Im my efforts to sort this car out I need to repair the cruise control and the AC/heater door controls. Getting there!
I have no 12 volt power at the dash cruise switch. Are these cruise systems repairable without going to a dealer for diagnosis? I know modules are involved, the understanding of which is above my pay grade.

I can do without cruise but find it quite nice on the Interstates. There were aftermarket systems available that could be mounted unobtrusively and worked off manifold vacuum quit well. Not sure that type is still on the market since computerization has become the default. This is unfortunate for anyone wanting to keep a car 30 or more years, especially an auto such as the XJS that saw limited production. Any ideas from experienced owners is much appreciated.
RagJag

 

@RagJag

From (bad) memory, I think the switch only gives a ground signal to the control unit, so not seeing 12V at it is probably "ok"

I also have inoperative cruise and I did do some diagnosis. To do it correctly, you would probably need an oscilloscope, because you need to be able to look at pulse widths going to the control unit from the speed sensor. Based on the fact I have had a marelli cat fire, I am scared about it happening again, and the thought of it happening while cruise control is set has scared me off fixing it, so I have removed all of the components from my car. I seldom drive the car further than 20 miles anyway, so I don't need cruise

The system has 4 basic components...... brain, actuator (that holds the throttle open) Vacuum pump (that moves the actuator) and dump valve (that releases the throttle when you press cancel) If I had to get mine working, I would probably have started swapping out components as they come up from time to time on ebay. Looking at the whole system, I would think the vacuum lines in the engine bay are probably the most vulnerable so I would check there first