Anti-Sink System - CITROEN DS series Technical Manual

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The Citroën Guide

Anti-sink system

Many contemporary Citroëns—including both reg-
ular hydropneumatic and Hydractive Xantiae and
XMs—have an anti-sink system (SC/MAC) fitted,
with the sole apparent purpose of tranquilizing
the average driver by keeping the car from lower-
ing when not used. The system does not interfere
with the normal functioning while in use. It at-
tempts to minimize leaks inside the system by hav-
ing only one element that can leak, the anti-sink
valve itself.
The layout is rather simple: there is an anti-sink valve fitted
for each axle, between the height corrector and the suspen-
sion struts (or the hydraulic control block on Hydractive sys-
tems). The valves operate on the pressure differences in the
system, without any electrical control: when there is signifi-
cant pressure in their control circuit, they keep their work cir-
cuit constantly open (in fact, they are analogous to a simple
electrical relay).
Under normal circumstances, the high pressure pump
supplies the pressure regulator and the main accumulator
with fluid. The output from these two feeds the whole sys-
tem with high pressure, going through the security valve
which keeps the brake circuit constantly under pressure, for
obvious reasons of security. If there is enough pressure in
the system, the security valve feeds the rest of the suspen-
sion via the anti-sink valves and the height correctors.
This pressure coming from the security valve appears in
the control circuit of the anti-sink valves. When the car
runs, the valves are constantly open, connecting the height
correctors to the rest of the suspension and brake subsys-
tems—everything functions exactly as in cars not equipped
with this anti-sink system. Even when the engine is turned
off, the valves remain open as long as the feed from the ac-
cumulator remains at a higher pressure than that of the sus-
HYDROPNEUMATIC
REAR
ONLY
anti-sink
sphere
rear
brakes
Suspension: Anti-sink system
security
valve
height
corrector
reservoir
pension. But as soon as the leakage in the struts, height reg-
ulators and the brake valve reduces the pressure in the main
accumulator below the suspension pressure, the closing
anti-sink valves isolate the suspension struts from the rest
of the system (see page 29 for a description of the Hydr-
active II control block). It is usually the front valve that
closes first as the front of an unladen car is much heavier
due to the engine and gearbox. Compared to a non-anti-
sink car, the leakage is quite drastically reduced. For in-
stance, a standard XM with its suspension in prime condi-
tion takes about 20-30 hours to sink completely, while with
the anti-sink system this would take as much as ten days.
The rear anti-sink valve is connected slightly differently:
in addition to feeding the rear suspension and the brake cir-
cuit, as usual, it connects to an additional anti-sink sphere
as well. The function of this sphere is to maintain pressure
in the braking circuit. As the brake valve is the most leaky el-
ement, it could exhaust the pressure between the piston
and the plunger while the remaining pressure behind the
piston (provided the high pressure and the front suspen-
sion circuits do not leak that much) stays rather high. In this
case the anti-sink valve might open again in error—this ad-
ditional sphere ensures that this will not happen.
The anti-sink system maintains the car height by counter-
acting the internal leakage of the various suspension ele-
ment that would make the pressure escape back to the res-
ervoir. Elements that are in constant motion—height cor-
rectors, for instance—leak past their seals on purpose to lu-
bricate themselves; brake valves, on the other side, start to
leak once they are worn. The anti-sink valves—which move
very rarely, need no intensive lubrication, thus are manufac-
tured with very close tolerances and hardly leak them-
selves—isolate all the struts from the rest of the system to
prevent any possible leakage to reduce the pressure in the
struts, allowing the car to sink.
HYDRACTIVE
anti-sink
valve
(closed)
REAR
ONLY
rear
brakes
security
anti-sink
valve
valve
(open)
height
corrector
reservoir
anti-sink
sphere
32
3 4
5 4
control
block