Update on Non-Working CC

O.K., I have now replaced the vacuum pump, in addition to my r/ring the vacuum actuator, the logic control module, and the switch, earlier on. And STILL my CC has not returned. :angry: The only component I have not r/red is the dump valve, but the intake on it looks pristine and the wiring connections are tight, so I would think it doubtful to be the culprit. In any event, is there any way to test its operation with a multimeter? I am at my wits end on this issue. :crazy_face: And yes,I checked and even replaced the fuse at the start, as well as made sure all my brake lights come on whenever I hit the pedal (which means the switch in same that disengages the CC is also working). :rage: Is there any way that the disengage button on the end of the stalk for the headlights could be at fault? I hate the fact that the CC system is apparently something that can only be tested with the car running @ 40 MPH on a chassis dynanometer (sp. ?), given the nature of the beast and the vacuums involved.

Vacuum pump? What vacuum pump?

Not to sure on the XJ-S but my switch (XJR) on the brake pedal has two micro switches, one for the brake and the other to kick out the CC.

  1. probably seeing the bellows and referring too it as a pump, which of course, it is clearly not. The only vacuum pump aboard these critters is the engine, itself. A very powerful vacuum pump!

  2. aye, my somewhat similar XJ wuzza 6 has two switches actuated by the brake pedal. But opposite in operation

A. Brake. Default is open. Closes on pedal application, brake light come on. if it gets juice from the nearby fuse panel.

B. CC. Default is closed. CC in operation. Apply pedal and it opens. CC out. Throttle no advisable when braking . That opposing forces thing.


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So, if it is in fact the microswitch at fault, then it would be stuck open and failing to close whenever the brake pedal is not being depressed, sounds like … :thinking: I’ll try to find some way to get down there and check it out, unless there is a simple way to remove the brake pedal for the assembly to be examined … (?)

I don’t know with the pre-face lifts, but the post have a tiny electrically-powered motor on the inside of the left fender in the engine compartment that generates the vacuum to “compress” the actuator bellows whenever the CC is on. I’m not sure how the dump valve works in conjunction with it, but I’m thinking whenever the brake pedal is pushed (or the CC shut off manually) it opens to “dump” the vacuum state existing in the system, quickly allowing outside air to rush back into the system so that the actuator extends back out to its relaxed (i.e. “off”) state. :thinking: I’ll try to post a diagram of the pump on here …

Here it is: “speed control actuator pump” DBC 3722


Actually, the application guide on Jaguar Classic shows this same part should be on ALL XJ-S and XJSes. btw, anybody else notice the new format that Jaguar Classic has gone to, re: using pictograms? :+1:

My '83 had no such pump.

Is this pump supposed to power the CC by itself, or merely supplement engine vacuum?

From the looks of it, it’s the whole ball of wax … I guess it’s maybe a “fail safe” system, so that if a vacuum leak develops in the CC system it doesn’t adversely affect engine performance, unlike it might if the source of the vacuum is from the engine manifold, perhaps :confused:

On second look, there is a second vacuum line “ted” into the system, but the diagram doesn’t show where it ends up. So, possibly it is a supplemental vacuum assist motor, connected in series from a port in the manifold. :question:

In trying to diagnose your non-working CC, have you tried simply pinching off that dump valve?

No. So if I apply a clamp to the vacuum line going to it, how would that show the source of the issue? :confused:

If you apply a clamp and the CC works, the problem is either the dump valve or whatever controls it.

Ah, I see what you’re saying … so the dump valve may be stuck open (default?) Thanks, Kirby … :smile: