As Prepared by Bruce Elsegood, Citroën Car Club of NSW, Technical Guru and all round good bloke. (Also known to many far and wide as Father Goose) This text was used as part of Bruce's presentation at Cit-In 2000 at Jindabyne.
You can freely reprint this article, but you must give credit to Bruce and the Citroën Car Club of NSW.
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Recently some members have asked me to explain how the high pressure hydraulic system works on their Citroëns. A look at the latest manuals from Haynes reveals both the Xantia and the XM have only four pages on the hydraulics. This is to explain how the system works and how to maintain it.
I can now see why these members are worried. Many have owned "normal" cars in the past. When they buy a Citroën the thing that concerns them most is the high pressure hydraulic system that makes the car work.
A look at earlier books like the DS book from Autobooks, and
the GS, CX and BX workshop manuals produced by Haynes in the past shows lots
more detail. Like about 20 pages each!
Does this mean that modern hydraulic systems need less understanding and maintenance? I don't think so. In fact I think they are more complex, particularly when they are allied to computer controls such as in the XM,Xantia VSX and Activa.
Citroën hydraulics started as "a load leveler" on the rear of the 1953 6cylinder Traction Avant we now call the Big 6H. In 1955 Citroën introduced the DS using their high pressure hydraulic system to operate the suspension, the clutch and gear-change, the newly developed front disc brake system as well as the power steering.
The fluid used in these cars was called LHS2 and is similar in many ways to modern brake fluid. This fluid was used until September 1966, in European and Australian cars, and until December 1969 in American models.
LHM (and its variants) is the replacement fluid for LHS2 and applies from the dates above. This fluid is less aggressive on metal parts, boils at a higher temperature and lubricates better. I will only deal with LHM system cars today but LHS2 cars use similar parts which work in a similar manner.
The typical hydraulic system consists of 8 main parts, these are:
1. A reservoir
2. The high pressure pump
3. A pressure regulator and flow divider (where fitted)
4. Pressure reserve system (accumulators)
5. The pressure warning / distribution / security valve
6. Power steering system
7. Brakes
8. Suspension system
PIC OF TYPICAL HYDRAULIC SYSTEM
To look at each one in turn, let's start with the reservoir. The main job of the reservoir is to store the hydraulic fluid. The reservoir also allows the oil to cool and give up any air. As well, any dirt, rubber or metal particles that may be in the LHM will sink to the bottom and lodge in the sump.
It is very important that the LHM is clean so there is usually a strainer on the LHM return and on the hydraulic pump pickup tube.
PIC OF CX RESERVOIR - also DS and Xantia Plastic types
LHM oil is very thin because it is designed to get between the finely machined parts of the valves. These valves do not use any seals. Sealing is achieved by making the clearance between the valve and the body just big enough for an oil film, the clearance is usually between one and three microns. A micron is 1/1000th of a millimeter.
An additional filter, on the return hose to the reservoir, while not essential is a good idea on cars with over 40,000 km. This is to trap the sludge and metal that circulates with the LHM. Such a filter is available from automatic transmission re-builders and should be changed every time the LHM is changed. One unit is the Magnafine In line filter
The next unit is the high pressure pump
The crankshaft or camshaft usually drives hydraulic pumps. Often by a belt or shaft. These pumps are positive displacement type pumps. This means the pump displaces a set amount of oil each revolution or cycle. Please note that the stated displacements are approximate only. There are generally three kinds of pump used:
Cars such as early ID, GS and non power steer CX can use a single piston pump because their high pressure oil volume requirements are relatively low.
Now let's look at the pressure regulator;
Positive displacement pumps discharge a set volume of oil on each revolution. It then follows that if the pump can meet the requirements of the system at an idle speed of say 1000 rpm then it will produce five times that amount at 5000rpm. This is where the pressure regulator comes in.
The pressure regulator is a series of spring loaded valves which open and close in turn to keep the system pressure between 14,500 and 17,000 Kpa or 145 to 170 bar.
The pump produces about 5 to 6 litres/min at 3000rpm. An accumulator and often an auxiliary accumulator is also required to compensate for any short fall of high pressure oil.
On Big 6, GS, DS and CX cars this was the only regulation of pump output.
Starting with the BX, a lower pressure power steering unit was introduced. This unit is simpler in design than CX or DS power steering units. This system uses a high flow and low pressure principle. The approximate oil consumption figures for this system are up to 5 litres per minute and 7000Kpa.
This steering system is quite different from the DS and CX systems which have about twice the pressure and are able to meet the flow rate at highway speeds. Because of the different power steering oil requirements on BX and some XMs, Citroën uses a flow divider. This device directs the pump output when steering assistance is required. The flow divider is fitted between the high pressure pump and the pressure regulator. The flow divider allows most of the LHM to go to the pressure regulator when the car is running straight, or supplies a high flow rate to the power steering when it is required. The remaining high pressure LHM passes to the pressure regulator where it is regulated as before. It is then sent to the brakes and suspension.
Xantia, and late XMs use a 6+2 piston rotary pump. On cars like Big 6, most D series, most GSs, CXs and BXs you can release the system pressure and make the car sink to low position by selecting low on the height selection lever and opening the pressure bleeder. In the mid 1970's, one of the owner complaints was that it was irritating waiting for the car to rise, this was particularly so with single piston pump GS cars or CX's with worn steering units.
The answer to this was for Citroën to fit a primitive anti sinking valve to the front suspension circuit of late GS and GSA cars, using a one way valve that could be manually released. Because these cars had no power steering the car rose quickly as the pump only had to fill the rear suspension as well as top up the brake accumulator. With the introduction of the more efficient 5-piston pump and the new steering in the BX, "rise time" was not a problem. In the late 1980s, however when the XM was introduced with Hydractive suspension, and later still when Xantias with Hydractive were introduced, "slow rising" again became a problem due to more spheres and valves. Citroën's answer was a new type 6+2 rotary piston pump of a completely new design which appeared in December 1993. The power steering is fed from six pistons and the high pressure circuit from the other two.
There is also an anti sinking circuit to stop the car going lower than normal ride height. This works well as the suspension and brake circuits need only to be topped up with pressure when the car is started. Note If you have an XM or Xantia and the hydraulic system needs to be de-pressurised to do any work, a new procedure is required. The car must have its weight on its wheels so it should be on a raised platform or ramps. Start and idle the engine, leave the pressure regulator bleed screw tight, set the height selector to low position, wait for the car to sink, then open the pressure regulator bleed screw about two full turns. The car should now be de-pressurised. Check this in the normal way (see if the spheres are loose).
The next thing to look at is the reserve of pressure system; A reserve of high pressure oil is kept in the system's accumulators. An accumulator is a means of storing incompressible high pressure oil to be fed into the system progressively and smoothly. This prevents hydraulic shock loading or hammering at the pump, ensures an adequate supply of high pressure liquid at times of high demand, compensates for internal leakage and avoids the need for the pump to operate under constant load. There is always one accumulator on the end of the pressure regulator and there may be one or more others to give a reserve to the brakes and/or suspension.
Up until the late 1960's accumulators and other suspension spheres were machined up on a lathe. This was rather labour intensive and the metal was only mild steel. After manufacture, the diaphragm was fitted and the sphere gassed-up. Over a period, the mild steel fatigued due to pressure fluctuations in the hydraulic system then the sphere would split near the screw joint in the middle. To overcome this problem and to make more spheres, the one piece pressed metal or "throw away" sphere was introduced (although DS cars came with machined spheres until 1975). The pressing process also "work hardens" the steel to make it stronger. As the picture shows, modern spheres are swagedor pressed in stages from a flat piece of 5mm steel plate. The flexible diaphragm and end plug assembly are inserted and the spheres are welded closed.
After this the spheres are charged with nitrogen gas.
"Throw away" spheres can of course be re-gassed, just like machined spheres, provided they aren't completely out of gas (flat). If a sphere is allowed to go completely flat, machined spheres usually need a new diaphragm and the one piece units are replaced.
OK. Now that the reservoir, pump, pressure regulator, flow divider and accumulators have been explained, let's look at the remainder of the system.
First is the low system pressure warning ;
From the pressure regulator the high pressure oil is sent to a low pressure warning and distribution valve. This valve, is known by names such as "priority," "security," "safety" or "low pressure valve". Its purpose is to warn the driver of low system pressure, usually by way of a light and / or buzzer, and also to ensure that, if a problem arises, the high pressure systems lose their pressure in the following order;
Suspension, steering and then brakes last.
To move the oil in the hydraulic system, a series of pipes are used. Steel pipes may be 3.5, 4.5 or 6.35 mm external diameter. They are made of mild steel sheet which is copper coated then rolled to double thickness then heated to give a bond like brazing. After this the pipe is galvanised. The pipe nuts are then fitted and the special sealing olives are formed on the ends. After market pipes supplied by Pleiades are made from a nickel copper alloy.
Flexible pipes are made of plastic or oil resistant rubber. These pipes are used on low pressure feed and return runs. It is interesting to note that most Citroëns have about five times their body length in steel pipe, and another five times their length in flexible pipes. For example the 1977 CX 2400 C-matic has a total of 52 metres of piping.
Next is the power steering;
As you might have noticed, the power steering rack and
pinion systems are different between DS, CX and later cars. There are three
quite different kinds of power steering systems used on Citroën cars, however
they all work in a similar manner. In all cases, high pressure oil is made
available to the control valve which directs this oil to the steering rack ram
when the driver turns the steering wheel. The high pressure oil then applies
pressure to the steering rack ram to assist the driver to turn the roadwheels
and so steer the car.
The braking system in your Citroën will be four wheel disc brakes. (Unless it is a DS which has drums on the rear). These are true "power brakes" where the system runs at 17,000 kpa and all you do is just open the brake valve, which is just a sophisticated tap, attached to the foot pedal.
Brake system pressure then acts on the pistons in the calipers and/or wheel cylinders to apply the brakes. To minimise brake lockup, Citroën uses a variety of measures. The DS has a brake valve that modulates pressure to the rear brakes using the height corrector position to tell the brake valve how much pressure to deliver to the rear brakes.
Most GS CX and BX cars use this principle as well, but use a different brake valve and a variety of extra valves to limit maximum fluid pressure to the rear brakes. Some CX, BX 16 valve, XM and Xantia cars also have an ABS or Anti-lock brake system.
Finally we come to the suspension system -
While the suspension system in your Citroën looks quite complex, it is really very simple. High pressure oil supplied by the pump and stored in the accumulators as previously described is fed to the height corrector via the low pressure warning device and anti sinking valves when fitted.
PIC OF HEIGHT CORRECTOR & LINKAGE
The height corrector is really just another tap that is linked to the suspension by means of a rod attached to the anti sway or anti roll bar.
PIC OF VARIOUS SUSPENSION CYLINDERS
As the car moves up or down, the height corrector either admits high pressure oil to, or releases high pressure oil from the suspension units. This oil then pushes against the sphere diaphragm at one end and the suspension unit piston at the other. This causes the car to rise or fall.
Acknowledgments
To prepare this article, I have taken information from Autobooks and Haynes manuals as well as various other sources including Archie Gordon-Graham who was the technical trainer with Franzcars and Joe Petane who is currently the technical trainer with Ateco. In addition I have used other material I have obtained on Citroën hydraulics over the last decade or so.
I hope this helps you in your understanding of these
fascinating systems.
