My cruise doesn't work. Is there a common problem with these (1985 Vanden plas) that I can start with?

Most common is vacuum leak in bellows. Check it closely for tear.

Agreed on bellows leaking. If not torn, sometimes the edges have curled up a bit a no longer seal against the end plates. A bead of silicone sealer will fix that.

Don't forget to check the fuse and the vacuum supply hose


More reading, the XJS system is the same as yours.......


Cruise Control



Cheers
DD

is the switch behind the Shifter set to ON when you press the turn signals stalk switch Button?

is the speed over 45 mph when you press the Button ?

More checks:


1. Cruise interrupt switch on brake pedal. Closed with pedal up. Open when brake applied.


2. Micro switch on shift tower. Open in P or D.


3. Cable from bellows to throttle pedal.


4. Double bullet connector in engine bay. Apply Jaguar mantra.


As for me, I rarely use CC. it isn't connected in my Jaguar


Carl.