Mysterious fast tick times ...

Frequently asked technical questions and common modifications/improvements
User avatar
AndersDK
BXpert
BXpert
Posts: 537
Joined: Sat Jul 23, 2005 4:41 pm
Location: Denmark West

Mysterious fast tick times ...

Post by AndersDK » Sat Aug 18, 2007 1:04 pm

After a consultation with Tom, I found that it was valuable information for many owners submitting a copy of his thoughts on fast regulator/pump tick times :
tom wrote:Why is the ticktime so fast when I just changed the accumulator sphere?

Your leakage rate is too high. Many owners do not understand the reason why a small amount of leakback has a large effect on the ticktime.
This is for them.

The hydraulic system of the BX has an accumulator sphere which contains a little under 400cc of LHM when it is fully pressurised to 175 bar. At this pressure, the pump flow to it is bypassed in the pressure regulator and when the pressure falls to 145bar, the regulator sends the pump output to the sphere again.

There is always a small amount of "natural" leakage in the system which is used to lubricate all the sliding components and which is then returned to the reservoir by the mass of small rubber pipes going into it.

The figures discussed here relate to an "as new BX". A new accumulator sphere has an ideal gas pressure of 62 bar so at 62 bar, there are 400cc of gas.

The formula P1 x V1 (62 X 400 in this case) = P2 X V2 means that for every doubling of pressure, the volume of the gas halves.

At the cut in pressure of 125 bar, there are 200cc of gas and 200cc of fluid in the accumulator.

To get another 100cc into the sphere would require the pressure to double again to 250 bar!


the amount of fluid that goes in between the pump starting to replenish the sphere at 145 bar and finishing it at 175 bar is given by

24800/145 minus 24800/175
171-142 cc.

That means that the system loses only 29cc between ticks.
In an ideal BX, the tick time is greater than 1 minute.

This means that 29cc are returned to the reservoir in that time from all sources if the accumulator is sound and the ticktime is exactly a minute.

Wear and tear increases the tolerances, reducing ticktime. That is why 30 seconds is considered acceptable for an old BX.

The Doseur valve is the biggest single leaker. It is out of spec if it flows more than 3.5cc/ minute. Discounting the doseur then, the system flows 26cc per minute back to the tank.

Suppose the doseur is worn, sending back 10cc/minute.
the ticktime will be down to about 45 seconds maximum ansd at 15 cc per minute, down to about 30. That is assuming that the doseur is the only worn component.

On an old car,the pump will be getting tired and take longer to refill the accumulator, everything else will leak a little more and tired LHM will halve the tick time.

That's why a leaky doseur should be replaced with the best one you can get.
It is the most likely reason why the ticktime is too fast once you have replaced the LHM and the accumulator sphere, provided you have already checked the strut return to the reservoir. It is neither cheap nor an easy thing to replace but it nearly always makes a hell of a difference.
C U / Anders - '90red16riBreak - '91GrisDolment16meteor - Project'88red19trsBreak
dead cars : '89white 16RS - '89antrasitTRDturboEst - '90white19triBreak

User avatar
cauchoiskev
BXpert
BXpert
Posts: 282
Joined: Mon Aug 15, 2005 11:36 am
Location: France

Post by cauchoiskev » Mon Aug 20, 2007 8:42 pm

"provided you have already checked the strut return to the reservoir"

Could I add a consideration on strut returns ?

In general, if you pull off the strut return pipe at the reservoir., you will see no return whatsoever. Why ? Because the return system only works when the suspension is being used, i.e. the LHM is pumped out of the struts by the up/down movement of the car. Normally, you don't check your strut returns while you are driving...

So, if you have checked all your returns and changed your accumulator, but still have a fast tick time, suspect your struts, particularly if the car falls quickly whan parked.

User avatar
Way2go
Over 2k
Posts: 7279
Joined: Fri Jan 13, 2006 3:15 pm
Location: RCoBerkshire
x 2

Post by Way2go » Mon Aug 20, 2007 10:50 pm

cauchoiskev wrote:"provided you have already checked the strut return to the reservoir"

Could I add a consideration on strut returns ?

In general, if you pull off the strut return pipe at the reservoir., you will see no return whatsoever. Why ? Because the return system only works when the suspension is being used, i.e. the LHM is pumped out of the struts by the up/down movement of the car. Normally, you don't check your strut returns while you are driving...

So, if you have checked all your returns and changed your accumulator, but still have a fast tick time, suspect your struts, particularly if the car falls quickly whan parked.
Agreed Cauchoiskev, I bet my strut return was passing a fair amount intermittently. This would be before the strut failed big time and had LHM gushing out of a blown off return like water out of a garden hosepipe! :cry: :wink:
1991 BX19GTi Auto

User avatar
AndersDK
BXpert
BXpert
Posts: 537
Joined: Sat Jul 23, 2005 4:41 pm
Location: Denmark West

Post by AndersDK » Mon Aug 20, 2007 11:51 pm

There is no return flow in the strut rubber hoses. These hoses are there only to catch the seepage leakback from the strut.
IF - and only IF - these rubber hoses show a flow of LHM, WHEN the suspension is at normal height, then its high time overhauling the struts with new inner cylinder seals.

The working return from the struts when suspension is lowered due to any compensation, is by way of the HC which opens another path. These returns are the 2 small flexible rubber hoses (1 for rear HC, one for front HC) vulcanised on to the next largest hose on the reservoir.

http://www.rwbsmith.plus.com/citroen2/H ... aulics.pdf

Tom is not at wrong in his submission above, but old backbone sayings makes him designate the strut rubber hoses as "returns" while they really only are lubrication seepage leakbacks to the reservoir. But still they do return some LHM to the reservoir :wink:
C U / Anders - '90red16riBreak - '91GrisDolment16meteor - Project'88red19trsBreak
dead cars : '89white 16RS - '89antrasitTRDturboEst - '90white19triBreak

User avatar
cauchoiskev
BXpert
BXpert
Posts: 282
Joined: Mon Aug 15, 2005 11:36 am
Location: France

Post by cauchoiskev » Tue Aug 21, 2007 1:17 am

Yes, I did mean "leakage return", not operational return, which goes back through the height corrector.

There's a ball-bearing valve at the end of the air-blowoff pipe and one on the rubber leakage return pipe on the strut which form a pump system for strut leakage.