Go LPG - XJ8 Cruise Control

Retro-fitting XJ8 / XJR Cruise Control

Large numbers of XJ8 and XJR V8 models built between 1997 and 2000 were not fitted with Cruise Control. The lack of this option seems to spoil an otherwise brilliant, well equipped car. I've seen buyers reject a car because of this only. I have done the same. Why put up with this omission when other cars have it?

It's just that sometimes a nice car may come along, ticking the boxes in every other category. It may be that we decide to turn a blind eye to the lack of Cruise and buy the car anyway.

"I don't use it that often anyhow......"

But still it niggles at you....

When I bought my 1998 XJR it had everything I wanted, including a replacement Steel lined engine fitted by Jaguar and the Romano LPG conversion we installed for the owner some 2 years earlier. The car had options not often seen in an XJR, the slide and tilt sunroof being the most rare. The one thing that annoyed me (as the car was so close to my ideal) was the absence of the steering wheel mounted Cruise Control key pad. It balances up the Audio controls on the other side so nicely. A steering wheel without it looks unfinished to me.

It just looks right.........

I decided to begin researching the possibility of retro-fitting a Cruise Control system removed from a car being broken up for spares. I didn't want to fit an aftermarket item. I was spurred on by noticing that most of the Cruise Control wiring seemed to be already present in the XJ8's and XJR's I converted for my customers. How hard could it be when most of the wiring is in place and all of the electronic signals are there for the purpose?

I wouldn't know until I tried. Now I had a Cruise deficient car of my own, I had the ideal opportunity to find out at my leisure!

So there's the background. I won't keep you in suspense any longer - I suceeded in all respects. The fact that I wanted a particular type of steering wheel (a piece of treasure discovered at a Jaguar Spares Day, Stoneleigh) made the task even harder, as you will see. Anyhow, pleased with a successful outcome I decided to tell the story so that it will help anyone else wanting to do the same.

So here are the details and pictures showing 'How to...'

Good luck with the task, but if you don't fancy DIY'ing then we will be happy to do it for you.

Firstly, Good News - The Job is Half Done Already....!

This should give you a little encouragement. As far as I know, ALL of the necessary wiring is included in every XJ8 and XJR V8 wiring loom. The connector plugs have just been taped back in cars that were not fitted with Cruise Control.

The Cruise / Throttle Actuator is part of every XJ8 Throttle Body - You already have it.

The main items that make the Cruise Control system work (Electronic Vehicle Speed Signals, Engine Manifold Pressure and ECU Controlled Fuel Injector Pulse Duration) are a natural part of every XJ8.

Your Cruiseless car has half of the system already in place. All you have to do is fill in the gaps!

Let's begin by looking at what parts are needed to fill those gaps;

Using a steering wheel with the cruise keypad already fitted makes the whole job far easier and, therefore, much quicker. This route is strongly recommended. Get a wheel that will match your Airbag / Horn push centre portion. Then you only have to fit the wheel to the column and plug into the existing wiring loom. It has the extra contacts in the column connector already. Hint - Set your Steering Wheel (and, therefore, the road wheels) dead straight before removal. It will be easy to re-align first time when re-fitting the original one or fitting a replacement. This is also very important for the alignment of the wiring cassette. This is mounted behind the steering wheel and must be correctly aligned, otherwise it will be damaged on the first corner you take after the wheel is re-fitted. When you have removed the steering wheel do not move the cassette spool from its home position.
If you want to use a particular type of steering wheel that hasn't got the keypad or pehaps retain the car's original cruiseless steering wheel, both Audio and Cruise keypads complete with their combined wiring harness will be required. Leave the wiring connected between both keypads. They can be fitted as an assembly to a non-cruise steering wheel but beware, a lot of careful and time consuming work is involved. This is definately doing the job the hard way!
Next thing on the list is the group of Vacumn Valves. They are sited under the bonnet, on top of the pedal box and to the side of the brake servo unit. It's best to remove the valves complete with the mounting bracket still attached. Notice that the connecting plugs for the valve wiring are colour coded which makes things easier, that'll become clear later. You'll gather from the picture that I cut the plugs from the donor car, a precaution that turned out to be unneccesary. It is a good idea to get the two pozi-headed self tapping bolts and rubber mountings that secure the valve block bracket to the pedal box though. The bolts tap straight into the holes provided on top of the pedal box for fitting at the factory.
It's better to remove the valves complete with all the pipes still attached. If you do that, you won't have to remember what goes where later on, especially if the project becomes shelved for some time after getting the parts. Remove the pipe close to the bottom of the throttle body (the vacumn supply pipe) complete with the tee piece and any other pipes it tees into. Your car will not have these if it didn't get cruise. Examine all of the vacumn pipes carefully - The first vacumn supply pipe I removed from a car had been chafed right through.
The last main component from under the bonnet is the Vacumn Reservoir pack, (centre of picture) an assembly of two plastic tanks and a mounting bracket. These are situated under the air filter box. These tanks also need to be undamaged, leakproof.
The next part required is not an obvious one - The Brake Light Switch unit. On cruise control models, pressing the brake pedal cuts off the cruise control at the same time. The cruise enabled brake switch assembly simply includes two switches of the same type, working in tandem - One switch turns on the brake lights whilst the other cuts off the cruise control. Because this item is safety criticial it would be better to buy a new one from a Jaguar Dealer.
The last parts are also from the interior. The centre console mounted Cruise Control switch, along with its mounting plate and the two-hole finishing trim.
Collect all of the fixing bolts and screws as you remove the parts you need from the donor car. You cannot have too many and some are bound to be lost when fitting the components to your car.

Now for the exciting part.....Installation!

There is absolutely no problem in doing this installation in stages if your time is limited. The order in which things are shown here is not cast in stone, you can re-arrange the jobs as you wish. The only things to keep in mind would be safety issues and runnning issues. Just for instance, if you fit the valve block it would be wise to make all the other pipe connections before fitting the vacumn supply. Then you won't have any vacumn leaks which the car won't like!

Fitting the Brake Light Switch It might be wise to begin with the most awkward job first. When that is done, you'll be coasting downhill from that point on! The switch is only held in place on the pedal box by only one nut and bolt although it is very awkward to get at. Move the Driver's seat fully back and adjust the steering column to its fully up position in preparation. In some cases (depends how big you are!) it might even be easier to remove the Driver's seat. Just to make things interesting and to provide you with a little entertainment, the head of the nut and bolt are of different sizes - 8mm AF and 7mm AF. The spanner in the picture is hanging on the bolt head. There is no room to get any type of ratchet that I possess so it's spannering all the way. When the bolt is removed the switch simply pulls downward and off. The top fixture (mercifully) is a peg and groove affair, making re-fitting quite easy. Simply plug in the wiring connector (white plug hanging on the right of the picture). The additional contacts and wiring are already in place in the orignal switch connector. When the fitting is complete, check the Brake Lights for correct operation before moving on. You can find a useful clue if they don't work - The gearshift may not allow you to select a gear from 'Park' if the brake lights do not come on.
Now for the easier parts... Locate the centre console Cruise Switch wiring which is tucked away and probably wrapped up (the white plug to the left of the picture). Plug in the cruise switch and fit up your finishing bezel.
Moving to under the Bonnet; Locate the Vacumn Valve cluster wiring. Mine was tucked under the Brake servo and wrapped up in grey foam.
Fit the Valve cluster to the top of the pedal box using the two pre-formed holes, the rubber mountings and the self-tapping bolts you so wisely brought with you. Simply connect the 3 liberated wiring connector plugs colour to colour - Couldn't be easier.
Now for the Vacumn Pipes, 3 in all. The first one (with a rubber 90 degree elbow on its end) goes to the hitherto redundant Throttle Actuator (pictured). You get to throw away the short dummy pipe that was fitted to the top of the actuator from new. The second pipe goes off to the Vacumn Reservoir Tanks (shown later). The third pipe goes to the Vacumn Supply connection, often tee'd into the fuel pressure regulator on the back of the OS (RHD drive cars) cylinder head and close to the back of the intercooler on the same side. NOTE that the car shown is supercharged - The normally aspirated 3.2 and 4.0 models are much easier to work on in this area.
Here's my temporary fit of the Vacumn Reservoir Tanks which live underneath the air filter housing. I couldn't get the reservoir mounting bracket from the donor car I got all the other parts from. The Vacumn connection leading back to the valve cluster can be seen leaving the picture on the bottom left, and at the other end are the two vacumn reservoir tanks, united from new by another tee piece.
Now for the final push.... Whether you have a Steering wheel with the cruise keypad already fitted or have converted your own (as described below) you'll need to identify the black plug seen in the lower centre. The column wiring harness enters the wheel through the oblong hole just above it. If you got the road wheels straight before you took off the steering wheel, you won't have any difficulty in positioning the cassette type wiring gadget which keeps things connected whilst the wheel is turned.
And now for the crowing glory... On goes my cruise enabled Steering Wheel (which took an hour or three to convert.) The wiring harness from the audio and cruise keypads will simply plug into the existing column wiring harness. Again, the wires and contacts are already in place in the car's original connector plug. If all is in place and connected up you should be able to start up the engine, press the Cruise button in the centre console and be greeted by the piercing glow of a red LED. If you get this result, the signs are good!

Testing the System

A little care and forethought will prevent you making holes in the scenery. Remember that proving the brake lights work does not ensure the cruise control switch will disengage the system. It is a seperate switch and not to be trusted until it is proven to work. Remember that you have 2 other ways of disengaging the cruise control - The centre console switch and the 'Cancel' key in the steering wheel keypad. In a real disaster, you could turn off the ignition, although you'd have to deal with a lack of power steering and possibly reduced braking efficiency.

When you are ready for testing, find a clear stretch of straight(ish) road. Drive at 40mph. Press the centre console Cruise button and check for a red light. If you get that and the road is clear, then press the '+' key. If the engine starts to increase the speed of the car, apply the brake gently but immediately to find out if the cruise disengages.

If all is well, do the same again, but this time check for disengagement by using the 'Cancel' Key, and for disengagment by pressing the center console switch to the off position.

If you have gotten this far, it looks like you have success on your hands!

Now test the system for speed holding, looking for any increase or decrease in selected and held speed. If you have any problems in this area, this is where I fail you. I have no idea (to date) how to adjust the system and don't even know if it is adjustable. All I can say today is that if I had that problem I'd begin by looking at the amount of free play in the throttle cable, or the lack of it.

Converting the Steering Wheel

If you want to keep the car's original Steering Wheel or want a particular colour of wood or leather trim, this may be your only option. Begin by removing both of the Horn Push bars. The keypad mounting hole and screws are behind them. Remove the original Audio keypad. Study the shape of the aperture it came from and mark out for cutting on the opposite side. Your aim is to mirror the Audio keypad aperture. Cut out the rough shape of your new aperture on the opposite side and finish the edges in your own way.
I used a B&D; Powerfile to make a nice tidy job of the edges.
Keep trying the keypad in the new aperture until it fits just right from outside and make sure that the keypad mounting bracket holes are touching their base on the inner face of the wheel.
When the Cruise keypad fits nicely it is time to fit the keypads and then the horn push bars. Check that the bars release properly before moving on........
.....but it's not over yet. The Airbag / Horn push will not fit. It doesn't have clearance over the new keypad.
To make the clearance, use another (cruise type) Horn Push as a pattern if possible. Carefully peel back the leather facing which has been rolled around the back face. Mark the cut out in any way that suits you, then cut out the unwanted portion of soft foamy plastic material. Take care to get a nice, even gap between the Keypad and the Horn push by trial fitting. Make sure the modified Horn Push has sufficient clearance over the keypad before beginning to re-fix the leather trim.
When you are happy with the cut out, remove the excess leather and re-attach it. Copy the way the leather was originally split into sections around the back edge of the Horn Push clearance curve so that it is possible to glue all edges down firmly. I used a contact adhesive to re-attach the leather (Bostik).
And there we have it. If the keypad has been fixed properly, using the original fixings and the holes provided, the job is as good as one that came out of the factory. The combined wiring of the Cruise and Audio keypads is a simple plug-in task.

I hope that that this article will help the many cruiseless XJ8 owners out there!

Steven P. Sparrow BSc (Hons)

www.go-lpg.co.uk

June 2008

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