Jaguar XJ-S. Manual - part 147

 
 

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Alternative plan B is to forget the stock bracket and simply fashion a bar across the top of the battery.  Longer J-bolts 
can be used, and two 1/4” holes made in the top of the original plastic cover.  Then, after the battery is secured with 
nuts and washers, the cover can be installed and additional nuts -- real pretty cap nuts or threaded knobs, they show 
within the boot -- can be used to secure the plastic cover. 

FYI: the battery compartment in the ’83 XJ-S is 10 inches by 6-3/4 inches.  The ideal battery height must be shorter 
than 8 inches, terminals included.  The height to the flat top of the Champion 78-2 is 7 inches, and it fits with room to 
spare.  It is highly recommended that you confirm the size of the compartment in your own car before shopping for a 
battery; the various documentation on what size battery to use, including those cute computerized battery selector 
displays, seem to be wrong more often than they’re right. 

One general observation about car batteries and warranties:  There are some excellent $60 batteries out there with 60-
month warranties (or longer).  However, some $60 60-month batteries are actually a scheme to sell you a $40 battery 
every 3 or 4 years; it dies, you bring it in and complain, and they happily install another battery just like it and charge 
you the “pro-rated” cost based on how long it lasted -- about $40.  If you think about it, you’ll realize they could just as 
well claim the same battery is good for 20 years and charge you $240 for it -- you’d still end up bringing it back every 
three or four years and paying about $40 for a replacement.  The only thing you’re buying by paying more than $40 for 
the battery is the “hook” that convinces you to keep buying the same battery rather than trying some other make to see 
if it lasts longer.  If you buy a 60-month battery and it dies without due cause in far less time, you might be well advised 
to simply write off that warranty and buy another make -- unless, of course, you don’t mind having to replace your 
battery every 3 or 4 years. 

There are some people who select batteries on the basis of cranking amps.  The logic works like this:  The car only 
requires a certain number of cranking amps to start, and almost any battery will start the car when new.  However, as 
the battery ages, its cranking capacity diminishes with time, until eventually the battery will no longer start the car.  The 
more cranking amps the battery has to begin with, the more it can deteriorate before replacement is required.  This 
theory assumes a lot, including that different batteries deteriorate at comparable rates and that the battery doesn’t suffer 
other types of failure such as internal shorts.  As such, this theory probably doesn’t apply to unusual batteries such as 
the Optima, but may be a fairly valid method of comparing conventional batteries. 

But to this theory must be added another monkey wrench:  In order to maximize cranking amps, the plates within 
batteries must maximize surface area, which is done by making them look like grids or screens rather than plates.  With 
successive charges and discharges, these grids get more and more deformed (metal is etched away when discharging, 
plated back on when recharging, possibly in a different place than it was before), possibly reducing the cranking 
capacity faster than flat plates would.  Also, the grids might fracture more easily with vibrations, causing internal shorts. 
 There are so-called “rough service” batteries available that go the opposite route: they sacrifice cranking amps in favor 
of a rugged plate design that won’t go to pieces if you install it in a truck and drive it across the Baja Desert. 

 

BATTERY TERMINALS:  If your XJ-S has the original battery terminals on the ends of the cables in the trunk, the 
5/16” nuts used to clamp those terminals down on the battery posts require a 1/4” Whitworth wrench size.  See page 24. 

 

BATTERY HOLD-DOWN NUTS:  Also Whitworth. 

 

DEAD BATTERY PROBLEMS:  If your battery is dead after the car sits for a few days, it’s time to check for current 
flow when everything is supposed to be shut off.  The standard method for doing this is to disconnect the battery and 
reconnect it with an ammeter in the circuit, and monitor the amps as you pull fuses one by one.  The current with 
everything off should be in the milliamp range; if it’s amps rather than milliamps, you need to find out what’s being left 
on when the car is off. 

Of course, the first thing you find out is that since the battery is in the trunk, the trunk lights are on while you’re 
fiddling in there.  So you’ve gotta shut off the trunk lights somehow to continue with the test.  It is simply astounding 
how often the test doesn’t find a problem -- because the problem was the trunk lights!  The trunk lid isn’t securely 

 
 

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pushing the button on the switch, so the lights stay on with the lid closed.  Or sometimes stay on with the lid closed, so 
sometimes the battery dies and sometimes it doesn’t. 

Craig Waterman fixed this problem by reinstalling the switch with a couple of washers under it to hold it up more.  
Others have fixed it by bending the bracket it’s mounted on.  The author went a slightly different route to address the 
same problem.  On the ’83, the flat metal surface on the inside of the trunk lid is supposed to be pressing that button.  I 
purchased a set of stick-on plastic chair feet from a hardware store and stuck one on.  Of course, now I have three spare 
chair feet left over.  This fix can be viewed at: 

 

http://www.jag-lovers.org/xj-s/book/TrunkLightSwitch.html

 

Then Stephen Edward Haley noted that on his ’89, adding a chair foot isn’t necessary because Jaguar already did.  They 
didn’t use a chair foot, of course; they manufactured a special part attached with two screws.  Probably cost you half a 
month’s salary from your local Jaguar dealer. 

Note that the early cars didn’t have this switch at all.  They apparently had the more simplistic light bulb in the trunk lid 
itself with a mercury switch operating it.  So, these cars don’t generally have this problem with the trunk lights killing 
the battery. 

 

TRUNK LIGHT CONTROL:  On any modern automobile, the trunk lights have a little switch so you can turn them off 
when you don’t need them.  Jaguar forgot to provide that nicety -- but it’s simple enough to add it.  Just purchase a tiny 
toggle switch and mount it adjacent to the trunk lid switch and wire it into the same circuit.  There’s plenty of room 
there, and it’s easy to find in the dark. 

 

AVOIDING A DEAD BATTERY:  Scott Jarvis suggests: “You can order a Battery Buddy from Outer Banks, (800) 
682-2225, part # batbud.  It mounts to the side of your battery and monitors the voltage.  When the voltage drops below 
a set level, it automatically switches off to preserve the battery.  You just reset a breaker and away you go!  Many of the 
Boating catalogs should carry this and you may find a better price.” 

Kyle Chatman responds: “You can also try Priority Start from BLI International, Dept HR12, 17939 Chatsworth St., 
Ste. 521, Granada Hills, CA 91344, 800-780-8276.  It monitors voltage drain when the switch is off and disconnects the 
battery if it gets too low.” 

Ned Blagojevic counters: “This is available in a kit form from Jaycar Electronics (Sydney), kit number KA-1739.  The 
kit was published in Electronics Australia (Jan 92 issue).  It connects to the accessory line and measures line voltage.  If 
voltage drops to preselect limit between 10.9 and 11.9 it shuts off the flow.  The kit includes a relay and specs.” 

John Setters provides a different alternative:  “Here in NZ an enterprising company called PowerBeat has been formed 
to manufacture the answer to starting problems.  They have developed an automotive battery which I believe is in two 
portions - one section runs the general electrics, the other is reserved exclusively for engine starting.  The URL for 
PowerBeat is 

http://www.powerbeat.co.nz

.” 

Sears also sells a battery with a switch on it, so you can switch to a “reserve” and get started. 

 

 

Fuses and Fuse Blocks 

 

If your XJ-S is old enough to have glass tube fuses, take note: the UK uses a different definition of the rating for these 
fuses than the US does.  The US rating is for how much current the fuse will carry without blowing; the UK rating is 
for the amount of current to blow the fuse within a certain time.  The difference is about 2:1.  Simon S. Johnson sends 
the following data:  “...the source: a 1974 edition of “Buss Fuse Car and Truck List” which has on the back cover a 

 
 

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section call “Foreign Car Fuse Replacement Data,” -- foreign to the US, that is.  It states that “English standards differ 
from US standards.  This accounts for difference in ampere ratings.”  Then it provides a list: 

 English 

Type Buss 

Replacement 

 50 

amp 

 AGC 

30 

 35 

amp 

 AGC 

25 

 30 

amp 

 AGC 

20 

 25 

amp 

 AGC 

15 

 20 

amp 

 AGC 

10 

 

10 amp   

AGC 7 1/2 

 

 5 amp   

AGC 3 

You may find that your fuses have a little paper label inside with both ratings; a Lucas rating (British) and a 
“continuous” (American) rating. 

If your car uses tubular glass fuses and the ROM specifies an amperage, it’s in the British rating; you will have to 
correct per the chart above to use the correct American fuse. 

 

UPGRADING TO PLASTIC FUSES:  Older XJ-S’s have glass tube fuses, both in three main fuseboxes and in several 
inline locations throughout the car.  Later cars have plastic fuses, a type found on many modern cars with the amp 
rating clearly marked on the top and color-coded as to amperage as well; these are called ATO/ATC fuses or “blade” 
fuses.  Now, there are those that will look at the plastic fuses and pronounce them “cheap” -- and they’ll be correct, they 
are certainly cheaper to make than the glass tube fuses.  But don’t confuse “cheap” with “inferior” -- the plastic fuses 
are much better than the glass tube fuses.  The small spade terminals make much more reliable connections with the 
fusebox than the metal caps on the ends of the glass tube fuses do.  And even when the connections are working just 
fine on both, the plastic fuses cause less voltage drop in the circuit. 

If you are new to Jaguar ownership, you’re probably thinking, “Yeah, right.  The glass tube fuses worked just fine in all 
sorts of cars for many decades, they can’t be that bad.”  If you feel that way, go ahead and skip the rest of this section -- 
but remember it’s here.  You’ll be back. 

While you’re pondering the superiority of plastic fuses over glass fuses in general, allow me to point out the superiority 
of generic fuse blocks over the OEM fuseblocks in the XJ-S.  While the fuse blocks in most cars are something you 
never pay any attention to, the fuse blocks in the XJ-S are a known trouble source.  This author had troubles with the #1 
fuse location in the headlamp fuse block in my ’83, the fuse that serves the small electric fan and the horn.  It seemed 
that every time I checked, this circuit was not working -- and I got to where I would tap the horn regularly to check.  
When it quit, usually the fuse itself had failed -- not blown, but rather had lost continuity within one of the end caps.  
Since the small fan was in fact a heavy-duty critter from a Mercedes, I thought maybe it drew too much power, so I 
went to a bigger fuse.  No help, quickly failed again, again not by blowing but by coming disconnected internally.  
Went to a larger fuse still, and the fuse block itself started to melt and issue smoke!  A fuse holder should never get hot 
-- it should either say cool or the fuse should blow.  I decided there was too much resistance in there somewhere and 
abandoned this socket.  I pulled that pair of wires out of the fusebox and connected them to a plastic 20A fuse.  No 
further problems. 

Unusual problem?  Iain Burgess:  “I had similar problems with the A/C blower fuse in the main fusebox on my XJ6.  
The plastic around the terminals that held the fuse eventually melted, for some reason, the contacts in the fuse box 
would heat up, regardless of whether the fan was blowing maximum or minimum.  The fuse would eventually fall out 
and the blowers would stop working (Lucas's second line of defence??)  The fuse never blew.  Irrespective of the 
current draw, there seemed to enough resistance between the fuse and the contacts to generate enough heat to melt the 
plastic.  I replaced the fuse a number of times, cleaned the contacts with contact cleaner, sand paper, you name it, but it 

 
 

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still got hot.  I fixed it like you did, replacing that section of the fuse box with an inline spade fuse - no further 
problems.  None of the other fuses in the box were affected either.” 

Donald Neff:  “You are on the right track - the heat is caused by the resistance of poor contact.  But as you know, since 
you cleaned the fuse tabs and holder sockets, it is not the fuse contacts.  They are probably the best contact in the 
system at any time. 

“I'll bet it is the wire-to-fuse holder connection.  These are usually poorly crimped connections with no protective 
covering.  They work fine when new, but deteriorate slowly over time.  Eventually the wire collects a high-resistance 
coating of oxide between it and the metal of the fuse holder creating a hot resistor at that point. 

“I cut the wire back to clean copper and pry the crimp open to remove the corroded section.  After cleaning the crimp 
tabs I insert the clean wire, recrimp it and then solder it.  The solder not only provides more surface area for current 
flow, it (crudely) seals the copper from oxidants.  It has worked well for me for a few decades.” 

Larry Lee talks about his 1979 XJ6:  “The S2 fusebox can be the source of a variety of seemingly unrelated electrical 
failures, thanks to an interesting design.  Several circuits are supplied by one wire from the battery in some cases.  
Electricity is supplied to adjacent fuse sockets through the ends of the fuses themselves.  Corrosion can be a killer here. 
 Be sure all of your sockets and the ends of your fuses -- particularly the ends on the "supply side" -- are clean.  I had a 
glitch involving my A/C and wipers a while back that was caused by this.” 

Walter Acker IV points out that it’s not that big a job to replace your old-style fuseboxes with new fuseboxes that use 
the better plastic fuses.  Obviously, one method for upgrading to the later style fusebox would be to obtain the 
fuseboxes used in the later XJ-S.  However, this might not be as helpful as one might expect.  For one thing, the 
electrical stuff apparently changed rather significantly over the years, and the later fuseboxes are likely to have fuses for 
components and wiring that didn’t even exist in the earlier cars.  Second, when Jaguar changed fusebox styles, they also 
changed the mountings, so you may have to do more upgrading than you planned just to get them installed. 

The best and easiest solution is to use generic fuseboxes.  In fact, Walter Acker IV and this author recommend basically 
the same type aftermarket fusebox in particular.  One source is 

Painless Performance Products 

http://www.painlesswiring.com/

 

Phone: +1 

817-244-6898 

Sales: 1-888-350-6588  

sales@painlessperformance.com 

Tech: 1-800-423-9696 

 

tech@painlessperformance.com 

This outfit offers several models of fuse blocks, with many having wires pre-attached and even pre-labelled as to which 
fuse serves what load, and some even have a relay for providing a switched source without loading up the ignition 
switch and existing wiring.  But for our purposes, Acker recommends the part number 30002.  This is a simple 4-fuse 
block, but it costs less than $8 and it has a clever arrangement by which you can assemble blocks together to form an 8-
fuse block or a 12-fuse block or any multiple of 4.  It also has no wires connected, just the terminals that snap into the 
box, preferred by people like Acker and myself who would rather terminate wires than splice them. 

A tip on acquiring such fuse blocks: Painless recommends you buy from their authorized distributors, and one of their 
authorized distributors is AutoZone; they don’t stock the Painless items, but will order them for you.  But if you just 
walk back to the rack, you’ll find the exact same 4-fuse block -- except that it’s made by Conduct-Tite!, part number 
115280, and sells for less than $4!  Such a deal! 

The terminals that are used in these fuse blocks come in a strip of four.  Hence, if you’re dealing with one common 
power source, you connect one wire and pop the entire strip into the block and go to work on the other four.  If you are 
fusing four separate power supplies, you merely snip this strip of four into four individual terminals and wire each one 
separately and snap them into the block.  However, Acker points out that you must be careful not to cut away too much 
of the strip.  It is the breadth of this strip that prevents the terminal from passing through the block, so if you trim too 
much of it away you can end up trying to pull a fuse and getting the terminal and wire coming right out of the block 
with it.  Just snip about a 1/8” wide section out between the terminals to avoid them contacting each other and leave it 
at that.