Hello, people... been a while.
I'm finally attending to the inoperative cruise control system on my 83. Hopefully, I have discovered the problem, and it's just a matter of connecting hoses correctly.
A couple photos:
The blue "mechanism", appears to be an electrically operated vacuum switch. Two available vacuum connections... one of which is not connected to anything.
The vacuum hose end, circled in red, is connected to the bottom side of that switch,
The larger vacuum hose, circled in purple, WAS connected directly to manifold vacuum. The other end is connected directly to the cruise control diaphragm ( bellows ) apparatus.



I'm hoping that somebody can either describe, or show me a picture of the correct arrangement for the hose connections..

I've never been able to stump you guys... and I bet I don't now either !

Thanks, people

 

Aubrey: that "blue device" is the redundant cruise control shut off valve. In other words, if the switch at the brake pedal fails, or the on/off switch fails, this unit guarantees that the vacuum in the bellows will be released.

I no longer have a V12 VDP in the garage but I might have a photo that will help - I will look later today.
The most common faults in the cruise control system are 1) failure of the rubber bellows to seal against the end plates - this is easily rectified with silicone sealant 2) loss of ground at the column switch 3) dirty connectors of the system in the engine compartment 4) failure of the electronic module found under the glove box side of the dash (held by on screw, obvious if you get your head under there.
But by far, the problem is most commonly at the bellows - and occasionally the issue is the solenoids sticking inside the bellows -wd40 is your friend for this.

 

This is fantastic, Gregory... thank you !
I'll tend to those things, immediately.

 

Hopefully pics from Gregory will clarify how the dump valve was originally plumbed. However, operationally, I don't think it matters. I think it's just an open-or-shut valve.

From your description it sounds like someone simply bypassed the blue valve, connecting the bellows directly to manifold vacuum. Nuthin' wrong with that...other than losing the safety benefit of the redundant release. I can't remember the details but a lot of cars didn't have the blue valve originally. it was added-on as part of a safety recall. At some point Jaguar began installing them as a part of regular production

Cheers
DD

 

My bellows had a crack in the rubber and was on the bottom where I couldn't see it. New, supple rubber ones are available for about $20 and it is quick and easy to replace.

My column stalk cruise control button was gummed up. I put tarps all over to protect the interior and sprayed electrical contact cleaner in it and kept pressing it until it worked freely.

Jeff

 

Also, check the switch on the brake pedal. Not the brake light switch, the other one, thats hot when brake is not apllied.. i replaced a few parts on mine and never could get it working, then when i did my engine swap i couldnt get torque converter to lock and it turns out that switch was defective...

 

This is correct. I forgot this one in my list - thanks for catching the omission! You can verify the operation of this switch simply by by-passing it (joining the two wires together) but NOTA BENE: if the cruise control now works, the switch is faulty, but the cruise unit will NOT be shut off by pressing the brake pedal. This should be, must be a test-only procedure.

This redundant valve was not fitted to the xj6 models, as I remember. It was a recall item for the V12 cars in Canada (can't remember the year) and then became a standard fitting for the late V12 saloons.

Time to wash our hands and stay apart from others.

 

Ah! do these photos help? these are photos of the valve on a 1992 V12 Vanden Plas, #92/100 - but the valve was fitted exactly the same way on all the cars. Note the Daimler sticker - at the end of manufacture of the Series III cars (built until December 1992) Jaguar was running out of some of the Jaguar trim items and substituted Daimler items - the Jaguar models were not built after the end of 1991 - but the Daimler Double Six continued to be built all through 1992. The 1992 Jaguar V12 Vanden Plas (made only for Canada) was in fact a fully-optioned Daimler Double Six - and this particular car was built in May 1992, when Jaguar items were being exhausted (the very late single knob mirror control - the chrome twin-stalk was no longer available, and another example from this car: it also had the Daimler signed driver's floor mat,)





and for general reference:


The mirror switch: