The speed control switch (located with the trip computer switches) is not working (unable to have it work when pressing the "set" button).

I opened the black "box" to check why.

Does anyone has images of the inside of the box as I have difficulties to reassemble all the little platic parts around the "set" button.

Cheers

 

Hi CHELMI,

I can't help with reassembling the trip computer panel, but on our XJ40s the cause of cruise control not engaging was failure of the dump valve, which is mounted in the engine compartment on the forward side of the left wheel arch. The dump valve has an electrical connector and a vacuum hose attached. See Part 2 in the diagram below from jaguarclassicparts.com. By the way, Jaguar calls the cruise control the speed control system if you want to look up the parts.

The dump valve is normally open, but it is supposed to hold vacuum when 12V is supplied (whenever cruise control is engaged). When you press the brake pedal or turn the cruise control off, the dump valve opens up to release the vacuum so the cruise control actuator will immediately release its link arm that connects to the throttle body spindle or cam.

You can test the dump valve by blowing through it to confirm it is open, then apply 12V to its electrical connector and try to blow through it. If you can, the valve has failed.






Cheers,

Don

 

The valve is ok after checking as indicated by Don.
The reason why I had to check the switch board is the Set button was "lax" and did not action the cruse control; after checking it, I found that the plastic part of the button itself has been a little bit damaged by PO which in turn prevented to effectively press on the contact of the electronic card; I corrected it by adding a very thin plastic piece to thighten the button into its location; it is now reassembled, and ready for life test.

Cheers