Jaguar XJ-S. Manual - part 146

 
 

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REPLACING THE AIR PUMP WITH A BIG-ASS GM ALTERNATOR:  John Napoli doesn’t cotton to those 
newfangled itty-bitty alternators, he prefers the earlier clunkers.  So he installed a “large case” GM alternator, which 
you can buy for a song since nobody else wants them any more.  Mounted on the air pump bracket, the tensioner lug 
was too high and cocked the tensioner rod in the clevis on the front of the right bank.  It also interfered with the right 
side radiator hose and thermostat housing.  And the centerline of the pulley was so high that interference with the fan 
belt idler was a definite problem. 

Napoli solved all these problems by making a simple plate to relocate the air pump mount bracket downward.  “Take 
the steel bracket that supported the base of the air pump.  Take a piece of flat steel or aluminum sheet (I used 
aluminum) about 5/32” thick (you want stiffness but too thick and you’ll need longer bolts, and who needs the extra 
weight) to use as an adapter plate.  The piece should be as wide as the stock air pump bracket and twice as long (high).” 
  That plate will need to be 4” wide and about 5½” long.  Of course, using a thicker plate and buying longer bolts 
(you’ll only need two) will also work; the one thing you don’t want to do is use bolts that are too short, since they are 
threaded into aluminum. 

“The stock air pump bracket has four bolts that secure it to the side of the block (via an aluminum block bracket casting 
itself attached to the block with two large bolts).  The plate you’ll make will be drilled for six bolt holes, and is mounted 
sandwiched between the alternator (via the steel air pump bracket) and the block bracket.  The upper two holes are used 
to bolt the adapter plate to the upper two holes of the block bracket.  The middle two holes are used to bolt the air pump 
bracket and adapter plate to the lower two holes of the block bracket.  The lower two bolt holes of the adapter plate bolt 
the air pump bracket to the adapter plate.  Got it?  Use the air pump bracket as a pattern for the bolt holes on the adapter 
plate. 

“Bore a large hole in the top center of the adapter plate to clear the forward bolt that attaches the block bracket to the 
block -- once again, use the air pump bracket as a guide.  Polish the adapter plate to a high gloss to add several miles 
per hour.” 

 

REPLACING THE AIR PUMP WITH A GM ALTERNATOR -- OTHER IDEAS:  If you’re removing the air pump 
and need to install a GM alternator to serve as an idler, you can hang the alternator off the bottom of the air pump 
bracket as described on page 576.  The difference: with that installation, the air pump stays and the alternator is spaced 
rearward from the front of the bracket to align the alt pulley with the groove on the damper.  But with the air pump 
removed you’ll want to install the GM alternator right up against the front of the air pump bracket so the pulley lines up 
with the A/C compressor pulley. 

You might consider flipping the air pump bracket upside down.  This will require drilling one large hole to clear the 
head of a bolt on the alternator bracket underneath. 

 

ALTERNATOR POWER WIRING:  After you’ve got your new alternator bolted in, you’ll need to connect the big 
power lug to the main power line to the battery -- which means either to one of the two posts on the bus on the firewall 
or to the post on the starter.  The original alternator was connected to the post on the starter with two wires, but those 
wires may not be long enough to reach the new alternator location.  David Osborne says, “An extension of the power 
cable from the alternator was required and a bubble-wrap Exide #6915608 6 gauge Quick Splice Battery Cable did the 
trick.” 

The wiring from the main power connection on the alternator needs to be suitable for handling the rated output of the 
alternator continuously in an already hostile environment -- things are hot in there without any current in the wires!  If 
you are replacing a 66- or 75-amp alternator with something over 100 amps, you will be loading the wiring and 
connectors with more current than they were intended to handle.  You may find that the two large wires are already a 
bit charred at the back of the alternator. 

If you install a more powerful alternator, you really should upgrade these power leads while you’re at it.  Just replacing 
these two large wires with two larger wires would be tough, since they go through a conduit to the starter terminal; you 
basically need to pull the exhaust system from that side to work on it!  Better idea: simply install an additional heavy 
wire from the main terminal on the alternator to the main bus on the firewall.  Just route it along the right side wheel 

 
 

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well, making sure to leave enough room for the wire to flex as the engine moves around on its rubber mounts.  Joe 
Bialy provides some guidance on what size wire is adequate:  “#6 gauge would be great, #8 is acceptable, #10 
marginal.  Three #14's are a wee bit better than one #10. Three #12's would work like one #8.” 

The wiring from the starter terminal to the main bus and then all the way back to the battery is already more than 
adequate, of course, since it must handle starter current -- several times what alternator current will ever be. 

Another idea: if you install an electric radiator fan with a control relay, you could run a wire directly from the alternator 
to the power input contact on that relay.  That way, the fan (usually more than 10 amps) is powered directly through 
this wire, and the load in the other wiring is reduced accordingly.  Of course, you can provide similar wiring to the 
OEM electric radiator fan relay and the headlight and fog light relays. 

 

ALTERNATOR GROUNDING:  Keep in mind that the full rated amperage of the alternator -- 66 or 75 amps with the 
OEM Lucas up to over 100 amps with the Bosch or GM alternators -- is expected to go through the bolts that hold the 
alternator to the side of the engine.  Believe it or not, these bolts are not always up to the task -- especially if there’s a 
bit of oil, Loctite, or corrosion involved. 

Rick Holland learned that there are reports of serious problems with grounding the GM CS130 alternator in particular.  
“Alternators supposedly were failing because of insufficient electrical grounds to the rear case of the unit.   My 2nd 
unit, a rebuilt from an outfit named "Columbia", said to install a heavy wire from the case to ground and cited a service 
excerpt allegedly from GM stating that a conductivity problem had been encountered with their "black oxide" mounting 
fasteners.  The recommended fix was to install a heavy ground strap from the usually unused ground boss on the back 
of the alternator (heat sink side of case) to an appropriate clean spot on the engine.  I used a basket weave ground strap 
with tinned ends.”  This is a cheap and easy way to make sure your alternator can send full rated current to the car -- 
regardless of which type alternator you have. 

Also keep in mind that the same current must then get from the engine block to the chassis; see the comments on adding 
a ground strap on page 553. 

Of course, even grounds that are not directly involved with the heavy current can cause flaky symptoms since they may 
effect the alternator’s internal regulation.  David Littlefield apparently had such trouble:  “I found that my gauge 
showed "discharge" most of the time.  For a long time, I ascribed this to just being another unreliable gauge. 

“On my '88 there is a black plastic shield covering a bracket with some electrical connections on the inside of the 
passenger side (LHD) front fender, towards the back of the engine bay.  I removed that cover and found a couple of 
ground wires connected to the bracket.  I cleaned a goodly amount of corrosion off these connections and off the brass 
(?) screw that held the bracket to the chassis.  Now, even at idle, the gauge shows "charge,".  I wish I had figured this 
out earlier, it might have saved me the cost of a new battery and a new alternator.  I'm convinced the old alternator died 
because of it, and it killed the battery, in turn. 

“There was no indication that anything was wrong, besides the gauge reading low.  Those of you that find your battery 
going flat overnight or similar symptoms may want to try this fix before removing the alternator.  Or if you are 
replacing your alternator, be sure to clean up these contacts to keep from killing the next one.” 

 

CHARGING VOLTAGE AND TEMPERATURE COMPENSATION:  Irrespective of Napoli’s claim that “volts are 
volts”, there actually is a design difference between British volts and Detroit volts -- or between early 80’s Lucas volts 
and late 90’s GM volts, anyway.  Section 86.10.01(c) of the ROM describes checking the regulator in the Lucas 
alternator for holding output between 13.6 and 14.4 volts.  When operating properly and within their load limits, the 
Lucas alternators seem to hold voltage near the low end of this range, around 13.6-13.7 volts.  According to Michael 
Aiken, the later Bosch alternator behaves similarly; “My 115 amp Bosch on my 89 XJ-S charges at 13.67 volts 
measured at the battery.”   

It turns out, though, that the ideal charging voltage for the typical automotive lead-acid battery depends on the 
temperature of the battery; the lower the temperature, the higher the charging voltage needs to be to maintain a good 
state of charge.  Modern GM alternators, including the CS130 this author installed, have “temperature compensated” 

 
 

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voltage regulators inside, meaning that the charging voltage is varied depending on the temperature of the air 
surrounding the alternator.  On the author’s car, the CS130 charges at 14.3V when started cold and the alternator is at 
room temperature, and the voltage drops back to about 13.8V as the engine warms up.  If the car ran hot, the voltage 
would probably get even lower. 

Of course, the reason GM alternators include this temperature compensation is that they presume the battery will be 
seeing the same ambient temperature as the alternator because it’s in the same engine compartment.  But the battery in 
the XJ-S is in the trunk, so really you’d be better off without the temperature compensation.  The original Lucas 
alternator evidently had no temperature compensation, but it charged at only 13.6V all the time; apparently it was 
designed for the battery to be in the engine compartment (as on Jaguar saloons and other cars using the same Lucas 
alternators) since it regulates at the voltage suitable for hot engine compartment temperatures.  Since there are quite a 
few applications where the battery is installed in a cool location (marine applications, for example), perhaps Delco or 
others actually make a regulator that lacks temperature compensation; if you find one, opt for it.  However, just 
installing the typical temperature-compensated GM alternator will generally work fine since the warmed-up voltage of 
13.8V is still as high as the Lucas ever charged. 

Combined with the sensing location difference discussed above, the installation of a GM alternator may result in the 
entire electrical system operating nearly a full volt higher than it ever did with the Lucas alternator in place.  This has no 
detrimental effect since the electrical components were all designed to operate up to that voltage anyway, but it does 
make lights noticeably brighter, window motors zip quicker, blower fans blow harder, etc. 

Don Neff says, “Jeep/Chrysler alternators also compensate for temperatures.  My newer Jeep has a thermocouple under 
the battery to measure its temp instead of relying on the alt's temp.  This alternator with remote temperature sensing 
might be a better choice for the XJ-S with its battery in the trunk.” 

 

 

Battery 

 

When your battery needs replacing, you will find that the XJ-S uses an unusual battery -- and that Jaguar wants $$$$ 
for it.  The Jaguar battery comes with an enclosed vent connected to a tube to route the vent out through the floor of the 
trunk.  Batteries are normally in engine compartments which are well ventilated, and things still corrode right around 
the battery.  A trunk is not ventilated at all, so the battery vapors will corrode the whole trunk.  Harry Trafford reports:  
“the DPO let the Jag dealer in Miami install a new one.  The dealer installed a standard Interstate mega-something and 
charged him $150 US for a $59 battery!  The thing out-gassed into the boot and now I have a major rust repair job 
ahead of me.  And no, the little red plastic Jaguar vent cover does not work.  There are large rusted out areas up in the 
buttresses and along the boot area on either side of the lid, including the area between the rear glass and boot lid.  The 
only thing keeping out the rain is the paint.  Push a finger into any of those places and you'll be looking at the spare tire. 
 We're not talking pinholes here.” 

Worse yet, batteries emit hydrogen gas when charging, so you run the risk of blowing the trunk lid off your car. 

According to Randy Wilson, an Audi 5000 battery will fit with the addition of a half inch plywood shim.  Audi put the 
battery in the passenger cabin, so it has similar venting provisions. 

Wilson also reports that Interstate offers an add-on vent kit for their batteries.  And there are some marine batteries with 
vent provisions. 

Charlotte Hand reports that the make of batteries sold at Pep Boys, Pro Start, offers a “group size 34” battery for the 
XJ-S, complete with the proper vent provisions.  There are apparently at least two to choose from, the cheapie and the 
high end.  

Robbin Lewis says, “Sears now sells an “International” DieHard Battery that is an almost exact match to the original 
Jaguar battery, but at a much cheaper price.  The only thing I had to do was move the 90 deg. fitting from the right side 
to the left... easy to do.  Just swap the blank plug on the left side with the angle fitting on the right.” 

 
 

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Delco Freedom batteries, as well as some Champion batteries, come with a flat top vent/cap assembly that has a vent 
opening on each end.  Each is sort of a flat oval shape, but it is possible to connect tubing to them.  Hopefully this is a 
trend among battery manufacturers; being able to remotely vent batteries is a plus, and it can’t cost them much to 
provide the capability.  Other Champion designs -- many of which are alongside on the same shelf, sometimes sharing 
the same part number -- have obscure venting, apparently coming out of whereever around the vent caps, and clearly 
impossible to properly seal. 

A flat-top Champion model 78-2 fits the ’83 XJ-S perfectly.  It is a simple matter to fashion a suitable vent scheme with 
some 1/4” ID clear vinyl tubing and a plastic tee.  This battery comes with an 84-month warranty, 24 month free 
replacement, 36 months free jumpstart, and is rated at 950 cranking amps at 32°F and 770 cold cranking amps at 0°F. 

The catch?  Model 78-2 is a side-terminal battery!  The original top-terminal cable ends must be cut off and side-
terminal cable connectors installed.  They cost only a coupla bucks, the cost is no big deal.  In truth, the XJ-S looks like 
it was designed for side terminals; there is precious little room for top terminals, requiring an unusually short battery 
height-wise.  There’s plenty of room on the front, however, and use of side terminals allows this full-size battery to fit 
under that plastic cover. 

A more traditional design battery can be made to serve, but you must find a way to vent the fumes outside of the trunk.  
Find or make a cover to completely enclose the vents on the top of the battery (being selective when you buy the battery 
may help here), or an airtight container for the entire battery.  Connect a vent tube and run it out through the floor of the 
trunk.  B. J. Kroppe suggests “install a DIN cover over your battery.  (DIN battery covers are found on BMWs and 
Mercs).” 

With careful selection, the vent cover from the old Jaguar battery can be used on a generic replacement battery.  
Georges Krcmery says: “The EXIDE Mega-Cell # E42 50W has a rectangular slot around its filler caps which exactly 
matches the vent cover with only a slight adjustment: I had to cut off about 1 cm of the slot’s lip to accommodate a 
similar widening under the nipple of the vent cover.  It then snapped right into place.  The battery is about 1/4” too wide 
to fit in the tray.  Fortunately, the bottom of the battery has extra plastic on each side and it is possible to carefully saw 
off 1/8” on both sides to make it fit.”  

Michael Neal recommends an Optima battery (see page 705).  This is a lead-acid unit but uses six separate coils instead 
of plates; it uses a gel electrolyte and is sealed, no vent required.  “So far they have proven nearly indestructible.”  The 
Optima is not cheap, but reports are unanimous that it is worth the money -- it lasts so much longer than a conventional 
battery that its real cost is comparable. 

David Littlefield elaborates:  “Optima only sells two batteries for passenger cars, one with side posts (800U) and one 
without (800S).  Either will work fine in the Jag, since the one with side posts also has top posts.  These batteries are 
both red-topped; Optima also sells a yellow top battery that is designed for deep-cycle operation, such as in electric golf 
carts.  You don't want the yellow ones.”  Optima also makes blue-topped marine batteries. 

William Noorloos adds his experience with installing this battery:  “...when I walked towards the trunk with the Optima 
in my hands it jumped the remaining 3 feet all by itself into position, and just sat there waiting to be connected!  It’s a 
total fit.  100% No fiddling.  As a bonus the original Jag venting thing (Non-metallic British part and Lucas, but it has 
not broken yet) fits 100% over the Optima’s 2 emergency pressure relief valves.  So in the event of an alternator 
thinking it has to drive standard household appliances, and thereby blowing the battery, the (probably messy) inside of 
your expensive Optima will not end up in your trunk.” 

Whatever battery you use, you will want to reinstall the plastic cover over it.  If you can make use of  the original 
battery hold-down bracket, it may make life easier.  With the Champion 78-2 described above, it would fit except that it 
blocks off the vents, and the tangs protruding downward on either end of the battery interfere with a built-in handle on 
the battery.  Easy enough to cut the tangs off.  The vents require a little more care, however; cutting enough of the 
bracket to clear the vents might render it two pieces!  However, with judicious cutting, the top edge of the bracket can 
be notched all the way around the corner and part way down the side, leaving a portion of the side intact.  If the height 
comes out too low it’d be a simple matter to put something under the battery to space the battery and bracket up enough 
to hold the plastic cover properly.