I just bought an XK8 1997 which unfortunately does not have ruice control which I can not do without. Therefore, I am considering fitting an unoriginal which I have done in several other cars with fine result. However, the XK8 is a little more advanced than I have been used to. I would therefore like to hear if anyone has experience with this in an XK8. I have observed that there is a traditional accelerator cable from the accelerator pedal and out into the engine compartment so that part can probably not be that difficult. I’m thinking about where you might. can retrieve the speed signal. On my XJ-S I picked up the signal on the Speedometer but with all the control boxes that are in an XK8 I am very much in doubt. I do not want to ruin anything. Is there anyone here in the forum who can help me along the way ??
Early XK8s with the mechanical guard type throttle body have a cruise control operated by three vacuum valves located in the left front wing (from memory). The vacuum supply is connected to the top of TB. The vacuum valves are driven by signals from the ECM which I suppose gets the speed information via the transmission control module and the instrument pack ( the TCM is a separate module on XK8).
Cruise and non-cruise ECMs are different. I thought I would be able to re-programme a cruise ECM to be non-cruise, which is what I needed at the time, using WDS. But I couldn’t do it. I could certainly tell it to be non-cruise but it it refused to save the setting without being connected to the factory database.
I went to a lecture by the Jaguar engineer who had been part of the engine and transmission management design team. He explained that they were not confident of running the car completely fly by wire and so came up the TB arrangement and a throttle cable.
I suspect that you might have to be a bit imaginative in where you get the speed signals from. How about tapping into one of the wheel sensor outputs? Much more accessible than trying to get into the conversation between the ECM, TCM and IP.
Would like to tell you that I have managed to install an original an unoriginal wired Cruise Control in my XK8. Let me just say that it was not easy. The first problem was finding a speed signal. I could not find any usable signal, neither from the speedometer nor the ABS sensors. Also could not find space in the engine compartment for the engine / vacuum unit that was to pull the accelerator cable. Eventually, however, I found a usable Speed Control on Ebay / Australia. It is called “Command AP50B” I placed the vacuum unit at the very top under the instrument panel on the left side (LHD car) and led the cable down to the accelerator where I extended the accelerator arm upwards to make a suitable place for cable from cruise control. I found the speed signal by 2 magnets with 180 degrees distance on the PTO shaft. The electrical control box was placed at the back of the center console. I should also add that the signal from the stop light switch I could not use either, but had to make a false stop light signal via a relay to make this work. had to go completely around the car’s own signals. Partly because they were impossible to measure, probably due to a lot of computer nicks and low voltage signals. Also because I was afraid of damaging some of the car’s original computer technology. All control of the Cruise Control is on the completely old-fashioned analog way.
I would not recommend others to try this, i.e. unless they are very patient, have some electrical insight and are willing to spend a lot of time on the project.