New accumulator

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mnde
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New accumulator

Post by mnde » Wed Feb 28, 2007 4:01 pm

As my BX now has a "tick-time" of 5-6 seconds, I'm going to get hold of a new accumulator sphere. I remember an XM sphere was recommended on here as it's a higher pressure... and I've tried using the search engine to try and find what I want to know... Is it the XM accumulator or the front centre sphere I need?

Cheers,

Mark.
1982 Citroen GSA Spécial Estate
1997 Renault Mégane RT 1.4e
1991 Citroen BX16 TGS Meteor - gone to Scotland

jeremy
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Post by jeremy » Wed Feb 28, 2007 4:03 pm

What's wrong with the proper one - they don't give much trouble - and what are you going to gain from the wrong one?

Just because you can do something doesn't mean its a good idea.

tim leech

Post by tim leech » Wed Feb 28, 2007 4:57 pm

You can a correct spec on from Euro car parts for about £10 I think?

M

Post by M » Wed Feb 28, 2007 5:52 pm

The reason for using an XM accy sphere is they opperate at a higher pressure / capacity and once up to pressure have more in reserve than the BX one. They make no difference to the ride and can help an aging pump.

The Xantia also uses the same rating accy sphere as the XM - so Citroen upgraded them for a reason.

I replaced mine with an XM one - £26 from ECP with a 3 year warranty - no problems at all.

http://www.bxclub.co.uk/diy/spheres1/

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Post by jeremy » Wed Feb 28, 2007 9:46 pm

Hang on a mo - something doesn't make sense here.

The ultimate system pressure is governed by the FDV. The regulator itself controls the pressure in the accumulator.

The accumulator holds high pressure LHM - which stretches the diaphragm and compresses the gas. Surely the higher the gas pressure the less the diaphragm stretches - and so the less the capacity of high pressure LHM in the sphere?

Where have I gone wrong?

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Vanny
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Post by Vanny » Thu Mar 01, 2007 8:23 am

operating pressure on the Xantia and XM is higher, i'm assuming that marty is refering to this rather than the sphere pressure? Operating and gas pressure still have no effect on the overall 'net' capacity of the sphere.

Ultimate system pressure is NOT governed by the FDV but (and quite obviously) by the pressure regulator of which the accumulator is a crucial part. Remembering also that the other purpose of the accumulator is to stop the pump pulsing the system, and that the XM and Xantia pumps are 7 port not 5 it stands to reason that they need a slightly larger capacity to absorb the pulses.


I have a Xantia accumulator on mine, but then i have a xantia engine. pump, hydraulic lines and no FDV

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Post by jeremy » Thu Mar 01, 2007 9:40 am

Vanny - Under the hex socket bolt on the FDV is a ball bearing, plunger and a spring. The bolt is sealed with an 'O' ring and can be screwed in and out - which has the effect of increasing and decreasing the pressure on the ball. This is in the circuit before the pressure regulator and is there to regulate the pressure supplied to the PAS and prevent amongst other things the ram splitting if the steering is held on full lock. Remove this ball and the pressure will drop.

I agree that the pressure for the brakes and suspension is controlled by the regulator and I think that normally the valve in the FDV remains shut - but when mine was incorrectly assembled (trying to seal on the plunger rather than the ball) the pressure in the rest of the system would drop and the light come on!

Under normal running the supply of LHM from the regulator/accumulator is not very large as its only topping up the suspension in response to inputs from the height correctors which is small due to the time delays built in and the very small amount of fluid moved when the brakes are applied. It is therefore able to function as a reserve supply with considerable capacity - unless of course there are major leaks from components that require topping up.

The later Xantia pumps are of course split with separate supplies for suspension and steering - and if I recall correctly only 2 cylinders supply the suspension and its for this reason that the accumulator is vital on these cars. the majority of the pumps output is for the steering which doesn not have an accumulator in the circuit.

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Vanny
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Post by Vanny » Thu Mar 01, 2007 4:26 pm

I see what your saying about the order of the FDV and PR, but the PR is the final restriction in the system and this is what determines when the accumulator is being filled or not (end hence the click), while the FDV may have control over where the pressure it is supplied goes (brakes, suspension and then steering) it is a pressure switch not a pressure valve, and adjustment will only determine at what pressure the FDV will begin to supply the steering. Still don't see how that makes any difference to the action of the accumulator, even if the switch in the FDV is locked out?

The steering rack is dependant on FLOW rather than pressure as the ram is open ended and would require more pressure than the hydraulic pump can create in order to make it split! It takes blocking the return passages and a bloody fat bastard tugging on the wheel to manage to snap it!!