Jaguar XJ-S. Manual - part 168

 
 

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If you really like that 10-second delay, you can opt to keep the green relay in the circuit.  Leave the WK and NP wires 
connected to the delay relay the way they originally were.  Provide a connection from the NP wire to the 12V power 
connection on the antenna, which may require tapping into a line.  Then connect a wire from terminal 4 on the delay 
relay (where the UW wire was originally connected) to the signal wire on the antenna.  Tom Graham says, “The Jag 
relay spade connector we want will have voltage (battery) on it when the radio is on.  Check this with a volt meter to 
chassis ground.  Then turn the radio off, the voltage will drop off in about 15 seconds (this is how the delay works).”  
Note that terminal 5 on the delay relay, where the UR wire was originally connected, is not used in this scheme. 

 

 

Miscellaneous Interior Stuff

 

 

SEAT HEATER:  Later XJ-S’s come with a seat heater, and apparently it lacks reliability.  Stefan Schulz sends a 
description of the repair of this unit: 

“The seat is connected to the car electrics through three different connectors, one for the seat belt logic (cable runs 
under centre console, pull carefully to expose connector), one for the lumbar pump, and one for the seat heater.  The 
latter two are under the seat and can be accessed most easily by moving the seat as far to the rear as possible. 

“Having disconnected the seat heater connector, check with a voltmeter whether it delivers power when the seat heater 
is switched on.  If it does, the problem is somewhere in the seat.  Remove seat. 

“Turn seat upside down in a clean area.  Locate the connector that connects the bottom seat heater in series with the one 
in the backrest.  Pull it apart.  Use an ohmmeter to figure out whether the bottom or the backrest heater is faulty. 

“If the bottom heater is faulty, remove the black rubber cover from the bottom of the seat.  See where the heater power 
supply wiring enters the bottom cushion ?  Good.  Carefully pull it apart at that point, exposing the top of a cheap and 
nasty heater element. 

“Cut the top covering of the heater element to one side of the thermostat and flip it over to the other side to expose the 
thermostat element.  Don’t cut it away, you’ll need to put it back later. 

“There are three joints within the seat heater element.  Orange/slate wire to thermostat, thermostat to heater element, 
heater element to black wire.  Examine all three joints.  Note that they get hot (hey, they’re part of a heater) and are 
moved and flexed constantly.  Solder joints should never be used in areas that get hot or which are under mechanical 
stress like the one these wires are experiencing.  So what did the cheapskate Jaguar designers use?  Exactly.  The 
thermostat is cheap too, and its connection lugs will be badly oxidized. 

“Take out the thermostat and subject it to the usual boiling water/ice water routine to test it.  Check with an ohmmeter 
that it opens when in hot water and closes when in cold.  Being more precise with a cheap part like that is a waste of 
time.  If you find that the thermostat is faulty, you’ll see that it is not a Jaguar part.  Jaguar wants you to replace the 
entire heater and cushion assembly.  But this ‘stat doesn’t do anything any other 45°C/12V/10Amp bimetallic ‘stat 
wont do, so get a replacement from an electronics shop if necessary. 

“Solder the thermostat back in, using weapons-grade solder wire with a high silver content and consequently high 
melting point.  You did remember to dry and clean the connection lugs first, of course.  Re-solder the third connection 
(heater to black wire) as a matter of course. 

“Squeeze the thermostat back into the cushion, make sure that none of the heater wires touch it.  Put back the top 
covering using solvent-free glue and a staple at the end.  If you use glue containing solvent, you will find that that 
works the same way as the naturally occurring rot of the seat foam, only a lot faster - seconds instead of years.  Use an 
ohmmeter to check the resistance offered across the seat heater connector now - it should be about 1.8 ohms. 

“Refit all the other components by reversing the removal sequence.  Put the seat back in the car and connect it 
(remember the seat belt logic connector!)” 

 
 

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GLOVEBOX LIGHT:  The XJ-S doesn’t have one!  What a cheap car.  It’s easy enough to install one, though.  Just 
buy a suitable light fixture at your favorite auto parts joint, and install it in the top of the compartment just behind the 
latch.  Wire it into the interior light switch just to the left, so when the switch is operated it will turn on both the right 
front interior light and the glovebox light. 

In the author’s ’83, there is an unused 2-conductor connector hanging out of the harness directly above the glovebox -- 
you’ll need to take the glovebox out to see it.  The wire colors are R and B.  This may have been intended to power a 
glovebox light at one time.  The B wire is a ground, and the R wire comes from fuse #15 in the auxiliary fusebox -- 
which is the fuse to the right front parking and marker lamps.  Hence, if a glovebox lamp were to be powered from this 
connector, the lamp would be on whenever the parking lights are on.  If you don’t like that, you would need install a 
glovebox door switch in the circuit so the lamp was off unless the glovebox was open. 

There are other implications of using that connector.  Any current drawn through it comes through the bulb failure 
sensor for the right front corner of the car.  Hence, the increased current could fool the sensor into indicating that the 
bulbs are all OK when, in fact, one is burnt out.  If you don’t provide a glovebox door switch, you probably should 
readjust the tiny screw on the bulb failure unit to put its sensitivity back where it belongs. 

To avoid getting confused by voltmeter readings, remember that bulb failure sensor has resistance in it.  On the author’s 
car, the voltage at that connector was about 10V while the battery was at a solid 12V. 

Yet a third power source possibility is a dedicated power source with a switch -- a glovebox door switch or even just a 
manual toggle inside the glovebox.  There are lots of solid brown wires in the area; all of them have 12V whenever the 
battery is connected, and all of them have enough capacity to handle a glovebox light with no sweat.  The main firewall 
power terminal isn’t too far away, but you’ll have to remove the rubber duct to get to it. 

 

CIGARETTE LIGHTER:  Apparently, some XJ-S’s were equipped with some sort of non-standard cigarette lighter.  
This causes two problems: first, when the element in the lighter quits working, it’s hard to find a replacement; and 
second, it may prove problematic to plug accessories into the cigarette lighter hole.  If you are having either of these 
troubles, the easiest solution is to drop by any auto parts store and buy a generic cigarette lighter and install it, and 
throw that hokey Jaguar one out. 

 

WHERE IS LUCAS TODAY?  “Lucas merged with the Varity corp in Sept. ’96.  Varity is what was left of Massey-
Ferguson after it was chewed up along with Kelsey-Hays.  The president of Varity is now in control of the new 
LucasVarity and it seems that he has no love for Lucas or the name.  US automotive operations will cease in the next 
few months for Lucas and will only be represented by a company called AutoSpecialty which was just recently 
acquired and based in LA CA. which will market no Lucas product but will be owned by LucasVarity.  AutoSpecialty 
supplies undercar and braking products.  LucasVarity will continue for a while in Europe but a team is taking surveys at 
present for a new name for the merged company, and it won’t be Lucas.” 

 

 

Cruise Control 

 

PRE-1992 VS. 1992-ON:  Richard Mansell quotes from a Jaguar publication describing the changes for the 1992 
model year: 
 

“New cruise control: The V12 models now feature the Hella speed control system as fitted to the pre-92 MY 3.6 
coupe in place of the AE Econcruise system. This gives more logical layout, more accurate control of speed, the 
addition of a cancel function.” 

As of now, this book only addresses the pre-1992 cruise control. 

 
 

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PRE-1992 CRUISE CONTROL:  Thanks for the following procedures for troubleshooting the Jaguar cruise control 
system go mainly to Tom D. Graham. 

 

CRUISE CONTROL DESCRIPTION:  An electrical signal from the drive train tells the cruise control system how fast 
the car is going.  On early XJ-S’s there was a dedicated sending unit near the input flange on the differential.  Later 
XJ-S’s split a signal from the speedometer transducer on the transmission.  On still later models, the speedometer 
transducer is built into the differential unit itself.  See page 590 for notes on these senders. 

The signal is processed by the cruise control electronic control unit.  This ECU directs the amount of vacuum in the 
bellows unit and the bellows unit operates a cable to the gas pedal/throttle. 

Within the bellows assembly are two solenoids, one which normally (cruise control off) vents the bellows to 
atmosphere and the other which normally seals off the vacuum line from the intake manifold.  The “vent solenoid” is 
connected to the yellow/white and black wires, and the “vacuum solenoid” is connected to the yellow/white and 
yellow/black wires.  When the control unit calls for speeding the car up, 12V is applied to the yellow/white wire, 
activating both solenoids.  The vent is sealed and the vacuum line is opened, and the resulting vacuum within the 
bellows pulls the cable, applying throttle.  The speed control unit modulates the ground connection of the vacuum 
solenoid to apply the proper vacuum to maintain a constant speed. 

On the brake pedal housing is a switch that breaks the yellow/white wire whenever the brakes are applied, effectively 
shutting off the system and allowing the throttle to return to idle.  This is actually a back-up feature; the power from the 
brake light switch is also sent to the speed control unit, which is supposed to drop the power to the yellow/white wire.  
Note that if the brake light circuit fails and doesn’t put the cc into decel mode and the backup switch is the only thing 
that disables the actuator, as soon as the brake pedal is released the cc will resume and the car will accelerate. 

 

CRUISE CONTROL RECALL:  As the result of a recall, there is usually a blue solenoid valve installed in the vacuum 
line to the bellows unit.  This device is designed to seal the vacuum line and vent the bellows unit in addition to the 
solenoids.  It’s mounted at the right front corner of the engine. 

Of course, it might not be installed correctly.  Dave Kautz says, “I've been fussing off and on with my cruise control, 
trying to figure out why it doesn't work.  This past weekend I bypassed the little blue solenoid valve mounted at the 
front of the right-side cam cover.  I believe this part was added as part of a retrofit as it doesn't show up in any of the 
documentation I have on the workings of the cruise control.  Well, bypassing it didn't fix my cruise control but it did 
lower the idle speed several hundred rpm indicating to me that it was behaving as an air leak even it the de-energized 
state.  It looks to be a three-way valve with the cruise servo side vented to atmosphere when the valve is (supposedly) 
closed so the air being drawn in was un-filtered - lovely...  I mention all this as one more place to check when folks are 
chasing down a high idle problem.  After hooking the vacuum line from the cruise control servo directly to the manifold 
tap it took more than a full turn on the idle "screw" to get the idle back up to 750 rpm.” 

Kautz’s solenoid valve had been installed backwards, probably by a dealer mechanic performing the recall.  It is a 3-
way solenoid valve, but when de-energized it’s supposed to connect the cruise control servo to atmosphere, not the 
intake manifold!  And it’s supposed to have a little filter on it so that even the cruise control servo doesn’t get unfiltered 
air. 

 

CRUISE CONTROL ADJUSTMENT:  If the cruise control is adjusted correctly, when the set switch is pressed the 
system will maintain the speed the car was doing at the instant the button was pressed.  If adjusted incorrectly, it will 
maintain either a higher or lower speed.  Hence, the test procedure is drive the car on a straight and level road and press 
the set switch, allow the speed to settle about ten seconds, and press it again.  If adjusted properly, it can be set over and 
over and still maintain the same speed.  If incorrectly, the repeated sets will result in gradually higher or lower speeds. 

 
 

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If adjusted incorrectly, adjust the speed control unit until correct; it is a simple matter to have the speed control unit 
hanging under the dash (it is located above the passenger side footwell) and the necessary tools along for the test drive, 
and preferably an assistant driving.  Although the repair manual describes a method of adjusting the cruise control, 
some of these units have no obvious adjustments.  If you pry the box open, however, there are two adjustable pots on 
the circuit board.  The one to adjust is the one in the corner, farthest from where the wires enter the box.  NOTE: The 
adjuster is very sensitive; it is difficult to move it a small enough increment. 

 

SERVO TROUBLESHOOTING & REPAIR:  If you have an ohmmeter, unplug the connector to the bellows unit and 
check the resistance across the bellows solenoids, yellow/white to black and yellow/white to yellow/black.  Each 
solenoid should register 25-30Ω. 

Connect the black wire from the unit to chassis ground and the yellow/white wire to 12V using a jumper wire.  You 
should hear a click.  Disconnect the black wire from ground and connect the yellow/black wire to ground, and once 
again connect the yellow/white wire to 12V.  You should hear another click.  This verifies that both solenoids are 
moving. 

Connect both the black and yellow/black wires to ground and the yellow/white to 12V to activate both solenoids, and 
suck on a hose connected to the bellows unit.  You should be able to move the bellows.  If you can’t seem to suck 
anything, the vacuum solenoid isn’t opening (or the hose is kinked or plugged).  If you seem to suck air easily without 
accomplishing anything, either the vent solenoid isn’t closing or the assembly is leaking. 

The following comments apply to the older cruise control actuators that had a bellows that looked like a bellows, and a 
flat metal disk that the cable attached to.  Newer cars use a different actuator, and some even attach directly to the 
turntable -- a no-no with the older throttle cable design. 

This cruise control actuator can be disassembled easily.  Remove one bolt at the front end and disconnect the throttle 
cable from the disk at the rear end (don’t lose the little cable attachment thingy!) and remove it from the car.  Then peel 
the bellows away from the disk the cable attaches to, and away from the solenoid housing the same way. 

These units seem to have several common failure modes.  The first and most obvious is that the bellows leaks around 
the edges where it snaps over the metal disks; even a small leak is enough to render the system totally inoperative.  If 
this problem is suspected, it is a fairly simple matter to peel the bellows off at both ends, apply some silicone sealant, 
and reassemble.  Another suggestion that’s been made: use a clamp intended for a dryer hose around the edge of the 
bellows to hold it securely against the metal disk. 

Another common problem is the bellows itself develops a tear or leak.  You can easily check if the rest of the system is 
operational by patching the leaks, using a bicycle tire patch kit, tape, or whatever.  It may not last, but it will tell you if 
the rest of the system is OK. 

It has been suggested that using Son Of A Gun or some similar substance on the bellows may help protect the rubber 
from aging. 

Gregory Andrachuk describes repairing another failure mode, sticky solenoids: after taking the bellows off, “I simply 
lubricated them liberally with Liquid Wrench (like WD40, but has Teflon).  The cruise functioned perfectly...” 

Yet another common failure is that the tiny rubber seats on the solenoid plungers fall off, and the solenoids no longer 
seal the ports.  Bill Farnsworth went looking for why his cruise control quit working: “I took the bellows unit apart 
today to reseal it.  Lying in the bottom of the bellows was a small thin metal disc about 5/16in. in dia.  It had a slightly 
smaller thin rubber disc glued to one side of it.  I took up the solenoid half of the bellows unit and gave it a close 
examination.  Guess what was missing from the top of one of the solenoid plungers.  Yep, that little tiny disc.  So I 
glued it back on, sealed the bellows and the Cruise Control works again.  If I just pulled the bellows off and tipped it 
over, the part would have fallen out onto the engine and I would never have known about the displaced disc.” 

After removing the bellows, the solenoids can be removed by prying them out of the housing to get at the plunger.  If 
you’ve lost or damaged the rubber seat, use a hole punch on a bicycle inner tube to make a new one.  Make sure the 
sealing surface is flat and smooth before reassembly.