Jaguar XJ-S. Manual - part 139

 
 

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Fig 20.2: Cruise Control, Resume switch and set switch locations reversed 

Fig 22.1: Door Lock Terminal Identification added 

Fig 25.1: Added Diode to EFI and Emissions diagram 

Fig 25.2: Added Diode component location 

Fig 6.1: New wiring diagram for Fog Lamps 

Fig 20.1: New wiring diagram for Cruise Control 

Both publications are numbered: S-57/90.” 

Jeff Strom mentions another source of electrical info:  “The Mitchell manual is a publication of all the wiring diagrams 
for all vehicles.  The older version that was printed had better information than the CD-ROM version out now.  These 
are bought by shops and customer service folks that need the electrical information and not the mechanical information. 
 Cost is in the $1200 range with quarterly updates, covers all cars and trucks.” 

 

JDS:  Later Jaguars are fitted with connections for an electronic diagnostic system.  A knowledgeable mechanic, who 
shall remain nameless here, sends the following words: 

“JDS stands for Jaguar Diagnostic System.  Basically it is a processor that ties into the serial ports in the car wiring.  
However good this may sound, it is no more than a glorified wiring diagram.  It sends you down the circuit you are 
checking and you end up more often with a car that is torn apart and not fixed.  All Jaguar dealers in the US were 
forcibly recommended to purchase one of these $23,000 units back in the late 80’s.” 

“The new P.D.U. diagnostic unit which is supposed to be the new JDS is a self-contained unit that can be taken on road 
tests.  Gen Rad is the manufacturer of both of these machines.  The P.D.U., already dubbed as “Pretty Damn Useless”, 
is a very complex unit.  It uses CD-ROMs instead of 3.5 floppies.  The screen is about 4” square, green display.  It is a 
very difficult unit to use.  The techs that have been to school for the P.D.U. still have very little understanding of it.” 

 

OPTICAL FIBERS:  If the wire is solid dark gray, it may not be a wire; it might be an optical fiber.  Illumination of the 
air conditioning control panel is via a single light bulb in a housing in the console, with the light carried by fibers to the 
various locations.  This makes the whole panel dark with a single bulb failure, but it’s a cinch to replace the bulb; just 
remove the ski slope (3 screws on early cars, see page 486 for later models) and replace the bulb inside the fiber optic 
unit within the console. 

Don’t cut those fibers; they’re not easy to splice.  They are a little brittle, so don’t bend them too sharply either.  They 
tend to get in the way when working on the radio, so be careful. 

If you need to disconnect one from the fixture at either end, don’t just yank.  Insert a tiny screwdriver into the slot along 
the side of the socket and twist to spread it a little, and the fiber will come out easily.  It has a little brass fitting on it 
with a lip.  To put it back, simply press it in until it clicks. 

Note that the sockets on the bulb housing are not all the same.  If you open it up and look inside, you will note that 
some of the sockets feature a colored filter.  Which socket used will determine what color light comes out the end of the 
fiber. 

If you need to try to splice a fiber, Don Mathis of the Lightguide Media Department at AT&T Bell Labs says: cut the 
ends of your plastic fibers with a razor blade.  This should give a very smooth cut.  You need to butt the two fibers 
together while you epoxy them in that position.  A “V-groove” works well.  If you come up with a means of clamping 
the two fibers together mechanically, index match grease between the ends helps to decrease the loss.  Silicone grease, 
clear, works well.  Vaseline is not bad either. 

If all else fails, Edmund Scientific has the fiber for approximately $.70/foot for 0.040” diameter.  You can also get 
genuine Jaguar fibers from several mail order outfits, but they aren’t cheap. 

 
 

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WIRE SPLICING:  Making durable, reliable wire splices is essential to working on a Jaguar; there are a great many 
electrical circuits, they tend to be rather complicated, and the Lucas components cause enough trouble.  When 
troubleshooting, it is important to be able to eliminate a previously-made splice as a possible fault. 

First, it is helpful to have a pair of wire-stripping pliers around -- a good pair.  A good wire stripper will remove 
insulation from the tip of a wire neatly, doing much less damage to the conductors than you can do with a razor blade, 
or your teeth, or whatever.  However, a cheap wire stripper, especially one in which the stripping slots don’t line up 
properly or are not sharp enough, can cut half the copper strands while removing the insulation. 

When splicing wires together, the best way is to solder them -- if they won’t be exposed to a great deal of heat, which 
may melt the solder.  A soldering gun of about 140W capacity is recommended; soldering irons are intended for circuit 
board work, and do not work well doing wire splicing.  On larger wires, an iron may not provide enough heat to make a 
secure connection.  And, the intermittent nature of wiring harness repair makes the instant heating of the soldering gun 
a big help.  Even the little light bulbs usually found on soldering guns tend to be helpful in automotive work. 

If your soldering gun isn’t heating like it should, loosen the nuts holding the tip and retighten securely.  These are 
electrical connections (a soldering gun is a transformer that provides low voltage and high current through the tip to 
heat it), and they need to be tight. 

Regardless of whether the solder used says “resin core”, you should use a separate tin of resin flux.  The first time you 
use it, you will know why this is recommended; relying on the resin in the solder doesn’t work nearly as well.  Do not 
use an acid flux; it is intended for copper pipe connections, not electrical work.  And, before doing any soldering, 
always dip the tip of the gun in the flux and apply a little solder to the tip as it heats up. 

Another workable splicing method, and the method to use when exposure to heat is a factor, is to use a crimp-on 
connector.  If the crimp-on connector is the uninsulated variety, it may be possible to combine methods; crimp the 
connector to the wires, and then apply solder. 

Crimp-on connectors can be purchased in automotive stores, often in a package along with other types of crimp-on 
terminals.  Some of the connectors will have a built-in piece of insulation, while others are bare.  Keeping a selection on 
hand is a must. 

Most of the available electrical connectors work well, but there are a couple specific types to avoid.  One to avoid is a 
tap connector that consists of a plastic device than is placed over an existing wire, a new wire is put in place alongside 
it, a slotted metal guillotine blade connector is squeezed into place with a pair of pliers, and a cover is folded over and 
snapped in place.  While slick, this connector makes weak and unreliable connections, especially on unusual wire sizes. 

Where possible, avoid the use of electrical tape.  With age, it tends to harden, while the adhesive gets gooey.  After 
some time, tape on connections can be found to have fallen off or slid up the wire, leaving the conductor exposed.  If 
electrical tape must be used, it should ideally be stretched a little as it’s applied; the stretch will pull it tightly around the 
conductors, helping prevent its coming loose anytime soon. 

Please don’t use friction tape.  Electrical tape is solid plastic, while friction tape is black cloth.  Friction tape is not 
intended for electrical work. 

If you are connecting separate ends of wire or can slip something over the wire from an end elsewhere, the best 
insulation method to use is heat-shrink tubing.  Heat-shrink tubing is available at some auto supply stores, hardware 
stores, and building supply stores, but the best place to buy it is at an electronics store.  At the better electronics stores it 
can be purchased in 4-foot lengths and in a great variety of sizes.  It also comes in various colors, including near-
transparent so you can see how lousy your soldering job was. 

Select a size of heat-shrink tubing slightly larger than the insulated wire, and cut a piece a little longer than your splice 
will be.  Slide this piece onto one of the wires before you connect the wires together.  After soldering, slide the tubing 
over the connection and use a cigarette lighter or match to shrink it down snugly.  Heat-shrink tubing can also be used 
to insulate uninsulated crimp-on wire connectors. 

 

 
 

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CONNECTORS:  Since the basic wiring connectors themselves are among the worst features of Lucas engineering, it 
is recommended that the owner keep a supply of Molex connectors (such as those sold at Radio Shack) on hand.  When 
a connector is intermittent or is otherwise causing trouble, don’t try to clean it up; simply cut the sucker off and install a 
suitable Molex connector in its place. 

Some hardware or building supply stores carry a substance for preventing oxidation and corrosion of electrical 
connections.  One such substance is called Ox-Gard, by Gardner Bender Inc. of Milwaukee; it comes in a 1 oz. tube and 
has the consistency of grease.  Since Jag electrical connections tend to corrode, it is suggested the owner keep a tube of 
this stuff around and use it.  The first place to apply it is on both ends of each fuse you can find. 

Tom Wagner says, “I have a number of associates that actually grease the lamp socket base.  They swear that it keeps 
the moisture out and prevents rust.  I have no emperical data to confirm this and really don’t have the guts to try an 
experiment.  I have noted however that some cars (especially Fords) have a type of white grease packed into the 
connectors.  Unfortunately every time I have seen this material it is because I have been replacing the device, so I don’t 
know if the device has failed due to it or whether it is just a normal failure.” 

 

GROUNDS FOR COMPLAINT:  This author has noted several places in the XJ-S where a ground connector is 
attached to the chassis with a bolt or screw that also holds a harness strap.  The harness strap is plastic, and is held 
between the ground wires and the chassis.  This is NFG for at least two reasons:  First, it means that the only way the 
ground connection can possibly be made is through the screw itself, which is less than half as reliable as a properly 
seated ground connection.  Second, since the strap is plastic, the screw simply cannot be tightened securely as that 
would only crush the strap. 

The car has enough electrical problems.  It is recommended that whenever such a ground connection is found, it should 
be revised.  One method is to provide two separate screws, one for the harness strap and one for the ground.  Be sure to 
scrape all the paint away under the ground connection before reinstalling it.  Maybe a little anti-seize compound would 
help, too -- both in assuring a good electrical connection and in keeping the bare metal from rusting. 

So far, such ground connections have been noted on the wheel wells just behind the headlight housings on both sides, 
and inside both doors near the hinge end. 

 

MORE GROUNDS FOR COMPLAINT:  Colleen Melton says, “the front subframe is grounded to the chassis through 
the ground straps (two straps, 1 from chassis to subframe, 1 from subframe to engine).”  Mike Morrin responds, “You 
just made the light go on for me.  I always wondered why my car has a ground strap with both ends bolted to the 
subframe.  There is another strap from the engine to the chassis.”  Morrin had been driving around with an ungrounded 
subframe due to somebody arranging these straps incorrectly.  There have been multiple reports of these straps being 
connected incorrectly or missing altogether. 

Having the subframe ungrounded may have little effect, but having the engine ungrounded is a disaster.  Check out the 
history of engine grounding problems through the throttle cable (page 267) and shift cable (page 323).  Just go ahead 
and buy a suitable ground strap and install it in addition to the existing one.  Convenient places to install a new strap 
include between a bolt on a front anti-sway bar mount bracket and anything on the bottom front of the engine on 
whichever side you choose -- power steering pump mount bracket, alternator mount bracket, whereever you find the 
right size bolt.  Remember to provide enough slack for the engine to move around on its mounts without straining the 
cable. 

 

SPADE TERMINAL INSULATION:  In the places that the XJ-S has female spade terminals to connect to male spade 
lugs on a device -- such as on the starter relay at the right rear corner of the engine compartment -- the female terminals 
on the ends of the wires have a milky white plastic sleeve that snaps in place over the terminal to prevent accidental 
shorts.  These insulators are guilty of misleading in two different ways:  1) When the terminal is pushed onto the spade 
lug on the device, the lug may enter the insulator between the flat side of the terminal and the sleeve rather than into the 
terminal itself.  This may actually feel like it was inserted properly, and will usually provide a connection when tested -- 

 
 

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but it will be intermittent, and you will have problems sooner or later.  2)  When the connector is pushed onto the spade 
lug, the terminal itself may slip backwards within the insulator without being noticed.  It looks like it’s in place, but the 
terminal itself is actually only touching the tip of the spade lug, again making an intermittent connection. 

A very workable plan is to take these insulators off and throw them away, and insulate the female spade terminals using 
heat-shrink tubing. 

If you are installing new female spade terminals on a wire, it is suggested that you use two different sizes of heat-shrink 
tubing to insulate them.  This works far better than the little plastic collars that come on crimp-on terminals, so it is 
suggested you rip the plastic collars off and use this method instead.  This also allows you to solder the wire to the 
terminal after crimping for a more secure electrical connection.  To insulate the terminals, take a piece of heat-shrink 
tubing about 1/2” long and a diameter that will fit over the wire itself as well as the crimp end of the terminal and slide 
it onto the wire before attaching the terminal.  After crimping and soldering, slide this piece over the crimp connection 
and use a match to shrink it down onto the crimp itself, leaving just the spade terminal exposed.  Then cut a piece of 
heat-shrink tubing of a diameter that will fit over the female spade terminal itself about 1/2” long and slide it on until 
the end is flush with the business end of the terminal, and shrink it in place so that it covers the terminal and overlaps 
the first piece over the crimp.  The end result is a two-level insulation job that looks professional and even provides a 
measure of strain relief to the wire connection. 

It’s possible to do a similar two-level insulation job to a male terminal using a very short piece of the larger heat-shrink 
tubing, but it doesn’t surround the terminal itself when disconnected, so unplugged terminals involve some risk of 
bumping into things and shorting.  It works just splendidly when plugged in, though. 

 

CONTACTS:  Vince Chrzanowski of Baltic, CT restores old auto radios for a living.  He recommends Channel Master 
COLOR contact Shield, Silicone Base, which is available at most electronic supply houses.  Model 9101 is the 16-oz. 
can; model 9100 is a 6-oz. can of the same stuff.  He claims many rocker switches, even many of those that appear to be 
broken, can be fixed by spraying this stuff through the cracks without even removing the switch from the panel! 

Chad Bolles likes LPS 1.  “It is greaseless and does not attract dirt and is made for elec systems.”  Tom Wagner adds, 
“CRC has a chemical for cleaning too.” 

Tom Wagner warns against confusing electrical contacts with connectors and using the wrong product:  “My major 
concern is that folk will generically use contact cleaners without actually knowing that fuses and the like are actually 
connectors.  Never, I repeat, never use a contact cleaner on them.  They are for rotary switches and will do more 
damage than good.  Often a cleaning with alcohol and a clean business card is all that is needed.  There are chamois on 
sticks for cleaning VCR heads at Radio Shack that are excellent.  Never use a Q-Tip, they will leave material behind.  
For pin connectors, clean all the dirt you can out and use alcohol followed by air to take any surface residue out.  Then 
a simple repeat removal and insertion of the connector a number of times is often all that is needed.  Stubborn cases 
usually require more drastic means.  A typewriter eraser used gently is a very good burnisher.  Round wood toothpicks 
can be used to clean the female part of sockets.   

“Check for broken or cracked parts.  They can also show where failures might be; the electronic circuits very seldom 
break down, but sometimes the plastic they are attached to allows the connections to fatigue, resulting in poor 
connections.” 

 

SWITCHES:  If you’re looking for generic switches to fit the existing cutouts in the console or dash panels, Craig 
Sawyers explains your problem:  “The aperture is 22.5x30mm, and you will find that the vast majority of rocker 
switches are 22.5x27.2 (or very close).  What you need is a switch specified as "European cut out", which is 
22.5x30mm.  Now I'm in Europe, and why these switches don't dominate the market is anybody's guess.” 

 

ROLLER MICROSWITCHES:  The microswitches on the throttle linkage and on the shifter look tricky with their little 
rollers and all.  However, they are in fact a standard item, and are readily available at your local electronics store -- 
complete with identical rollers.