Jaguar XJ-S. Service manual — part 169

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It must be pointed out that a failure of a cruise control could be dangerous, although that would typically require the
two openings to atmosphere to fail closed and the opening to vacuum to fail open -- highly unlikely. You should make
repairs carefully, using a very reliable glue (such as JB Weld) to hold the rubber seat on the rod. Use of unreliable glue
such as cyanoacrylate (superglue) is not recommended.

Robert I H Egerton adds some tips for putting the solenoids back in. “When I put em back they didn't work.
Eventually I realised that the horseshoe-shaped bits of copper or brass under them are springs which hold the moving
core down so you must ensure that they clip into the core when you put em back. I get my pension at Xmas so it took
me some little time to suss it out .”

CRUISE CONTROL CABLE ATTACHMENT: Bill Trimble managed to lose the little part that screws onto the end
of the cable to connect it to the bellows unit. “I took the car over to Rick the Mechanic's to see if he had any ideas, and
sure enough he remembered that one of the local auto parts places had barrel connectors for hood release and choke
cables. I bought one for a couple of bucks and we fitted it in about ten minutes, the only modification needed was to
gently pry out the round end on the bellows side to fit the wider diameter of the new barrel connector. We also
wrapped some safety wire around it so that if the cable slips out again, the connector won't fall out.”

Safety wire is tricky to get right here. Before trying to assemble this thing, take the tiny screw out of the barrel
connector, push it through a tiny piece of cardboard, and screw it back into the barrel. That way, when your grip on the
thing fails, the barrel will only drop 1/4” and sit right there on the loop instead of falling into the dark abyss. A washer
would work, too, but it’d jingle with engine vibrations.

Another idea would be to tie a piece of string to the screw, and tie the other end to the bellows end plate itself. You
could actually do that before taking it apart.

HEAT SHIELD INSTALLATION: There’s a heat shield attached to the bracket that wraps around the bellows and
holds the cable housing. This heat shield should be underneath the bellows to protect it from radiant heat from the
engine. Steve Wilke’s car had it on top when he got it. If your car has been similarly misassembled, you’ll want to
correct it.

SERVO RELOCATION: To get that bellows assembly to last longer, it would probably be a good idea to relocate it
somewhere cooler; this will not only keep the actuator cooler, but it will make working on the distributor easier. Ron
Whiston relocated his to the area in front of the radiator, mounting it on the same bolt that holds the horns. He
fabricated a new cable, similar but longer. He notes that removing the mount bracket, which is held to the back of the
A/C compressor with the same bolt that holds the hoses on, will require recovering and recharging the freon circuit -- a
lot of extra effort and expense. Therefore, he suggests you can either wait until you’re working on the freon system
anyway, or you can fabricate a new bracket and abandon the original one in place until the opportunity arises to remove
it.

BELLOWS REPLACEMENT: Jim Isbell says, “If you ever do this make sure you get the metal ring out of the center
fold of the old one and put it into the new one which may not have the ring in it.”

SERVO REPLACEMENT: If you have to buy a new servo unit from Jaguar, be prepared to open your wallet a long
way. You may find better prices if you shop around, though; Randy Wilson says “Those of us not locked in to the Jag
parts distributing system, i.e. the independents, have an alternate source for this servos, plus we can get the bellows as a
separate service item. They still aren’t cheap, as they come through a different British car parts network (RR!), but the
servo is about half what Jag wants.”

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CRUISE CONTROL TROUBLESHOOTING -- WIRING AND SWITCHES: To check the wiring, disconnect the
wiring connectors at the speed control unit. The speed control unit is located just under the fuse panel above the
passenger footwell. It has two electrical connectors, a nine pin connector and a single pin connector. Pull the
connectors apart and perform the following checks on the wires going out to the car, not to those going into the speed
control unit. Make sure that the bellows unit three wire connector is connected properly if it was disturbed by previous
testing.

Connect one lead of a volt/ohmmeter (VOM) to a convenient car chassis metallic ground, and test to make sure you
have a good ground using the ohm setting on the meter. The other meter probe will be used on the pin connectors.
Unless stated, all measurements are made with the ignition off.

1)

Set VOM to ohms and test the black wire; it should check as zero ohms, since it is a ground wire.

2)

The yellow/purple wire is connected to the “inhibit switch” mounted on the gear selector; this switch opens to
allow the cruise control to work only when the selector is in D. The same switch serves to allow the kickdown
solenoid in the GM400 transmission to be activated only when the selector is in D. As a result, when the
selector is not in drive, the wire is not truly grounded; it is routed through the kickdown solenoid in the
transmission to ground, which is effectively grounded as far as the electronic circuits in the speed control unit
is concerned. With the selector in P (or anywhere other than D), check that the yellow/purple wire reads low
ohmage; it might not be zero, but it should be low. With the selector in D, it becomes open; check that it
measures infinite ohms.

If the console is opened, the physical operation of this switch can be observed. These type microswitches,
complete with the rollers, are available at electronic shops.

3)

The black/pink wire is connected to the set switch on the turn signal stalk. It normally reads as open (infinite
ohms) but should indicate zero ohms while the set switch is pushed. See below.

4)

The black/slate wire is connected to the master control switch just behind the selector lever on the console. It
should read open (infinite ohms) when the switch is in either ON or OFF position and zero ohms when the
switch is moved to RESUME.

5)

The yellow/white wire connects to the two bellows unit solenoids through the brake-operated switch which
acts as a backup to cut out cruise control when brakes are applied. It should read the 25-30Ω of the vent
solenoid. When the brake pedal is pressed, it should read open. Note: the brake pedal switch intermittently
shorting to ground is a known problem, so you might want to wiggle the brake pedal while here and see if you
can get this wire to read zero ohms. If it does -- even for an instant -- read the section on fried ECU transistors
below.

6)

The yellow/black wire is connected to the vacuum solenoid. It should read 50-60Ω; this is because both
solenoid coils are in series to ground referenced to this wire.

7)

The yellow/orange wire actually connects to two different pins at the speed control unit connector. It is
connected to the master control switch. Turn the ignition on, select a suitable voltage scale on the VOM, and
check that this wire reads 12 volts with the switch in the ON position (center) and zero volts with the switch in
the OFF position.

8)

The green/purple wire is connected to the brake light switch. Normally, this wire should read zero volts; when
the ignition is on and the brake pedal is pressed, it should read 12 volts.

9)

The yellow wire to the single connector is the signal from the pulse generator mounted on the transmission.
Pulses from this wire signal the cars’ speed to the speed control unit. This pulse signal also works the
speedometer, so if the speedometer is working it is likely that the pulse is also getting to the speed control unit.
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, and even at very low speed the signal will measure around 4 volts AC.
And the voltage does not increase with speed.

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SET SWITCH GROUND: The set switch on the stalk has a ground wire within the column that connects with other
grounds. This wire has a nasty tendency to break off right where it attaches to the stalk, rendering the entire cruise
control system inoperative. Any scheme that will provide a ground wire connection to the stem of the stalk itself will
fix it. The broken wire can be spliced, or a new wire can be clamped onto the stalk just inside the column housing.

BRAKE PEDAL SAFETY SWITCH ADJUSTMENT: There are two different switches used on XJ-S’s, but both need
to be open when the brake pedal is depressed and make a connection when the brake pedal is released. One type switch
is located on the brake pedal housing within the engine compartment, right next to the driver’s side bonnet latch. It is
threaded into the housing with a locknut. The locknut can be loosened and the switch screwed in or out until it works
properly. The other type switch is located in the footwell, and its operation is obvious.

CRUISE CONTROL TROUBLESHOOTING -- SPEED CONTROL UNIT (IN CAR): If the bellows unit is OK and
no problems are found with the cruise control wiring or switches, the only remaining component to check is the speed
control unit. The tester shown in Figure 38 is simple to make, requiring only two flashlight bulbs, two resistors and
some wire. It is also used for the bench test of the speed control unit.

Notes-

a)

Length of the three wires is about
three feet each.

b)

The two bulbs are 3V flashlight type
PR2.

c) The

33Ω and 22Ω resistors are 1/2

watt or larger.

d)

The bulbs and resistors are soldered as
shown.

e)

Small alligator clips will help in
connecting.

f)

All components can be purchased
from Radio Shack, about $6.

Mount this assembly on a piece of cardboard about 4 by 6 inches.

Disconnect the connector from the bellows unit and connect the three wires from this tester to their respective colors on
the wiring harness (not to the bellows unit). The bulbs and resistors take the place of the two bellows solenoids so that
we can observe the operation of the speed control unit. Route this assembly out from under the hood and use a
windshield wiper to hold it against the windshield so you can observe the bulbs while driving.

Next go for a drive. At about 30-40 mph push the “set” cruise button. The bulb on the left (33Ω) 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 speed control unit and associated wiring/switches are good. If this test fails then
the speed control unit is possibly at fault.

CRUISE CONTROL TROUBLESHOOTING -- SPEED CONTROL UNIT (ON BENCH): Bench testing the speed
control unit is not complicated but you do need some experience working with electronic circuits. Essentially, what we
do is connect up the speed control unit with power and grounds as it would be in the car, then feed it a pulse generation

Figure 38 - Cruise Control Test Circuit

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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.

First, fuse the 12v battery supply with a 1 amp fuse; this will hopefully save you from any nasty smokings. The speed
control unit should only draw around 1/2 amp.

Working with the speed control unit and its associated nine pin connector make the following connections:

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 speed control unit. Next a pulse must be sent into it via the single 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.
The generators’ 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 80 Hz, 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 not, replace the speed control unit.

FRIED ECU TRANSISTORS (SALOONS): One of the two switches on the brake pedal is a “last chance” safety
feature for the cruise control system; it is connected between the system ECU output and the actuator itself, and
when the brake pedal is pressed it positively disconnects power to the actuator no matter what else is going on. The
switch in question is item #2 in the speed control schematic on page 86-13 in the ©1982 Supplement, and is
connected into the YW wire between the ECU and the actuator. Hence, when you find the switch, it will have YW
wires on both terminals. It doesn’t matter which wire connects to which terminal on the switch.

The switch used in the XJ-S, C42525 or SMB420, is shaped like a bolt and is located on the left side of the pedal box
inside the engine compartment. This type switch appears to have no problem. The troublemaker is DAC1895, used in
the saloons. This rectangular switch mounts down in the footwell. Craig Sawyers helps identify it: “On the side of the
switch I could easily see were the numbers 1533447, which isn't a Jag number, so it must be Lucas's code to hide the
guilty.” This switch may occasionally short the ECU output to ground, which causes a direct short which flows too
much current and blows a transistor or two in the ECU. It’s not terribly difficult to fix the ECU, but if you don’t
address the switch problems it’ll just get blown again. Even if your cruise control is working, it might not be a bad idea
to check the switch and make appropriate corrections to prevent trouble.

What to do about the switch? There are many possibilities, starting with merely replacing it with a new one. Bruce
Segal says, “We got into the habit of always replacing this switch when replacing the ECU. Comebacks went way
down.”

Sawyers found this fix unsatisfactory. “Some dumb-ass designed the switch body out of steel, which is firmly earthed.
So when the plastic button wears a little in its guide, the conducting end wobbles sideways and shorts to the case.
When I replaced my switch to cure this problem, I found the new switch could be coaxed to short by wobbling the