I ordered a Rostra cruise kit and will let all know how I get along on the install.
RagJag

 

Got a 90 XJS and my cruise doesn't work since I bought it a year ago. Got any ideas where t start in order to get it working again?

Tony

 

dtony, I ordered the Rostra unit because I don't know how to fix the OE cruise. Unfortunately, the Rostra has a rather large box and there is very limited space in which to mount it under the hood. I may still do it but as yet I haven't found the time. A few weeks ago a member posted that he replaced the vacuum dump valve and fixed his.
Possibly some senior members will post what you need.
Cheers, RagJag

 

I Just fixed my cruise. Last year when I did my spark plugs and cap I noticed that the metal caps on each end of the vacuum diaphragm were lose and would leak vacuum, so I removed, cleaned and siliconed them back in place. While I had the unit off I jumped the electromagnet valve on the front plate to ensure it was operating (it was).
When I road tested after reinstalling still no go.
How the heck do you test a system that doesn't turn on till 60 km/hr?
I bypassed the vacuum control valve for the cruise control (small valve with electrical plug and a vacuum line in and one out to the diaphragm) and road tested and It worked as it should.
When I jumped the valve with 12v I could hear it click, so I assume its a fault in a switch or wiring.
Seems to work without so I'm good for now.
Possibly you have a similar problem

 

Resealing the servo/bellows end plates often does the trick. Sometimes it's just a blown fuse (suspect the release switch shorting of the fuse blows again). Sometimes as simple as a soft vacuum hose that collapses shut.

More reading:

Cruise Control

Cheers
DD

 

On my '92 v12 the cruise stopped working because of this small plastic washer........part 4. It goes soft and pulls out.


Speed Control Actuator-5.3 Litre - Parts For XJS from (V)179737 to (V)226645 | Jaguar Classic Parts UK