The Jaguar XJ40 ride levelling system (SLS) on a 1993 XJ40 [page 1]
september 2016
Text to be revised - January 2017
The Jaguar XJ40 ride levelling system (SLS) on a 1993 XJ40: introduction and operation (this page)
– Components of the XJ40 SLS system on a 1993 V12
– Typical failures of the ride levelling system
Introduction
When the Jaguar XJ40 was introduced in 1986, most models were equipped with the ride levelling system or self levelling system, well-known as SLS. It is a hydraulic system with a sensor on the right rear lower wishbone. As soon as the height of the rear wheels stays below a certain level, the hydraulic pump is activated. Oil is pumped to reservoirs in both rear struts and the car rises, till the level is high enough. In this way the position of the car is always optimal, independent of load, for example by passengers, luggage and petrol. This also has the advantage that the carefully designed rear suspension always operates in its optimal layout, leading to the optimal combination of the famous comfort and roadholding of the Jaguar XJ40.
However, the SLS system is notorious for failing in several ways. This not only leads to many complaints and theories of XJ40 owners all over the world wide web, but also to all kind of descriptions of how the system works and what exactly fails, including all kind of nonsense, I have to say.
On this page I will describe how the system works and in what way it often fails. Later on I will add information on how to repair it and on my plans to restore the original SLS system on my Double Six.
Operation of the ride levelling system (SLS)
Later to be inserted here, partly from the texts below. About the separate operation of the shock absorber and the ride levelling, about the measurement of the ride height, the low-frequency feedback loop and 20 second threshold and the 40 seconds test cycle on start on the latest model. Etc.
The SLS system on my 1993 Daimler Double Six
When I bought my 1993 Daimler Double Six in Switzerland in 2008, is was in very good condition and had done only 70,000 km. However, the previous owner made clear that the rear shock absorbers had to be renewed. On the long way home to The Netherlands, I experienced that this was very true.
My Double Six was still equipped with the original ride levelling system. By searching the web for information, I found out that is was absolutely advised to remove the SLS system from the car and replace it by a conventional set of shock absorbers. In fact, Jaguar got so many complaints about failing SLS systems that it was an official recommendation to remove the SLS system and replace it by conventional shocks.
However, I wanted to understand how the system worked and what part failed. It soon became clear to me that even Jaguar professionals and web pages disseminate nonsense about this (we could call this fake news 
). This statement found on the web, for example, is not right:
The typical failure mode is where the rear end of the car 'hunts' for the correct ride height, but doesn't find it, and the rear of the car ends up bouncing up and down continually.
I can imagine that if you have struts with the levelling system in it, you could easily think that if the car 'bounces' this will be caused by a failing self levelling system because bouncing has to do with the height of the car, albeit in a high-frequent way. However, bouncing is caused by completely worn-out shock absorbers and this has nothing to do with the ride levelling system, although they are in the same struts.
From Jaguar documentation it became clear to me how the system works. I will write on it later on. But let me now show what a failing system looks like.
A failing 1993 SLS system with test cycle
When I had my 1993 Daimler Double Six car for several weeks, I made three films showing the behaviour of the car. (These are filmed with a 2003 Nikon digital camera, so the film option is rather primitive and there is no sound.)
The latest generation of the ride levelling system (SLS) of the Jaguar XJ40 has a test cycle, that raises the car 40 seconds after the engine was started. It is meant to prevent the valves in the hydraulic valve block from sticking, but can also be used as an indication whether the system still works. In this film I started the engine and then the camera. It became clear that the SLS system worked flawlessly.
However, at the same time the rear shock absorbers were completely worn out, as the following film demonstrates. It is dangerous to drive with a car like this! If you drive over a bump, the rear wheels loose contact with the road and in particular in a bend you loose control over the direction the car goes.
For comparison, I also pushed the front of the car. The shock absorbers function as they should. The movement of the car is damped to zero after the car went up after being pushed down.
In this way I concluded back in 2008 that the SLS was still perfectly working, but that the struts should be renewed because the 'shock absorber department' in them was shot. I first installed (with a mate) an other pair of rather new struts, but although the shocks were sligtly better they behaved differently left and right. In the end I had the whole system replaced by the original new Jaguar replacement set with conventional shock absorbers. The tandem pump (more on this later) was also replaced by a used steering pump because it leaked.
However, it stuck me that the car sits rather low and if loaded it hits the bump stops every time you drive over a bump in the road. I read elsewhere that other people also complain about this and taking, for example, three people with you leads to embarrassing situations. Moreover, since they were installed in 2008 and have done only 30,000 km, the new shocks are worn out already.
The original Jaguar replacement seems not to be that good. As I like the SLS system and it worked perfect except for the shock absorbers, I consider rebuilding the original shocks by a specialist and reinstalling the SLS. I still have the original parts, I found an other tandem pump and all hydraulics are still in place, including the valve block, only the hydraulics main line is possibly bent on the end.
I have plenty of texts and photos of the process and parts, but need to find time to publish them.
Next page: components of the self levelling system
Kind regards,
Rens Swart