is take off accumulator/pressure regulator assembly and add the required shims to bring those pressures to the required standards. Anybody knows if those shims are a service spare part on Citroen's catalogue, or I should start visiting scrapyards, or maybe they are just plain common shims of some type that I can get anywhere???

thanks
George

PS I also established that the pressure regulator leaks within acceptable levels, it lost 7 bars of pressure in 3 minutes (from 155 to 148), Citroen says if it drops more than 10 bars below the cut-in read out (ie 138 in my case), then it's too leaky.

[jorgy9]

Right, as Brian from the XM List kindly let me know, the shims I need are numbered 5412 540 (0.3mm) and 5412 595 (0.7mm) on Citroen's parts list. I'll visit my local agent to see if I can get hold of them and then do the job. The least I should get with this adjustment is a pump under less strain. Iterestingly, again on the XM List, Priit from the Nederlands wrote the following:

" 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.

Regards,
Priit "

regards
George

[noz]

Hi George,

You've really opened a very interesting can of worms here with your experiment. I have never even heard of someone doing the check on regulator pressures before although it seems perfectly logical to do it. I measure the pressure left in spheres all the time. I have all the kit but never done the pressure measurement at the regulator. Where did you finally connect the guage into the system? If its relatively easy I might try it myself at the weekend just to provide some comparative results.

Priit's comments are interesting. If I understand his english he's saying that you can only be sure that the electrovalve will open the way to the centre sphere only if the supply pressure is above 150bar. I'm going to have to study the hydraulic schematics and read the manual very closely to see if that statement is indeed true. I have never heard of it before.

Keep up the sterling work and let us know how you get on with the shims.

Cheers

noz

[jorgy9]

The theory behind is simple really: if you see how the whole hydractive set-up actually works, it's obvious that the electrovalves we speak about all the time do only half of the job of isolating the middle sphere(s) from the other two. In reality, it works indirectly: when it's open (ie with current), it allows the system's pressure to press against the hydractive valve per se and hold it open. When the electrovalve is closed (current off), it allows this system pressure to escape (return to tank), so the pressure the middle sphere contains at any moment has no counter anymore, and it self-pushes the hydractive valve to a position that isolates the middle sphere. The result of this set up is that, even if all your electronics and electircs work perfectly, if the pressure your system produces is below some point -let's say "too low" in general, it won't have what it takes to counteract the hydraulic shocks it's receiving through the middle sphere every time the car hits a bump or so. The hydractive valve will then be instantly moving or even fully "closing" (isolating the middle sphere). The hydractive block(s) receives the system's pressure through the flow distributor (for cars with one, don't know the set up of a 6+2 pump car), which comes from the pressure regulator through the security valve. So, take a 10 year old car whose various components after the PR probably leak more than they should, add a PR that anyway provides a lower range of min/max pressures because it's out of tune, and...how much pressure do you think that the hydractive block(s) actually receives at any moment? Not the required, probably.

All this makes absolute sense to me, but nobody I know really knows what this critical pressure above which the hydractive system as described above will work properly; in practice it depends on many factors, pertaining to the car and to the...bump. Priit's comment is empirical and very valuable to start with.

My motive for doing this is that I can't believe my car is so smooth when a whole axle is attacking a hump-bump or whatever, and so bad when only a single wheel meets a hole ect. I've read various comments and articles about what seems to be a weak point of the hydropneumatic syspension, ie some kind of rattle and harshness over particular formations of the road, but it can't be that bad as in my car. Moreover, I've heard various people complain about this, now labelled "a characteristic of the H2", ie harsh reaction when a single wheel hits a bump.

The test is very easy to do, you guys here do much more difficult stuff. The way I measured: on the accumulator block there are 3 pipes, one
is the inlet pipe from the pump (the thicker one, high pressure),
other next to it is the PR outlet (controlled pressure within 145+-5
to 170+-5 bars normally), and a third thick rubber pipe is the return
to the LHM reservoir. Jack the car *securelly* -I work under it-
,start engine, put it to low position, undo by 1 turn the 12mm bolt
on the accumulator body (u'll hear the pressure "escaping"), switch
off engine, undo PR outlet pipe, in its place plug your gauge set up,
start engine, tighten the 12mm bolt, immediately you'll see the
pressure jumping to the max (cut out) point on your gauge. Untighten the
12mm a bit to allow pressure to drop slowly, at some point it'll jump
again to the max and start dropping again. You can repeat this cycle
again and again and see the pressure regulator working, and that's
you really, with your cut in and cut out pressures.

For testing the PR wear, allow the presusre to stabilise (engine
running, 12mm closed), switch off engine, and observe pressure drop
for 3 mins. If it drops on excess of 10 bars below your min (cut in)
reading at the end of this period, then it's too leaky and looses too
much pressure.

You can also measure the static presusre of the accumulator sphere on place if u allow pressure to stabilise, switch off engine, untighten slightly 12mm to allow a slow pressure drop, it'll go slowly all the way down till a point where it will suddenly drop very fast: that point is the accumulator sphere pressure. Was 62 bars in mine (brand new sphere). Plus, next critical point of measuring should be the FDV really -which is known to wear and leak-. But on my car the DIRAVI performs fine, so I don't really have a reason to believe the FDV is leaking too much.

I'll keep you posted.

George

[jorgy9]

In my local Citroen dealer they said that Citroen doen'st make anymore the shims numbered 5412 540 (0.3mm) and 5412 595 (0.7mm), so, without bothering more with them, I'm having my indy open a couple of accumulator blocks they have around so I can get my shims (I need around 10 of them in total). Citroen however still make the 2 o-rings that seal the pressure regulator.
G

[jorgy9]

Right, with some lag I'm reporting my doings with suspension.

As planned I spent an hour at my indy's like 3 weeks ago and we tuned the pressure regulator to the correct pressure range. We added a number of shims to the cut-in and cut-out valves of it. The result is now spot-on, cut's in at 145 bars and cuts out at 170 (exactly what Citroen describes). Pump cuts in every 2 minutes approximately (!), Diravi steering is impecable, and car rises in 10 seconds. On that day, from the first metres of driving the car out of the rough and damaged garage's cobblestone, I felt the difference: I couldn't feel much of the surface. I was extra happy as for the first time I felt what was the Citroen comfort we speak about. I drove like that all day.

Then, next morning I decided to bleed the brakes since I thought the investigation of my suspension problem was finally over, so wouldn't touch/remove any component anymore (because air enters each time you remove a sphere or pipe).
I took the car for a drive immediately after doing it and disaster struck: suspension was again like before (hard). It still remains like this till today.

So, the pressure issue being corrected (and the result being fantastic on everything hydraulic), and having seen how comfortable the suspension can be, I now have to investigate the only two causes that I can see as possibly remaining and giving the fault:

1/electrovalve not working well -electrically.

2/electrovalve not working well -hydraulically.

Today I checked the voltage on both electrovalves. I did this by pinching the 2 yellow cables that come out from the white plug of the suspension ECU. Pin 1 is the front electrovalve positive feed, and the pin 2 is the rear. The circuit has to be "closed" (i.e. electrovalves plugged), if not the only thing you'll read is about 9 volts on both pins. With a digital voltometer set in "20 volts" and DC, the tension shown if all is OK should be between 2,5 to 3 volts on each electrovalve -when this is energised-. Indeed, the feed on both of them was good, 2.9 volts. So the feed to the electrovalves is right, and the fix I did on the diodes half year ago still stands. Presumably also, the electrovalves are working, but I'll check this easily by measuring their resistance easily on the electrovalve itself (it's the front that is problematic in, my car).

So, I am now leaning towards the senario that something hydraulic-related is wrong, since problem was fixed automatically after the presure regulator was tuned, but again automatically appeared after bleeding the brakes. I suspect the valve in the electrovalve is not fully shuting when energised, resulting in some system pressure escaping to the LHM tank and the middle sphere being constantly half-cut off (that's the feeling it exactly gives). This senario is supported by a small but constant LHM leak that has appeared on the near-side very thick metal piping that connects the middle sphere to the struts. I.e. most of the pressure has to be taken by the corner spheres and the connecting point in that pipe can't take it (strangely, the near-side of this pipe is in two pieces that connect just before the sphere, the off-side one is a single pipe from the hydractive block to the strut). How this relates to the brake bleeding? I suspect the bleeding has made dirt in the pipes move and block somewhere in the hydractive block.

How I will test this? Easy: I'll detach the return to the LHM tank of the electrovalve and plug a rubber pipe of mine on it, driven in a bottle or something. Starting the engine, if my hypothesis is true, there should be some LHM leeking from there -when it shouldn't leek at all if things were correct-.

If this is not the case, I'll measure the output pressure on the security valve (one that has 1 inlet and 3 outlets [rear brakes+suspension, front brakes, and front suspension, I think]) to make sure the full system pressure indeed arrives to the hydractive block at it's full (145 to 170 bars). It might be that some component is loosing significant pressure on the way, and that this was covered by having an amount of air in the dead-ends of the brake circuit (although I can't see how).

If nothing shows there, the only thing I can do is undo and detach to pieces the hydractive block and find if there is dirt somewhere else in it or if some valve is simply sticking or something.

The feeling the car currently gives is like the middle sphere cutting-valve is half open, i.e. adding extra damping . I am clear on that because, if I pull out the hydractive suspension fuse, I then feel how it really is when you are left with the two corner spheres (awful and undrivable, teeth-shaking vibrations). The front of my XM feels something like a hard sprung car, with the difference that it is accompanied by a kind of high-pitched vibration that would dismantle the plastic trims of the interior if driven like that for a about a year!

So, next tests and news to follow soon, as I don't want to be driving around like that -it's bad for the car-. I'm close to the bugger now!
George