Stop Light: Time to go off

Unread post by **CitroJim** » 07 Apr 2007, 20:46

This is not a problem but something I cannot quite understand. I'm sure there is a very simple explanation but it has not been forthcoming in my tiny brain.

When I first got my Activa, the Stop light went off rapidly on first start-up of the day. Too quickly for a 6+2 pump. The Accumulator was flat. Replaced. The Stop light then took considerably longer but not excessively so.

Just recently, the Stop light has been taking for ever to go off on initial start-up after an overnight and even if stopped for an hour would take a fair time to go off, particularly if the Activa Rams had tilted the car over to one side whilst stopped.

On Friday, I gave the Activa Linkages a good clean and lubricate and also cleaned and lubricated the Roll Corrector and both Height Correctors. The Roll Corrector mechanism was partially seized, allowing the Activa system to correct in one direction only. This work has made the Activa system work better by an order of magnitude but it had little immediate effect on the time the Stop light took to go off. It did stop the car leaning when at rest though.

Yesterday Xac and I replaced the rear suspension spheres on his car and mine. We wanted to change my rear Hydractive and Activa Balancing spheres but they defeated Xac's Sphere tool because they're so tight and and not very accessible. Xac's Sphere tool works really well for normal spheres though as both his and my rear suspension spheres were really tight but they came off easily with the tool. Mine defeated my webbing tool comprehensively.

Since then, my Stop light is a lot quicker going off. After an overnight I guess it now takes about 20s and even after a two or three hour stop it goes off in about 5-10s. Sometimes it beats the airbag light in going off.

So how could changing rear suspension spheres and bleeding the rear brakes possibly significantly affect how long it takes the Stop light to go out? I must add that overall, the suspension is a good bit smoother now and I'm looking forward to getting my other two spheres replaced. The car will go to my local indy Citroen specialist for those though.

Please, someone, put me out of my suspense. It is fourrowing my brow a little

AndersDK · Unread post by **AndersDK** » 07 Apr 2007, 21:02

A flat sphere is rarely a completely collapsed sphere.
This means there are some residual (but still too low) pressure to press out the hydraulic fluid on the fluid side - meaning the gas then expands to fill the sphere cavity. This only happen of course if the suspension is allowed to fully depress - i.e. parked over longish time or set to lowest.

When at start up the pump pressure must overcome the suspension weight, which means the sphere is filled up on the fluid side of the membrane, until the HC detects correct height and disrupts the fluid pressure charging.
It comes by logic that if the residual gas pressure in the sphere is low, then it compresses a lot more than a correctly inflated gas volume - for the same pressure applied (by the fluid). This is the gas constant in effect.

Thus the sphere may be completely filled with hydraulic fluid, compressing the low gas volume to almost nil.

The more volume you need filled in to replace the missing gas - the more work and time the pump needs to bring up the required pressure.
Add to this that you may have a couple of (near) flat spheres - and you get the red STOP light disco show for long periods.

OTHO if the sphere had collapsed completely with no residual pressure left at all, then there is no pressure left to press out te fluid when the system comes to rest. It then takes very little volume - and hence time - to re-pressurise the sphere after a pause. This is most commonly seen on the accumulator, but may as well happen with other spheres on the car.

jeremy · Unread post by **jeremy** » 07 Apr 2007, 21:07

With flat spheres the light will go off very quickly as no fluid is returned to the reservoir when the car stands.

Its the volume of LHM returned as the car stands that influences the pump up time. With a nearly flat sphere the diaphragm will stretch nearly to the end of the sphere and will therefor return nearly a sphereful of LHM.

Ultimately when its virtually flat it won't have the strength to return the LHM - but enough strength to soak you as you undo the sphere!

Unread post by **CitroJim** » 07 Apr 2007, 22:00

Ahh, thanks both. Brow now fully unfurrowed

An excellent explantaion that make a lot of things very clear now.

So that explains why despite appearing to loose quite an amount of LHM in sphere changining, very little was needed to replenish the reservoir. In fact I bled out more than I lost in sphere changes.

Xaccers · Unread post by **Xaccers** » 07 Apr 2007, 23:01

jeremy wrote: Ultimately when its virtually flat it won't have the strength to return the LHM - but enough strength to soak you as you undo the sphere!

Which is what happened when I took off my rear driver's sphere, was a bit of a suprise!