The Jaguar XJ40 power hydraulic system
February 2020
Introduction
There is a lot of confusion about the operation and components of the Jaguar XJ40 power hydraulic system. Almost every Jaguar enthusiast knows that the XJ40 self-levelling system (abbreviated SLS) is very infamous for its malfunctioning and that complete removal and change for a conventional system with conventional shock absorbers without ride height levelling system is often executed and even advised by Jaguar itself. But many know that there is a connection with the XJ40 braking system, but not in what way the SLS and braking system are connected.
Well, this depends heavily on model year. On these pages I will try to explain and illustrate.
One of the reasons you might be interested is that you want to know what to do if you want to get rid of the self levelling system. If you really want to remove the SLS parts, you have to keep in mind that on early cars the power hydraulic system also serves the brake booster! Jaguar provides technical guides that describe what to remove, what to disconnect and what to plug and although I did not do this myself apart from my Daimler Double Six, it seems that most parts can stay in place.
Warning
Please keep in mind that the explanation below is based primarily on my 'desk research'. I do not have a lot of hands-on experience. Beware of errors! Please point out misunderstandings to me! The reason I took a dive into the subject is that the self-levelling system of my Daimler Double Six from 1993 was malfunctioning when I bought it.
Principles of the power hydraulic system
Let me first point out why the Jaguar XJ40 utilizes a 'power hydraulic system'.
The most conventional application is the power boost of the brake system, necessary because human force only is not enough to do (heavy) braking of a heavy luxurious car in a convenient way. Most cars use the vacuum of the engine inlet to boost brake operation, but most XJ40s use hydraulic fluid, pressurised by a hydraulic pump. When the XJ40 was introduced, Jaguar claimed that the power hydraulic system provides a 450% increase in potential boost energy over conventional vacuum systems.
The second application of the power hydraulic system is the operation of the ride levelling system, if present. Most earlier XJ40s were specified with a system that measures the height of the rear axle and, if it drops too much as a result of loading of the car, increasing the height of the rear of the car. This self-levelling system (SLS) was regarded as a pro for a luxurious car the XJ40 was meant to be. As the SLS proved to be rather unreliable, on later cars it was only specified as an option.
It is rather straightforward that increasing the ride height of the rear of the car can be done by pumping hydraulic oil in special struts. For this high pressure hydraulic operation, a power hydraulic system is necessary. The struts look familiar to conventional shock absorbers, and indeed the shock absorbing is done by a conventional shock absorber, but added to this unit is an extra chamber that, dependent on how much high pressure hydraulic oil is pumped into it, increases the length of the strut and hence the ride height of the rear of the car.
See my other pages for a description and fault finding of the self-levelling system. There is a lot of misunderstanding and pure nonsense on the web on its operation (we possibly call it fake news, nowadays).
The power hydraulic system: three generations
The power hydraulic system for the XJ40 comes in three generations. They do not differ in the way they work, but in the functions of the car they influence, i.e., ride levelling and/or braking.
1987-1989
The first generation, used from 1987 up to and including the 1989 model year (MY) to VIN 594575, there is one hydraulic pump (red in the figure below), mounted on the front of the engine, supplying high-pressure hydraulic fluid (green) to a valve block (blue), that supplies the pressure to the special rear suspension struts and the brake boost cylinder over the brake pedal.
(Figure taken from one of the Jaguar reference documents listed below; colours by me.)
So, the following components are shared by both the brake boost and ride levelling functions:
- hydraulic pump;
- hydraulic fluid reservoir;
- hydraulic fluid: Hydraulic System Mineral Oil (HSMO);
- valve block.
Please note that the power boost function of the brake system utilizes hydraulic oil, but the braking itself is done with conventional DOT4 brake fluid. Don't confuse them! In the figure above I coloured the hydraulic oil circuits green and the brake fluid circuits yellow.
You can use the LHM green oil for Citroëns, it is the good specification and apparently a lot cheaper than specific Jaguar-spec oil. I always use MPM fluids for my cars. Back in 2008, one litre cost me € 6 ex VAT. See the MPM lookup page.
This is what the three-cilinder hydraulic pump looks like. It is not large and it is mounted on the front of the engine (apart from the later V12).
1990-1992
With the introduction of model year 1990, Jaguar converted to use the Teves anti-lock braking system instead of Girling, and the complete setup was changed. The 1990 ModelYear starts with VIN 594576. This is the same moment the 3.6 litre engine was replaced by the 4.0 litre.
From 1990, the brake master cylinder is combined with the booster, pressure accumulator, brake fluid reservoir, anti-lock unit and hydraulic pump into a single unit. The pump operates electrically. So from MY 1990 on, the power brake system is completely separated from the power hydraulic system for the ride levelling, as described above for 1989 and before.
As for the ride levelling: a revised pump is used as of MY 1990, but it looks about the same. The reservoir for the Hydraulic System Mineral Oil is now also used for the power steering! Do not use power steering fluid (ATF)! The only component the ride levelling system and the power steering share, is the reservoir. So the power steering keeps using its own pump and hydraulic circuits.
1993-1994
From VIN 667829, the XJ40 was updated in a drastic way. Actually, this last generation of XJ40s share almost more with their successor X300 than with the previous XJ40!
This third generation can again be divided into two subgenerations. Model Year 1993, starting in September 1992, has a dramatically changed bodywork: approximately one third of all body panels were new! This was not only to supply bodywork strength necessary to utilize airbags, but also to "accommodate for future engine and transmission installations", as Jaguar called it. We now know what they meant: the XJ40 bodywork was changed to be able to shoehorn the new six litre V12 in. This introduction of the XJ81, as the V12 model is called, was referred to as 'Model Year 1993.25'.
For the MY 1993 power hydraulic system, not much was changed compared to the MY 1990 installation, apart from another location of the valve block. The main difference is that no longer the power steering used the same reservoir and HSMO mineral oil as the ride levelling. From MY 1993 on, it has its own reservoir, using conventional Dexron IID/E automatic transmission fluid (ATF).
I could show the Jaguar figure here, but as it is rather confusing I will leave it out.
For Model Year 1993, the braking system did not longer use a unit with all functions combined as described above. The electric pump and pressure reservoir were now moved to their own location near the right side of the bulkhead (independent on 'handedness'). Anyway, just like in the previous generation, the brake system has nothing to do anymore with the power hydraulic system, but as it had in the first generation, I thought I should include this information.
For the V12, the whole setup of ride levelling and power assisted steering remained unchanged, apart from the pumps. For the V12, the power steering pump and the ride levelling pump share the same shaft. It actually is a 'tandem pump': it is one unit, but both pump functions and both oil circuits are completely separated. Even the technical type of the pumps is different (as in the previous generation): the ride levelling pump is a three piston pump, the power steering pump is a vane pump. In the photo below the tandem pump of my Daimler Double Six in front and the V12 pump for power steering only in the back.
References
All references are scans to pdf of Jaguar handbooks and documentation, issued by the Jaguar Daimler Heritage Trust.
Power Hydraulic System, FOCUS publication number S-73, © 1992 Jaguar Cars Inc. (apparently for the North-American market)
XJ6 & Vanden Plas 3.6 technical introduction, © Jaguar Cars Inc. 1987 (apparently for the North-American market)
Sedan range update 1990, Publication number S-66, © 1989 Jaguar Cars Inc. (apparently for the North-American market)
Jaguar technical guide. XJ6 model range power hydraulic system, publication part no JJM 10 15 05/01
XJ6 Sedan Range Model Year Update 1993, publication number S-78, © 1992 Jaguar Cars Inc. (apparently for the North-American market)
Dealer training 1993my introduction
Jaguar technical guide. 93.25 model year 6.0 litre saloon introduction, Jaguar Cars Limited, December 1992
Kind regards,
Rens Swart