OK, I know no one will believe me, but at the moment EVERYTHING works properly on my XJS, for a few days anyway---EXCEPT the cruise control. Does anyone know how to troubleshoot the cruise rather than just replacing parts until you find one that succeeds??

 

RagJag asked a similar question a few days ago..... https://www.jaguarforums.com/forum/x...996-xjs-80705/

If you have the circuit diagram it tells you what signals should be present at the control unit. I was able to check that and all was well. I also tested the vacuum pump and the dump valve on their own and they all worked. At the point I concluded it was the control unit and then something else on the car broke and I got distracted on that and didn't go back to the cruise control

 

I have the Repair Operation Manual Published by Jaguar Ltd, but no other. Can find nowhere in manual that shows which signals should be present. Been using the manual a long time, but don't remember ever seeing that information. Am I losing it, or using a different circuit diagram than you? Could be losing it, after all, I love Jaguar's....says something about ones sanity I suppose. Being nuts is fun though, so I'll keep doing it :-)

 

Yes, the ROM is weak on circuit diagrams for our later cars.

Go here Jaguar electrical reference, then download "XJS '92 Electrical Guide". It's invaluable.

Page 53 gives you "Inputs / Outputs" for the control module both for key on and key off. The control module is in front of the passenger seat under the little metal box cover that also hides the seat movement controllers and the seat heaters.

I can tell you that on my car, everything worked out correctly, including the road speed, which was a 2 man job to test (1 driving, the other probing)

Good luck !

 

Thanx Sarc

 

SARC, could shift gate being slightly out of sync at the linkage (top of trans at shifter) cause an issue?

 

The control unit needs to see 12V on pin 12 to operate. This 12V is wired through both the gear selector switch and the brake switch, which is at the brake pedal. If your gear selector micro switches are not set up properly, that could be your problem.

Very quick test you could do --> while driving (obviously at a sedate speed, ie 40mph), shift into "2"...... see if cruise works then..... if it does, that suggests your micro switches on the gear selector are out of alignment.

Another test........ At the control unit, probe pin 12 with the engine off, ignition on, gear selector in D, "2" or "1", and foot off the brake pedal...... does it show 12V ?? If it doesn't wire pin 12 to 12V ONLY FOR THE PURPOSES OF TESTING then go for a run and see if the cruise works now. If it does, you know you have a fault somewhere in the gear selector or brake pedal switch area.

 

Yo Sarc. As soon as I typed that about the shift gate I decided to check it out. It was the shift gate. Shifter wasn't lining up with switch when in "D". Works beautifully now. Thanx for trying to assist mate!!!

 

Awesome. That was an "easy" one :-)