Hi all, the cruise control of my 1987 XJS V12 is not working. In my previous Vandenplas I always drove the car as much as possible on cruise control so as to avoid extra shifting and general ware of the vehicle. I intend to do the same on my recently purchased V12, as I want it to last a long time. Unfortunately this cruise control is not working. I tried everything as instructed in the manual, but no luck. Any suggestions?
BTW, I a so impressed with this forum and the kind suggestions from different members. I was helped to get my new oldtimer to run, suggestions on injector cleaners to stop a nagging knocking sound, etc.
Thanks,
Daan

 

This might help

https://www.jag-lovers.org/books/xj-s/16-Cruise.html

Cheers
DD

 

Doug is correct. You need to test everything listed there. The downloadable version has a circuit diagram of a test rig you can make up which is useful. However the resistors in the diagram need to be at least 5 watts as they get very hot.
One of the most common non-electrical failures is the bellows. It gets hard and leaks around the edges. Rockauto carries an aftermarket replacement. Seal the edges with RTV or similar.
The switches or switch adjustments are also a common issue. Pretty easy to test with a multimeter and the light tester mentioned above. Note that the cruise only works with the transmission in D. When you test that switch, wiggle the shift lever around a little to make sure the switch stays engaged and doesn't break the circuit randomly. I had that issue which I chased for a week; drove me crazy.
Please don't baby the engine. The V12 needs to be taken near redline occasionally for an Italian tuneup once warmed up. This helps keep carbon down and the plugs clean. Your engine will thank you for it. The V12 is a rugged engine if you keep the cooling system maintained and change oil regularly.

 

Great suggestions. I see I have some work to do to sort it out. Tx

 

Yes, this drove me crazy too. Once the cruise control worked in position D, once it didn't. For proper functionality, the lever must be pushed forward in position D. If you want to turn off the cruise control (no need to press the brake), just slide it back slightly. All in position D. At least that's how it works for me. My main problem with the cruise control body was a bellows leak. I bought a new one and sealed it with high temperature silicone. The filters in the solenoid body still need to be cleaned. If the electricity is fine. There is still one component - EAC7905 the cruise control dump valve, which should be functional. It's a safety solenoid.

 

That's not the intended operation.

There is an inhibit switch on the gearshift. I suspect yours needs some adjusting.

Cheers
DD

 

Yep, absolutely. I had to adjust mine. Pretty easy to do with the ski slope removed.

Jon

 

If not, then the cruise control switch on the shifter is in the wrong position. But from my point of view, it's a nice feature that I'll probably keep.

 

Thanks for all the advice, folks. I had the bellows replaced and now it works great. I like my cruise on, because I feel that one should drive a 35+ old car gently. I live in the country side and like to drive with the cruise control on as much as possible, to avoid the car shifting back to a lower gear. Now that the cruise works fine, that avoids a great deal of shifting, while the car automatically stays a just a little bit above the speed limit even in the hilly terrain where I live.

 

daanv,

The cruise control doesn't operate any differently with the gearbox, compared to your own foot being on the throttle. If the gearbox drops down gears when you're pressing the throttle, compared to driving with cruise on, it's because you're pressing the throttle more! All the cruise is doing is using the throttle to hold a set speed. You can do exactly the same with your right foot if you wish!

Cheers

Paul