Jaguar XJ-S. Manual - part 140

 
 

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POTENTIOMETERS:  Tom Wagner says, “There is another product for variable resistors like volume controls and air 
position sensors.  It is a pretty good “stop gap” solution for intermittant radio controls and sensors, but has to be 
sprayed directly on the carbon track.  It does work, have used it for years on noisy radio controls.  Check the can and be 
sure that it is for controls not switches.  In the old days we used “carbon tetrachloride” (just tapped the shop fire 
extinguisher), but that wrecks your liver and can actually be absorbed through the skin.  Illegal and dangerous!” 

 

ELECTRIC MOTOR LUBRICATION: Stefan Schulz and Chuck Johnson Jr. forwarded this procedure, originally 
from Chuck Johnson Sr., for oiling a “permanently lubricated” electric motor: “It is possible to lubricate a 
‘permanently’ lubricated bearing by oiling the wicking that surrounds the bearing.  To do so take a sharp awl (punch) 
and with a hammer punch a hole into the ‘bell’ shaped cover over the bearing housing.  Do this through the vent holes 
in the motor and not in the end of the motor itself.  The wicking is housed on the inside of the motor in a ‘bell’ shaped 
tin cover so it is easy to poke a hole in it.  Then just take an oil can (I use a PLEWS oiler so I can get some volume in 
there but almost any oil can that can put some pressure on the oil will work), and ‘flood’ the wicking.  This way you do 
not have to take the motor apart to get the bearing soaking in oil.  After this you can periodically lubricate the bearing 
by just re-flooding the wick through the hole you have made.  This technique works with all motor types, auto as well 
as small appliance and large appliance motors.” 

The bearing cover that you are punching a hole in is very thin metal, much thinner than the housing of the motor itself.  
If you punch near the center, you may hit the bearing itself, and possibly damage or misalign it.  Punch the hole near the 
outer edge of the cover; there will be nothing under there except the felt that’s supposed to hold oil. 

Of course, some motors don’t have suitable vent openings, so you may have to open the motor anyway.  This method 
still applies, though, since the bearing inside is almost always retained by a permanently-attached cover of this sort and 
oiling is almost impossible without punching a hole. 

Another favorite item for applying the oil is a hypodermic syringe, preferably one with a fat needle.  With a little luck, 
you can buy one in your area without being arrested for drug abuse. 

Now that you have a procedure, you can oil motors periodically or you can wait until they seize up.  Your choice.  Do 
you really believe “permanently lubricated” means forever? 

In the specific case of the XJ-S electric radiator fan motor, Schulz adds “the motor is of the “definitely no user 
serviceable parts inside, so do not open me” variety.  Then again, you can open the thing by forcing the pry slots at the 
top and close it again be replacing the cover and punching down a bit more metal from the side.  Look at one and you’ll 
see what I mean.”  Of course, bending the metal back and forth regularly might result in needing a new motor sooner 
than not oiling it at all.  In these cases, you might try a different idea: drill a hole through the housing itself, aiming for 
the same area adjacent to a bearing, and apply oil without disassembly.  If it is important to keep water or dirt out of the 
motor, cover the hole with a piece of aluminum tape when you’re done. 

 

WIRING HARNESS RENOVATION:  Richard O. Lindsay sends this innovative method:  “Tie the harness into 
position with tie-wraps thereby preserving all of the original bends and more importantly, break-out points.  Remove all 
of the jacket leaving the wires only in position.  This is a good time to clean and degrease all of the insulation.  Then cut 
each wire, one at a time, about a foot or so back from the connector end.  This cut should be well back into the jacket 
away from the breakout point.  This allows you to splice in a piece of generic wire of the appropriate gauge and turn the 
original cut off wire around leaving the nice clean color-correct wire sticking out.  The addition of a correct connector 
makes for a functional harness that, when vinyl wrapped, will look new and be color code correct!” 

Dave Covert sends the following: “The cloth cover is not something you can really buy, but must send your harness to a 
shop and have it wrapped.  The shop has a braiding machine that weaves 32(?) strands of cotton thread around the 
bundle.  Sixteen strands in a clockwise direction, sixteen strands in a counter-clockwise direction.  The cotton strands 
are usually black, but if your original harness had a colored tracer thread(s), send a sample along with the harness and 

 
 

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the shop will switch some of the 32 strands out for colored strands to match the original tracer.  The shop will also want 
you to mock up your harness with a few pieces of electrical tape to hold it in the proper shape. 

“Cost is modest, and varies a bit from shop to shop.  I had good conversations with two different shops, each with 
different pricing schemes.  The first shop was Class-Tech of Bend, Oregon, 1-800-874-9981.  The second shop was 
Harnesses Unlimited of Oreland, PA, (610) 688-3998.” 

If a complete rewrap isn’t called for, Bruce Snyder sends these suggestions: “I’ve had a lot of success with the large 
sizes of heat-shrink tubing available at electronics suppliers.  It’s available in long lengths and a large variety of 
diameters, and looks quite nice when installed.  Of course, you have to be able to slip it over the wires.  The other thing 
that has worked well for me is the dry vinyl and cloth wrapping tape from Eastwood, and the cold shrink tape.  These 
work very well, and have no adhesive to make that sticky mess we all love so well.  These all take a little time to install, 
but look good, are durable, are considerable cheaper than a new harness and don’t involve extricating the old harness 
for re-wrapping.” 

If your problems happen to involve the injector harness, read all about it under the EFI discussion starting on page 275. 

 

RODENT DAMAGE:  One of the members of the XJ-S online discussion group happened to mention that his wiring 
had been damaged by rodents, and it was simply amazing how many members responded with similar experiences!  
Apparently Lucas wiring, along with all its other shortcomings, is found delectable by rats!  The problems usually seem 
to occur up in the V between the heads; it’s probably a nice, warm, cozy spot for a rat to curl up in, and there’s an 
assortment of tasty wires to chew on.  Simon Gray reports, “I spent yesterday replacing spark plug leads, you guessed 
it, mice.  It may have been a rat, either way it took one night to eat through four cables and totally ruin my day (I had 
renewed them three months ago).”  They don’t limit themselves to spark plug leads, either; there are also reports of 
chewed fuel injection wiring and ignition pickup wires. 

Matt Dillon suggests, “My solution was to leave a cheap radio on in my garage all the time.  Apparently the mice don't 
like the noise.  My radio's been on for 2 years without any further rodent attacks.  Until I started playing the radio, my 
cat was attacked twice!!!” 

Perhaps you should get a cat to protect your cat, Matt. 

 

MAIN 12V BUS AND TERMINALS:  On most cars, the battery is in the engine compartment, so it is pretty obvious 
where 12V comes from and how to tap into it.  On the XJ-S, however, the battery is in the trunk.  There is a serious 
cable coming from the trunk along the bottom of the car and up to a post on the firewall, directly under the rear bolt 
attachment of the left side diagonal strut.  From that point there is a short bus directly across to a similar post on the 
right side.  Both posts actually mount in plastic insulators through the firewall, and protrude through on the interior side 
of the firewall to provide similar attachment points for electrical loads inside.  Basically, everything in the car is 
powered from these two posts.  Both ends of both posts are covered with rubber boots.  If you need to tap into a 12V 
supply to power a new stereo or something, these posts are the place to connect to. 

Note that there are no fuses in this supply; if you get a screwdriver between any metal on the car and one of these posts 
with the boot pulled back, you are gonna make some serious fireworks.  It is highly recommended that the battery in the 
trunk be disconnected prior to fiddling with those posts. 

The nuts on these posts -- both ends of both posts -- have a hex that is 0.525” (13.3mm) across.  That’s right, the only 
thing in your tool box that will fit is a pair of pliers!  Craig Sawyers tells us these nuts are 1/4” Whitworth - 5/16” BSF 
wrench size, even though they are neither 1/4” Whitworth nor 5/16” BSF threads.  See page 24 for help finding a 
wrench or socket. 

If you’re not interested in buying oddball tools, perhaps the easiest thing to do is to get the nuts off with pliers and then 
file the flats down until a 13mm socket will fit.  Or, just replace the nuts with normal coarse-thread nuts; the post is 
5/16” with 18 TPI, the same number of threads per inch as 5/16” SAE coarse thread, although the thread face angle and 
root details may be different.  Brass nuts are recommended, even though the originals appear to be plated steel.  If 

 
 

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you’re concerned about the perfect mating of threads, you might consider replacing the post in entirety, if you can find 
a piece of 5/16” brass threaded rod and a selection of nuts. 

 

CONTROLLING INDUCTIVE LOADS:  This book is not intended to be a primer on electrics, but in this case a 
review of some basics is warranted.  Inductance is the characteristic of an electrical circuit that causes it to resist 
changes in current flow.  If the current is zero and trying to get to 3 amps, it will take a little time to get there because of 
inductance; similarly, if the current is 3 amps and trying to get to zero, it will take a little time to get there, too.  
Inductance can be thought of as “inertia” of electrical current; it takes time to get it moving, and it takes time to stop it. 

Inductance is a result of the relationship between electricity and magnetism.  When current flows through a wire, a 
magnetic field is built up around the wire.  Since the magnetic field is actually a form of stored energy, it takes some 
time to build up that energy, which means it takes some time for that current to get going.  Similarly, when the current 
is cut off, the magnetic field remains briefly, but in collapsing attempts to maintain the current in the wire. 

Once you understand the nature of inductance, you really don’t need to have written data on components to know 
which ones have high inductance; it’s the ones that involve wire wrapped around an iron core to form a magnet.  Such 
loads are called “inductive loads”, even though they always have some measurable resistance.  By comparison, a light 
bulb is called a “resistive load” because it has lots of resistance and very little inductance.  In an automobile, electric 
motors (fans, wipers, pumps, etc.), solenoids, and the A/C compressor clutch are inductive loads; ironically, the relays 
handling such loads are also inductive loads in themselves, although obviously on a smaller scale. 

When you initially “turn on” an inductive load by closing a contact, things work well.  The current is initially zero and 
builds smoothly to steady current flow.  When you shut it off by opening the contact, on the other hand, things can get 
very messy indeed.  The current in the inductive load attempts to continue flowing, but the circuit has been broken so it 
has no place to go.  The dead-ended current results in a sudden spike in voltage between the leads to the inductive load, 
which means a sudden spike in voltage across the contacts.  This spike has the opposite polarity of the power that was 
previously connected; if it was +14.4 volts, it is now negative volts, and it can be many times higher than 14.4 volts. 

This spike is really quick.  So quick, in fact, that a set of relay contacts opening that may look instantaneous to the 
human eye looks like a train slowly pulling out of the station to this spike.  When the contacts first break open, the 
voltage will rise so quickly as to jump the gap long before the contact has time to move a couple of thousandths of an 
inch farther away.  The result is arcing at the contacts as they open, which of course chars the surface of the contacts 
and wears them away a little each time.  Some electrical switches and control devices actually have two separate contact 
ratings, one for resistive loads and one for inductive loads -- and the inductive load rating will always be lower, 
reflecting the additional stress of breaking contact with inductive loads. 

Is this a problem?  Well, you might want to think about that.  If the contacts controlling the inductive load are in a 
standard relay in a place that’s easy to get to, probably not; the contacts within a standard relay are often much heavier 
than called for and may last the life of the car, and if they eventually crap out a new relay costs $3.  On the other hand, 
if the contacts are within something expensive and/or difficult to get to (the climate control system, for example), you 
might prefer that the arcing was avoided.  The serious concerns arise when the loads are controlled by transistors rather 
than mechanical contacts; some of those electronic control boxes are astoundingly expensive, and reverse voltage 
spikes don’t do them any good at all.  Problems also arise when a single set of contacts controls both an inductive load 
and something with delicate electronic circuits; disconnecting the contacts can cause a spike from the inductive load to 
go zap the electronic circuits rather than jump the contacts. 

Fortunately, controlling these reverse voltage spikes is really easy.  All that’s required is a diode, which is an electronic 
component that permits current to flow freely in one direction but blocks its flow in the other direction.  Just wire a 
diode across the terminals of the inductive load; when the power is disconnected, the current flowing in the coil will 
simply come out one end of the coil, through the diode, and back in the other end of the coil.  This provides a path for 
the residual inductive current to take without having to jump the gap at the contacts.  The voltage spike is almost 
entirely eliminated, reduced to the gate voltage of the diode -- something less than one volt. 

It is obvious which way to connect the diode; if you connected it the other way, it would form a bypass around the load 
when the power was turned on (and usually a direct short to ground, so your diode vaporizes in short order or a fuse 

 
 

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blows somewhere).  The symbol for a diode includes what looks like an arrow pointing in one direction; this is the 
direction that current will be allowed to flow from + to -.  So, the diode will need to be installed around a load with the 
arrow pointing towards the +12V power source so current won’t flow through it when the power is on.  Some diodes 
are marked with a simple band at one end; this end should point towards the +12V power source. 

Alan Heartfield says, “On modern cars with electronic circuitry, a diode should be placed around every inductive load, 
even relay coils.  In fact, P & B and Bosch sell 'ice-cube' relays with the diodes built in.  These are used as standard on 
most modern transit vehicles to protect the electronics.  The diode doesn't only protect the switching device, but also 
reduces RFI (caused by arcs) and eliminates large reverse polarity voltages from migrating around the car, potentially 
(pun intended) punching pin holes in insulation and reducing the life of electronic parts.” 

So: would it be a good idea for owners to add such diodes to earlier cars that didn’t come with them?  Like they say 
about chicken soup:  it couldn’t hurt.  Diodes are cheap; you can get small ones suitable for relay coils from Radio 
Shack for 25 cents each or so.  The biggest expenditure on your part would be the time and effort to install them.  For 
that reason, you might consider just buying the later design relays that Heartfield mentions and just swapping them out. 
 Note that a basic relay will work no matter which way 12V is connected to its coil, but once a diode is involved 
polarity becomes important; you might want to check that the +12V connection is on the correct terminal before 
plugging in the uprated relays.  If not, you’ll need to reverse wires, which can usually be done by popping the 85 and 
86 spade terminals out of the socket, interchanging them, and popping them back in. 

If you decide to buy diodes, you’re gonna want to know what to look for.  Diodes seem to come with lists of 
specifications as long as your arm, but many of them are unimportant in this application.  One specification that is 
important here is the Peak Inverse Voltage (PIV), which obviously needs to be higher than the 14.4V that the diode will 
be subjected to -- but it’s difficult to find a diode with a PIV rating less than 50, so this isn’t a problem.  The continuous 
current rating is unimportant, this diode will never see a continuous current.  The surge current rating needs to be at 
least equal to the continuous current draw of the inductive load it’s attached to, but the loads on relays are some fraction 
of an amp while the larger loads may be a few amps, and the surge ratings on even tiny diodes can be 30 amps or more. 
 All in all, it’s hard to go wrong; just select some tiny diodes for the relay coils and something more substantial for 
motors and other loads, and they’ll work fine. 

Finding a suitable diode for the A/C compressor clutch is especially easy.  See page 503. 

If you will compare the electrical schematics of early XJ-S’s with those for the 90’s models, this is one of the changes 
you will note; circuits containing a diode and a resistor have been added around many relay coils.  Why the resistor?  
That brings us to more electrical theory.  The magnetic field built up within the inductive load is a form of stored 
energy, and sooner or later that energy must be dissipated.  If a simple diode is wired across the load, then when the 
power is cut off the current will continue to flow around the circuit until the energy is dissipated in the resistance of the 
wire and the diode itself.  This might actually take too long.  Some inductive loads have relatively heavy wire and low 
resistance, and so may take some number of milliseconds to decay.  This can have detrimental effects.  In the case of a 
relay, the slow dissipation of the magnetic field may cause damage to its contacts as they slowly drag open instead of 
popping open as they should.  In the case of fuel injectors, Roger Bywater says, “Some years ago we measured injector 
closing action which with injectors like those on the V12 normally takes about 1 millisecond.  Putting a flywheel diode 
in slows the injector closing down to about 3 milliseconds which is why you never see diodes used that way on injector 
drive circuits.” 

If you install a resistor in series with the diode, the initial current flow when power is disconnected will be unchanged; 
it is still the same amount of current that was flowing when power was on.  However, this current is now flowing 
through an additional amount of resistance, which means it is dissipating more energy.  The current flow will drop off 
more quickly.  Electrically, the way to envision this is that the current through the resistor results in a voltage drop 
across the resistor -- which means we have reintroduced the reverse voltage spike, only now hopefully at a tolerable 
level.  Since the initial current is a fixed value, the higher the resistance, the higher the reverse voltage spike.  The 
voltage then acts against the flow of current and brings it to a stop; the higher the reverse voltage spike, the quicker the 
current stops. 

The ultimate case would seem to be an infinitely large resistance: leave the diode out altogether.  Infinite resistance 
results in an infinite voltage spike, which in turn stops the current in zero time.  Doesn’t happen in practice, however.