I'm happy to report success in trouble shooting and repairing the cruise (speed) control on my 1984 XJ6.  Because the system had multiple problems, I had to test all major components and thus acquired an understanding of its operation and how to trouble shoot it.  So, the following is a system description and trouble shooting plan.  My cruise control had three problems; 1) the brake switch was intermittent, 2) the inhibit switch was broken, 3) the vacuum bellows unit leaked, would not hold vacuum.

JAGUAR CRUISE (SPEED) CONTROL SYSTEM DESCRIPTION AND TROUBLE SHOOTING Series III XJ6

The following Jaguar cruise control system description and trouble shooting procedure results from repair of my 1984 XJ6 Series III.  I do not know for what other years or models this information will be directly applicable.  If nothing else for other models, maybe this will give you some areas to check.

SYSTEM DESCRIPTION

The cruise control uses these same three human principles.  Instead of an eye, an electrical signal from the drive shaft operates the speedometer and thus also becomes the signal about how fast the car is going.  This signal is processed by the brain which is the cruise control electronic control unit (ECU).  The ECU directs the amount of vacuum in the bellows unit and the bellows unit acts as the foot by operating a cable to the gas pedal/throttle.

RECOMMENDED TROUBLE SHOOTING PROCEDURE

I) VACUUM BELLOWS ACTUATING UNIT
The vacuum bellows unit controls the gas pedal/throttle.  It does this under control of the cruise electronic control unit (ECU) and with a cable running from the bellows to the gas pedal.  The ECU controls the bellows by closing and opening two solenoids on the bellows.  One solenoid, which we'll name "yellow-white" normally (no cruise control) opens the bellows to the atmosphere.  The other solenoid, which we'll name "yellow-black", normally closes off the vacuum hose from engine vacuum.  When the cruise is activated the ECU closes off the yellow-white solenoid to atmosphere and controls the yellow-black solenoid connected to engine vacuum.

1. Unplug the three wire connector from the bellows unit.  Note that the wires going to (and from) the bellows unit are color coded black, yellow-white, and yellow-black.  At the bellows connector, connect the black and yellow-black wires to engine/chassis ground.  Connect the car battery positive terminal to the yellow-white wire.  These connections should energize both solenoids so that the yellow-white solenoid is closed to the atmosphere and the yellow-black solenoid opens to the vacuum hose.  When you clip on battery, the solenoids can be heard to click.  If not, recheck the ground connection.
2. With your mouth or a vacuum pump, suck on the vacuum hose connected to the bellows.  The bellows should move inward as vacuum is drawn.  If all you accomplish is sucking air, then the yellow-white solenoid is not working or the bellows is leaking in air.  Check the hose; if OK then it is most likely that the bellows is leaking air at the end plates.
3. If vacuum draws quickly and stops, as if it's not getting past the hose, then the yellow black solenoid is not operating to allow access into the bellows.
4. The solenoids can be checked for electrical continuity (but not mechanical operation) easily with an ohm meter.  The solenoid coil resistance is 25 - 30 ohms as measured from the yellow-white wire to either the black wire for the yellow-white solenoid or to the yellow-black wire for the yellow-black solenoid.
5. Should it be necessary to remove the bellows unit, it is easily accomplished by removing one bolt at the front and the pedal cable at the rear.
6. A vacuum leak is most likely where the end plates fit into the rubber bellows.

II) WIRING AND CONTROL SWITCHES TO THE CRUISE CONTROL ECU
The cruise control ECU is the brain that monitors the speed signal from the pulse generator (speedometer sensing unit) and controls the vacuum bellows.  In essence the ECU modulates the yellow-black solenoid to move the bellows in or out and thus move the gas pedal.  There are however some additional factors the ECU must be aware of, for instance, if you put on the brakes the cruise control should shut off.  These additional controls are performed by open/closed switches mounted and wired for that function.  The following tests using an ohm and volt meter will check the wiring and the function of all switches and the ECU wiring harness.

1. The cruise control ECU is remarkably easy to access.  It is located just under a the fuse panel on the passenger foot well.  Unscrew two small screws and the ECU will drop out below the panel.  It has two electrical connectors, a nine pin connector and a single pin connector.  Pull the connectors apart; we need access to the connector that has the car/chassis harness wiring (not the connector on the ECU).  Note, that the vacuum bellows three wire connector is connected as normal at the bellows (if it had been disconnected from the bellows only test).

1. Connector pin wires black and green-purple should meter ground.  Both wires are straight wires to ground (no switches).
2. Connector pin wire yellow-purple is connected to the "inhibit switch" mounted on the gear selector, so that cruise control works only when the selector is in Drive.  With the selector in Park, the yellow purple pin meters to ground.  With the selector in Drive it becomes OPEN.  Check that it measures OPEN.
3. Connector wire pin black-pink is controlled by the Set switch on the column, you push it when you want to enter cruise control mode.  It normally reads as OPEN but meters to ground when pushed to set the desired speed.  Meter it for open and then for ground when pushed.
4. There are two connector yellow-orange pin (with two wires on one pin) wires, all are associated with the master switch mounted on the center console.  In the normal center position all three measure around 14 ohms or less to ground.  However with the ignition ON, all three will see the 12 volt battery.  And, when switched to OFF the 12 volts will drop off.
5. The black-gray connector wire is also associated with the master switch and it meters OPEN normally and around 4 ohms to ground when the switch is moved to "resume".
6. The connector wires yellow-white and yellow-black control the two vacuum bellows solenoids.  The yellow-white wire is also in series with the brake operated switch which cuts out cruise control when brakes are applied.  This brake switch is normally closed, it is mounted on the brake pedal.  Ohm meter measurement to the yellow-white wire should show the 25 - 30 ohms of the yellow-white solenoid to ground.  If this circuit is open to intermittent, check the brake switch.  The yellow-black wire (is not switched) will meter 50 - 60 ohms to ground (not 25 - 30 ohms because both solenoid coils are in series to ground referenced to this wire).
7. Finally there is the lone yellow wire connector.  This is the signal from the pulse generator mounted on the transmission.  Pulses from this wire signal the car's speed to the cruise ECU.  This pulse signal also works the speedometer so if the speedometer is working it is most likely that the pulse is also getting to the ECU.  However, the presence of the pulse at this connector can be checked.  Connect a voltmeter to chassis ground and the connector pin.  Drive the car; even at very low speed the voltage will measure around 4 volts AC (and also a DC reading of about 5 volts, why?).  And, perhaps surprisingly, the voltage does not increase with speed (due to the type of meter used).

III) CRUISE CONTROL ELECTRONIC CONTROL UNIT (ECU)
If the vacuum bellows works and no problems are found with the cruise control wiring or switches, the only remaining component is the ECU.  The final test of the ECU is on the "bench", and we can do that, but first let's test it in the car.  The following tester is simple to make, requiring only two flashlight bulbs and two resistors and some wire.  It is also used for the bench test of the ECU.

--------  vacuum bellows wire colors ----------------
black                      yellow-            yellow-
(ground)                   white              black
    ↑                      ↑                       ↑<--- mini-alligator clips
    |                      |                       |
    |                      |                       |<--- three feet wire
    |                      |                       |
    |__ bulb ____ 33ohm ___|___ bulb ____ 22ohm ___|

The bulbs and resistors take the place of the two bellows solenoids so that we can observe the operation of the ECU.  Mount this assembly through the windshield wiper arm so you can observe the bulbs while driving.  Connect the three wires to their respective colors on the vacuum bellows chassis wiring connector (not to the bellows unit).  (The bellows connector is unplugged and the ECU is connected in the car normally).

Next go for a drive.  At about 30 - 40 mph push the "Set" cruise button.  The bulb on the left (33ohm) should light but rather dimly and stay lit.  This bulb is taking the place of the yellow-white solenoid that closes the bellows to the atmosphere.  The right bulb should light but flicker, and, as you slow down the bulb will light brighter, and, as you speed up it will grow dimmer.  This bulb is taking the place of the yellow-black solenoid that controls the vacuum from the engine.

If this bulb test circuit works, then the ECU and associated wiring/switches are good.  If this test fails then the ECU is possibly at fault.  The ECU can be bench tested as follows.

IV) BENCH TESTING THE CRUISE CONTROL ECU
Bench testing the ECU is not complicated but you do need some experience working with electronic circuits.  Essentially, what we do is connect up the ECU with power and grounds as it would be in the car, then feed it a pulse generation to simulate speed and observe its operation with the above two-bulb tester.  Thus, to perform this bench test you will need; 1) 12V battery source, 2) about 8 alligator clip leads, mini-size with boots, 3) a sine wave frequency generator.

Working with the ECU and its associated nine pin connector, make the following connections.  But FIRST, fuse the 12V battery supply with a 1 amp fuse, this will hopefully save you from any nasty smokings, the ECU should only draw around ½ amp.

1. Black-red and brown go to battery ground (negative).
2. Brown-black is not connected (simulating the inhibit switch in drive).
3. Brown-white is not connected (used only for "resume speed").
4. Brown-yellow is attached to a clip lead but left open (it is the "set speed" switch).
5. The two-bulb tester is connected, black to battery ground, middle wire to yellow-white, and outside wire to yellow-black.
6. Yellow and black-green are connected to positive 12 volts.

This completes the basic wiring of the ECU.  Next a pulse must be sent into it, via the lone green wire connector, to simulate the car speed.  A simple sine wave generator that can put out 5 volts at 60 to 200 Hz will work.  (Note: 60 Hz seems to be the minimum frequency required, so the AC output from a simple wall powered AC transformer converter unit may or may not work).  The generator's signal lead is connected to the green wire connector and the ground to battery ground.

With all connections in place and the signal generator set at 80Hz and 5 volts, touch the brown-yellow wire to ground for a second and remove it.  This simulates pushing the "set speed" switch in the car.  The two bulbs should light with the left one remaining steady while the right one changes its flickering as the signal generator frequency is lowered or raised.

If the ECU seems to have failed this test, double and triple check all the clip lead connections.  It is easy to get a bad connection with alligator clips.  Also, are you sure that the two-bulb tester is connected correctly?

CRUISE CONTROL - ANOTHER CHAPTER
Well, about two months after my first fixes on my cruise control, it stopped working.

However, remembering the most excellent cruise control trouble shooting guide by Tom Graham (hey, that's me!!!), I found that the cruise control Set button on the stalk was not making ground when pushed.  OK, so probably another example of funky Lucas switches.  Now question is, how do I get inside the little plastic box on the turn signal stalk to look at this button???  So, I went over to the local independent Jag repair shop and asked them same question, how or can you get into that box on the stalk?

Here's how, you have to take off the front aluminum plate.  Actually, I had guessed as much but knew that any amount of prying on the plate would bend it all up making it unusable.  The secret to getting it off safely is using a hair dryer, blowing hot air on the plate/box for about 2 minutes, the adhesive softens and you can then gently pry off the plate.  With the plate off you see another flat black plastic piece that the aluminum plate was glued onto.  This plastic plate pops out and underneath is the “switch”.  The switch is simply a spring loaded metal plate that when pushed makes contact with the metal stalk which then should make the ground.

Well, my switch worked fine, but the metal STALK was NOT grounded.  So, getting to the short strokes, I took apart the directional switch inside the steering wheel column.  How?  Very carefully!  There are many little springs and balls and stuff.  Found the broken stalk ground wire and patched in a new one.  Cruise control now works.