Jaguar XJ-S. Service manual - part 144

 
 

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LUCAS ALTERNATOR:  From the introduction of the XJ-S until engine number 8S57571, it was fitted with one 
Lucas alternator or another -- 66-amp on the pre-H.E., 75-amp on the H.E.  There are also reports of Motorola 
alternators on these cars, possibly even fitted from the factory.  Bob Johnson says the number is A5000/12. 

If you’re interested, there is a performance chart for the 75-amp alternator at: 

 

http://www.jag-lovers.org/xj-s/book/Lucas75Aalt.html

 

If a Lucas alternator seems to be charging intermittently (fully charging one minute, discharging the next as indicated 
by the voltage gauge) or has simply stopped charging but has no shorts or burnt wiring, it might be fixable by replacing 
just the regulator itself.  This is much cheaper than replacing the whole alternator, and is easy to do by removing the 
plastic cover from the back of the alternator. 

 

LUCAS ALTERNATOR REPLACEMENT:  Below you’ll find several pages on replacing the original alternator with 
a GM alternator.  Most of these were developed to replace the Lucas alternator -- before Doug Harper broke this news:  
“My Lucas 75-amp alternator was suspect, took it to a rebuild shop to see what my options are.  He ended up selling me 
a rebuilt 1995 Land Rover 110-amp alternator, #13697 Lister or CAE A2051, that was a direct replacement for the 75A 
Lucas, just bolted right in.  Had to replace the pulley.  Made by Marelli!!  Using the Radio Shack 270-1509 lighter 
adapter to the volt meter trick I was getting 14.2V at 1000 rpm, engine sounded better.  The Lucas when working was 
putting out 13.66 Volts. The needle now sits 1 bar above mid point, rock solid.  50% more power as a direct 
replacement, no bracket required, no messing.” 

“The unit has an internal fan, same hole locations for bolts, and appears to be volume-wise smaller.  Beside the power 
out and sense lugs is a timing output pin for diesels (?) and a 12VDC secondary power source on some units.  The first 
one they showed me had cracks in the metal housing where the regulator bolts connect, on all 3 bolts, one serious.  The 
second one they showed me was perfect - well, almost, but no cracks.  Make sure you want the crack-free units.” 

Frankly, if you’re replacing a Lucas alternator for any reason, you’d be well advised to go ahead and upgrade to this 
Land Rover/Marelli alternator even if you don’t need the power.  It’s one of those “couldn’t hurt” kinda things. 

You probably should upgrade the power cables as described on page 579. 

 

 

BOSCH ALTERNATOR:  Beginning with engine #8S57572, the XJ-S was fitted with a Bosch 115-amp alternator.  
This alternator is driven by a multi-groove belt instead of the V-belt found on earlier cars.  Since the alternator is driven 
from the crank damper itself rather than from the pulley that bolts onto it, this change involved the replacement of the 
earlier crank damper C36013 with one with the multi-groove pattern, EAC9248 (replaced sometime thereafter with 
EAC9693 to fit a timing disk for the Marelli ignition).  The pulley part number didn’t change. 

 

ALTERNATOR LOAD DUMP MODULE:  Reportedly, the 115-amp alternators fitted to the late 80’s XJ-S will not 
begin to charge until the engine has been revved up.  Although not really a problem, it is somewhat irritating to see the 
charge light on when everything else seems OK.  According to Michael Neal: “Actually, there is a fix for this.  There is 
a device called an alternator load dump module that was fitted to the later XJ40’s and XJ-S’s with the high output 
alternator.  Fitting the module will fix the problem.  The load dump module will cause the alternator output to function 
properly at idle without having to raise the idle speed.”  The part number for the 115-amp dump module is DBC 5896. 

 

NOISY ALTERNATOR BELT:  If you have the later Bosch 115-amp alternator with the multi-groove belt, chances 
are that sooner or later you will have trouble with that belt squealing.  Neither tightening the belt nor even replacing it 
will help for long.  The problem here is a flaw in the design of the multi-groove pulley -- not just this one on the Jag, 
but basically with all multi-groove pulleys.  The author discovered this design defect on his ’84 Honda Accord (his ’83 
XJ-S has V-belts), but the problem has been confirmed on the later Jaguars as well. 

 
 

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V-belts and multi-groove belts work basically the same way: a V-shaped section of belt is wedged into a mating groove 
in a pulley, and the load is transmitted by friction.  The wedging action is what provides enough friction to handle the 
load.  A conventional single-V belt wears on the sides, making it narrower and therefore sitting deeper in the groove, so 
it needs periodic tightening.  This works fine until the belt either breaks or is so worn that it sits in the bottom of the 
groove instead of wedging between the sides, in which case it will slip badly. 

The multi-groove belts, unfortunately, cannot wear anywhere near that much before they begin to slip.  In fact, after the 
pulley itself has a little wear on it, they cannot wear at all before they begin to slip -- the brand new belt will slip almost 
immediately.  This is because the belt will contact the pulley on top of the ridges between the grooves and ride on these 
edges rather than wedging down into the grooves.  Sitting up on these smooth edges, it can barely transmit any power at 
all without slipping.  In fact, even when it’s not squealing it’s probably still slipping. 

It is easy to confirm this is what’s happening: look at the alternator pulley with a flashlight.  If the top edges of those 
ridges are bright and shiny, you’ve found the problem.  Usually, the inside surfaces of the V’s are much duller, 
sometimes even rusty, indicating the belt isn’t even touching them. 

The fix is easy, too, except that it requires getting that pulley out -- which isn’t easy.  With the pulley removed from the 
alternator, mount it on something that will spin it (a lathe, or find a way to chuck it up in a drill if you don’t have a 
lathe) and grind the edges of those ridges down.  See Figure 28. 

 

Figure 28 - Multi-Groove Pulley Correction 

Don’t worry about grinding off too much.  Yes, you are reducing the contact area of the wedge area, but the fact is that 
once the belt is actually wedging rather than skimming around the ridges, contact area isn’t a problem -- the multi-
groove belt will easily handle all the load of the alternator and then some without complaint.  Just make sure to grind 
the ridges down enough that they won’t be a problem any more, even with a worn belt. 

Obviously, the grooves on the damper pulley itself would benefit from similar treatment.  It’s not really called for, 
however; the larger diameter means that the belt generally won’t slip on this pulley, even when it is riding on top of the 
ridges. 

There is yet another possible cause of alternator belt squeal.  See the report on failed crankshaft dampers on page 90. 

 

RETROFITTING THE BOSCH ALTERNATOR TO EARLIER CARS:  The alternator mount bracket EAC4181 was 
replaced with EAC9320 at the same time that the Bosch alternator was fitted.  Perhaps the purchase of this bracket will 
permit the upgrade of the earlier cars to the Bosch unit.  Since they all use internal regulators, the wiring connections 
should be fairly straightforward. 

 
 

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  To deal with the multi-groove pulley on the Bosch, you can either purchase and install a new crank damper, or you 
can simply replace the pulley on the Bosch alternator with a V-belt type.  Note that there might actually be some 
problem with this latter scheme; historically, alternators driven by a single V-belt never exceeded 60-70 amps or so.  
Larger alternators used two V-belts prior to the introduction of the multi-groove belt. 

Scott Horner of New Zealand retrofitted the Bosch alternator to his UK-spec pre-H.E. XJ-S.  “The Bosch alternator is 
bigger than the Lucas and Motorola in most dimensions.  This meant having to drill out the original mounting bracket 
‘swing’ (technical term!) hole, definitely needs to be done using a drill press.  Trying to find a replacement pivot bolt 
with a larger diameter was entertaining; I eventually got a suitable bolt from a motorcycle store (but that could just be 
good old NZ and the complete lack of parts). 

“The unit I got had a grooved belt pulley originally (although the Jaguar parts manual lists both grooved and V-belt) so 
I had to remove this and fit a normal V-belt pulley.  The shaft size on the Bosch is larger than the Lucas and Motorola, 
so you can’t use the original.  I was lucky enough to be able to rummage through a collection of dead Bosch alternators, 
which all have the same shaft size, and found a suitable replacement, even down to the offset (so the belt lined up with 
the crank pulley). 

“I was able to re-use the original power connectors on the back of the alternator (although I had to drill these out as well 
to make them fit).” 

 

ALTERNATIVE ALTERNATORS:  Regarding the whole general idea of replacing one alternator with another, John 
Napoli offers a viewpoint:  “It is a good idea, IMHO, to go with a common alternator for your geography.  Volts are 
volts, what you want is dependability and maintainability.  A large Lucas infrastructure exists in Britain.  In the US, a 
large infrastructure exists for GM stuff.  The same can probably be said for other marques, but I have GM experience, 
and there is no shortage of ridiculously cheap GM components in the States!” 

Alternators are rated according to the number of amps they can produce.  There is no such thing as too many amps; 
since the regulator will limit voltage to a set value, an alternator with excess capacity will simply put out whatever is 
necessary to meet the load.  The only reason to opt for fewer amps is to save money, since higher-amperage alternators 
generally cost proportionally more.  A good rule of thumb, obviously, is to provide at least as many amps as the 
alternator the car came with was rated at.  Before you decide how many amps are enough for you, you might want to 
read the sections on high-wattage headlights (page 653) and electric cooling fans (page 219) and think about whether 
you might add loads in the future.  You might also think about whether you’ll be considering a high-power sound 
system. 

Note that in the old days when engines had carburetors and windows had cranks, a typical alternator might be rated at 
40 amps or less.  In modern automobiles with EFI and electric everything, the capacity of alternators has increased 
considerably.  Meanwhile, the designs have become more efficient, so they don’t appear any larger than their 
predecessors -- often smaller, even. 

Any 12-volt internal-regulator alternator of suitable amperage would serve in the XJ-S if it could be mounted.  Because 
they’re cheap and plentiful in the US, several schemes have been cooked up for installing a GM alternator as described 
beginning on page 576.  But first we’ll tell you more about GM alternators. 

 

GM ALTERNATORS:  There are lots of varieties of GM (Delco) alternators, but for our purposes there are six basic 
“types” that we might consider for the XJ-S:  The SI-10, the SI-12, the SI-15, the SI-17, the CS130, and the CS130D.  
All six mount basically the same way, which is not the same way the Lucas alternator mounts.  This is why such a 
retrofit either requires the John’s Cars GMALT alternator bracket or fiddling with the air pump bracket. 

When talking about GM alternators, positions of the tensioner lug and the regulator terminals are defined by clock 
positions looking at the rear of the alternator with the pivot lug pointing downward (6 o’clock).  Some GM alternators 
have the tensioner lug at 12 o’clock (“straight across”), some have it at 10 o’clock, some have it at 2 o’clock; the CS130 
is available with tensioner lugs at 10 o’clock and 2 o’clock.  The lugs (both pivot and tensioner) can extend farther 
from the centerline of the alternator on some models, termed a “long lug” case.  However you decide to mount your 

 
 

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alternator, you might want to drop into an alternator rebuild shop and review the lug schemes available to select a 
configuration that will fit best. 

The regulator terminals can also be located in any of several clock positions, but who cares?  Connect the wires to 
whereever they end up. 

The SI-10 comes in amperages up to about 70, and can be rebuilt to put out over 100 -- but the shops don’t recommend 
this and even refuse to do it.  Apparently when this alternator is set up for high current it won’t charge well at idle, and 
they get nothing but complaints from customers. 

The SI-12 looks to the untrained eye to be the same thing as the SI-10 -- in fact, many of the parts are interchangeable, 
and one shop reported that they occasionally find rebuilds that have SI-10 and SI-12 parts mixed up in the same 
alternator.  One visible difference is that the SI-10 has a metal fan blade while the SI-12 has a plastic impeller that looks 
like a solid disk with openings around the edge.  The core parts of the SI-12 are much better than the SI-10, though; this 
alternator comes standard up to about 108 amps, and can be rebuilt to put out 140 amps -- a very common rebuild for 
folks that have high-power sound systems in their cars.  It reportedly charges just fine at idle. 

The SI-15 is physically quite a bit larger than the SI-10 or SI-12, and the SI-17 is difficult to tell from the SI-15.  These 
typically can be had up to about 108 amps as well.  A suitable unit can be purchased by going into an auto parts store 
and asking for an alternator number 70897.  Note that if you’re planning to hang the alternator off the bottom of the air 
pump bracket as described below, the SI-15 or SI-17 will probably not fit; it’s too big, it might run into the chassis, 
possibly when the engine moves around on its mounts.  Better to choose an SI-12, CS130, or CS130D alternator for 
such an installation. 

When fitted with an internal regulator (the only configuration that should be considered for the XJ-S, since the Lucas is 
also an internal regulator type), all four of the “SI” alternator types above will come with three terminals.  The big one 
is the main power terminal that is connected directly to the battery with heavy wires.  The other two are 1/4” spade 
terminals numbered 1 and 2.  You can buy a standardized two-terminal plug to connect to these terminals on a GM 
alternator at any auto parts store, but you can also use normal insulated female spade terminals.  If you buy the 
standardized connector, they always seem to use a black wire for terminal 1 and a red wire for terminal 2. 

Terminal 1 connects either to the indicator light on the dash or to a switched ignition power supply, depending on the 
alternator and the car, but apparently any of them will work connected to an indicator light.  So, just connect this to the 
NB wire in the XJ-S. 

Terminal 2 is a “sense” connection; the regulator will control the alternator’s output to maintain a desired voltage 
(13.6V or so) at this terminal.  The simplest method of connection is simply to connect it to the big terminal mere 
inches away on the back of the alternator, which will work fine; the Lucas alternator did this internally. 

But this isn’t the optimum configuration.  The reason a separate sense connection is provided is so you can run a 
separate sense wire directly to the battery terminal; that way, the regulator will be sensing the voltage right at the 
battery, and will push the alternator’s output a little higher to overcome the voltage losses in the cables between the 
alternator and the battery.   Going all the way back to the battery would be inconvenient on an XJ-S, but it’s not too 
difficult to connect this sense wire to the main power bus post on the firewall.  Since the cable between there and the 
battery in the trunk is seriously massive, the losses there are likely to be minimal.  However, the wiring between the 
alternator and that post includes some marginal size wires (unless you’ve upgraded them) so sensing beyond these 
losses may be beneficial. 

Note that section 86.10.01(b) of the ROM describes a method for checking the losses in this line on the XJ-S; the same 
test will work with the GM alternator, regardless of where the sense wire is connected.  The Lucas setup merely accepts 
losses up to 0.5V while the sense wire setup tries to overpower them, but in either case excessive losses in this circuit 
are not good. 

There are also a few GM “SI” alternators that come with one more terminal for connection to a tachometer. 

The CS130 is a newer GM design, used on most GM cars from 1987 into the early 90’s; there is also a CS121, but it’s 
hard to tell the difference, and apparently cars with fried CS121’s are often advised to replace them with a CS130.  The 
CS130 is physically smaller than any of the “SI” alternators described above.  It has two cooling fans -- one external