Cruise Control Testing 83 XJ6

I’m trying to get the cruise to work on my 83 XJ6 VP. I have removed the bellows unit from the engine compartment. Both solenoids are clicking when powered. I used my vacuum pump at the nipple. The solenoid holds vacuum, but the bellows don’t contract. Do both solenoids have be powered? I also noticed I hear air escaping when I depress the bellow. The bellow is new, but I assume I need to use silicon to seal it to the metal plates?

Any help appreciated to complete my testing!


This was written for the XJS guys but info pertains to your car as well. It covers a lot of ground


It is not supposed to be necessary, Richard, but it doesn’t hurt - bellows leaking is a common cause of cruise failure…

The document referred to by Doug is worthwhile downloading - it is fairly comprehensive, and it is fairly easy to make test rig for checking out the electric functions of the valves. Briefly; one valve opens by computer applying 12V to let in vacuum - the other valve’s ground connection is controlled by the cruise control to ‘vent’ out vacuum as required for controlling speed. Ie, you need one valve open and the other one closed to get vacuum in the bellows,:slight_smile:

Seal the bellows and see what happens…?

xj6 85 Sov Europe (UK/NZ)

One solenoid is NO (Vent) and the other is NC (Vacuum)

And there are three states vacuum wise:
Vent open - Vacuum closed: System Off
Vent closed - Vacuum open: Acceleration
Vent closed - Vacuum closed: Cruise

The extra bue solenoid will cut vacuum and bleed to the atmosphere when you apply the brake or turn the CC Off as an additional safety feature.


I did more testing today and still no joy!

I sealed the bellow on both ends. Both solenoids click with power. Using my vacuum pump on the nipple, I have perfect vacuum on one solenoid powered;when powering the other solenoid, the vacuum instantly drops completely. During this testing, the bellow does NOT move at all (when the solenoid is powered, and it holds perfect vacuum). It is a simple system, but I can’t understand why the bellow does not contract when using my vacuum pump (and the solenoid is energized).



The solenoids inside that module have little rubber bumpers on the ends to help them properly seal against the ports to control the vacuum. On my '83 both of those little bumpers had fallen off and were rattling around in there. I fashioned new bumpers by using a hole punch on a bicycle inner tube and glued them in place – today I’d recommend E6000 glue for that job. IIRC, I even polished the surface of the rubber a bit because I wasn’t confident enough the surface of the bicycle inner tube was good enough.

Yes, something is wrong with the solenoids.
When you power both the bellows should retract.
It seems that you found the problem.