Jaguar XJ-S. Manual - part 34

 
 

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VACUUM ADVANCE MODULE:  On most cars, it is a simple matter to determine if the vacuum advance module is 
intact:  Connect a hose and, using your mouth, suck and watch the mechanism move.  While in advanced position, put 
your tongue over the end of the hose and see if it holds vacuum and stays in position. 

On most cars, but not on the XJ-S!  On the H.E., there is a vacuum regulator in the line to the vacuum advance module, 
and such regulators cannot be depended on to work properly with no flow.  Therefore, the vacuum advance module on 
the XJ-S has a deliberate bleed hole, so the module will not hold a vacuum even when in perfect condition. 

According to Michael Neal, “The early units hold vacuum.”  This makes sense; if the vacuum advance plumbing 
doesn’t include a vacuum regulator, there’s no need to put an orifice in the diaphragm. 

Roger Bywater says, “Of course the high temperature situation also gives the vacuum capsule a hard time and they 
usually need replacing every couple of years or so but a lot of so-called mechanics seem to miss that one.” 

Note that a vacuum advance module that isn’t leaking may still need replacement.  Val Danilov says, “My 
diaphragm was fried rock hard, I broke it trying to test the rod movement (CRRRACK!), so I think it wasn’t 
leaking.” 

The earlier vacuum advance modules came with an adjustment screw on the top.  This screw permits adjustment of 
the limit of travel of the module -- but was supposedly set at the factory to the correct travel.  This is not intended as 
a user-adjustable feature; the only reason to be messing with it is a suspicion that someone else has already messed 
with it.  Later replacement modules don’t even have the screw. 

 

VACUUM ADVANCE MODULE REPAIR:  The following procedure for rebuilding a vacuum advance module is 
credited to John Napoli and Val Danilov. 

Before removing the vacuum advance module from the engine, you might want to take note of the best position for 
the vacuum line connection when it is reassembled.  And after removing the vacuum advance module but prior to 
taking it apart, it is suggested that you first carefully measure the distance which the actuator rod extends out of the 
unit.  Also, push the rod back into the unit (fully retracted position) and measure that as well. 

Raw material is a generic replacement vacuum advance from a Chevy V8 -- dirt cheap at any auto parts store.  Take the 
Chevy unit apart by prying open the case; Napoli suggests it may be easier to open the case by grinding the case all 
around its periphery to weaken the metal, but be careful not to get it too hot and damage the diaphragm.  What you 
want to get ahold of is the diaphragm/rod assembly. 

Open up the stock Jag vacuum advance by prying around the crimped-on case.  In this case, you’d like to reuse 
everything except the diaphragm/flat link assembly, so try to do as little damage as possible to the casing; if you’ll read 
ahead to understand how you’ll be reassembling it, you may decide to simply cut or grind the lip off the edge to avoid 
mangling the dome itself.  Even though the diaphragm/flat link won’t be reused, keep it on hand for taking 
measurements.  

Cut the rod from the Chevy diaphragm/rod assembly to the same length as the Jag original, then set it down on an anvil 
or something and pound a flat spot at the end with a hammer.  No problem -- it’s a Chevy part, remember? 

Reassemble the vacuum advance unit using the new diaphragm/shaft assembly and all the old Jag parts.  If the casing 
wasn’t damaged when disassembled, you may be able to simply clamp the unit togeher and gently bend/hammer the 
dome shut.  This works but the cannister will become work-hardened so you will never be able to do this repair this 
way again. 

If the edges were too mangled (or you cut them off), drill the body and dome for eight fasteners; Napoli suggests 3/4” 
long 3-48 fillister head screws and nuts, but notes that the exact sizes aren’t critical.  Sandwich the body, diaphragm 
(poke a hole through for the screws with a needle), spring, and dome, and bolt it all together, being careful that the 
flattened section of the rod is aligned properly.  This method has the distinct advantage that it permits repeated repairs 
later -- important, since the heat in the valley of the V12 will eventually bake the Chevy diaphragm as assuredly as it 
did the original. 

Check that the unit responds to vacuum and pulls the shaft in. 

 
 

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Measure carefully the extended length of the rod, mark, and drill a hole in the flattened area the same size as the hole on 
the original part.  Check the retracted length as well, and if there is an adjustment screw on the dome adjust it to provide 
the same total travel (if your extended length was off a little bit, make the retracted length off by the same amount; the 
variance can then be corrected by setting the timing normally).  If you don’t have an adjustment screw, don’t worry 
about it, the total travel will probably be close enough. 

Reinstall on the distributor.  Be sure to check the timing; the timing procedure requires that the vacuum be 
disconnected, but the module still has an effect -- you might not have gotten that extended length as perfect as you 
thought you did. 

Note that the original diaphragm may have had a tiny orifice built into it to provide a deliberate leak.  This will usually 
be evident as a tiny hole through the metal disks on either side of the rubber diaphragm itself.  The vacuum advance 
plumbing includes a vacuum regulator and such regulators don’t work reliably when there is no flow, so the orifice is in 
there to provide a little flow.  It appears that this orifice may not be necessary, since some genuine Jaguar replacement 
vacuum advance modules reportedly don’t have it.  If having an orifice proves to be necessary, it can be added 
anywhere in that branch of the vacuum advance plumbing; you might wanna make the hole in the dome rather than in 
the diaphragm itself.  It might make more hiss, but you can seal it later if you decide to with a small piece of aluminum 
tape.  Or you may be able to fit a separate device with an orifice with a tee connection into the vacuum line.  You could, 
for example, take a piece of brass tubing, drill a tiny hole in the side, and connect it in the line to the vacuum advance 
module. 

If, for some reason, you are unable to reuse other parts of the original Jaguar vacuum advance unit, you may be able to 
use the corresponding parts from the Chevy unit.  Note, however, that doing so may result in differences in the advance 
curve and/or limits.  Use of a different spring will change the advance rate.  Use of a different dome with a different 
depth, and therefore a different place where the spring seats, will also affect the advance rate.  And use of a different 
dome may also affect the retracted length, allowing the diaphragm to travel too far or preventing it from travelling far 
enough. 

If the Chevy dome happens to include a limit adjusting screw, you may be able to adjust the full-retract position after 
assembly to make sure it is the same as the original was.  If there is no adjusting screw, you might actually be able to 
add one, being careful to seal it when done to avoid vacuum leaks.  If the Chevy dome is shorter and compresses the 
spring too much, you may want to add a spacer ring between the diaphragm and the dome to space the dome back.  If 
the Chevy dome is taller and compresses the spring less, you might want to insert something within the dome for the 
spring to sit on. 

Napoli adds, “I have seen in this month’s Jeg’s catalog (see page 714) that Accel manufactures a replacement 
Chevy vacuum advance that is adjustable.  The ad copy states that the unit is adjustable for total advance and rate.  
It comes with instructions.  The unit looks just like the generic replacement I used, so the same repair approach 
should work.”  This fancy aftermarket Chevy part is still cheaper than the stock Jaguar vacuum advance module by 
a long shot. 

If you tore things apart and then read the suggestions to take measurements first, Napoli sends some reference 
measurements from a junk unit he had laying around:  “For reference, the measurements were taken with 1/16” of 
the adjusting screw exposed.  That is, from the face of the adjusting nut to the top of the unmolested screw is 1/16”. 
 Also, this is a stock ’88 V12 advance (non-Marelli) from my donor engine and the unit does not operate (won’t 
build vacuum).  With the unit relaxed, the length from the end of the housing to the center of the hole in the 
actuating rod is exactly 2 inches.  Full compression of the rod reduces the length by 5/16” (i.e., 5/16” maximum 
travel).” 

 

VACUUM REGULATOR AND DUMP VALVE:  A vacuum advance mechanism applies more advance as vacuum 
increases, which means there will be minimal advance at full throttle (to avoid knocking) and lots of advance at light 
throttle (where knocking isn’t a problem, to provide optimum fuel efficiency).  This also means a lot of advance at idle, 
but this can be problematic.  The manifold vacuum at idle may vary as the RPM rises or falls a bit, and this will move 
the vacuum advance in a direction so as to accentuate those fluctuations.  The result can be an unstable idle, sometimes 
even stalling. 

 
 

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The time-honored fix for this is to use a “throttle edge tapping” for the vacuum source.  The vacuum line to the vacuum 
advance, rather than simply connecting to a port on the manifold, is connected to a port located near an outward-
opening edge of the throttle butterfly.  Whenever the throttle is open, the edge of the butterfly itself is on the air filter 
side of this port, so the port is seeing manifold vacuum.  As the butterfly approaches the closed (idle) position, however, 
the edge of the butterfly passes over the port, leaving the port exposed to atmospheric pressure rather than manifold 
vacuum.  The result is no vacuum advance at idle.  This results in a stable idle, and also makes it a bit easier to adjust 
the timing -- you don’t need to disconnect the vacuum line first.  This setup was very common on carburetted American 
V8’s of old, and unfortunately resulted in an entire generation of mechanics who don’t believe it’s necessary to 
disconnect the vacuum advance when setting the timing. 

If you’d like your Jaguar V12 to operate this way, it’s very simple.  Just throw away all the vacuum advance control 
devices and connect a single hose from one of the ports on top of the right hand butterfly housing directly to the 
vacuum advance module.  This will work fine, although the total lack of vacuum advance at idle will make it use more 
fuel and generate more waste heat when idling.  This shouldn’t be a problem; the cooling system should be able to 
handle the additional heat, and the fuel economy reduction is small -- especially if you don’t sit around idling much.  It 
may affect emissions tests, but ironically it may help them; in general, retarding timing seems to make it easier to pass 
emissions tests.  Note that most of the 1976-80 V12 models (except California and Australia) apparently did operate 
their vacuum advance exactly this way, and some later models did as well except that they added controls to shut off 
vacuum when the engine was cold to help it heat up quicker. 

With later models, Jaguar used a better idea here.  Rather than providing no vacuum advance at idle, they used a 
vacuum regulator to provide some vacuum advance at idle.  Because the regulator provides a fixed amount of vacuum 
regardless of how the manifold vacuum is fluctuating, the instability problem is avoided.  In such schemes, one hose to 
the regulator comes from the manifold itself to provide a vacuum source, while another hose connects to the throttle 
edge tapping to provide a “reference” to regulate to.  When the engine is at idle, the manifold vacuum is high but the 
signal from the butterfly housing shows no vacuum, and the regulator throttles the vacuum to the advance capsule down 
to a fixed amount above that reference.  When the engine is at part throttle, both the manifold vacuum and the signal 
from the butterfly housing show high vacuum, so the regulator does nothing; the advance capsule gets full manifold 
vacuum.  At wide open throttle, both the manifold and the throttle edge tapping see very little vacuum, so the advance 
capsule doesn’t get much either. 

This is a really good system, providing a goodly amount of advance at closed throttle so the engine idles efficiently.  It 
also results in less of a step coming off of idle; rather than the vacuum advance going from no advance to full advance 
all of a sudden, it merely goes from partial advance to full advance.  The effect is smoother throttle response. 

There is one problem, however:  A vacuum regulator is a throttling device, and therefore cannot provide a great deal of 
flow suddenly when needed.  The specific case of concern here is when the throttle is suddenly floored.  In a car with 
simple hoses connecting to the vacuum advance capsule, the sudden lack of manifold vacuum would result in a similar 
lack of vacuum advance just as quickly, and the engine could proceed at full throttle with the timing properly 
positioned for that condition.  But with this regulator in the line, a sudden opening of the throttle will require a second 
or two for the vacuum to bleed off the advance capsule through the regulator.  For that second or two, the engine will 
be running at full throttle with too much advance, and the engine will be knocking something fierce. 

The solution is another device, the dump valve.  This is a vacuum-operated valve that merely opens the line to the 
vacuum advance capsule to atmosphere when there is no vacuum in the manifold. 

There is one other minor detail, and that is the fact that a regulator generally works better with a little flow through it; it 
may have difficulty regulating vacuum in a deadheaded line.  So, the vacuum advance capsule may have a tiny internal 
bleed orifice, small enough that the vacuum system can still apply a vacuum but preventing it from holding a vacuum. 

A little guidance on plumbing:  The North American models use a vacuum regulator EAC5157, and it has three hose 
connections.  One connection is labelled “DIST”, and this line is connected to the vacuum advance capsule; a tee in this 
line should lead to the dump valve EAC4069 and connect to the fitting farthest from the flat side.  Another connection 
is labelled “CARB”; this should be connected to the throttle edge tapping, which is a port on the top of the right side 
butterfly housing.  The third connection is labelled “DELAY”, and this one should be connected to the intake manifold; 
it typically is connected through a whole series of valves and controls that provide less vacuum when the engine is cold. 

 
 

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 Basically, the regulator and the dump valve are the only things in the vacuum advance control system that are 
operating when the engine is warmed up; everything else in the plumbing has to do with cold starting. 

Note that other markets use a different vacuum regulator, but the idea is the same. 

The connection on the dump valve nearest the flat side should be connected to the intake manifold; in some cars it 
appears to be connected to the bottom of the right side butterfly housing, but this fitting is a ways behind the butterfly 
and will see manifold vacuum in all but nearly wide open throttle, and at wide open throttle it should see nearly 
atmospheric pressure either way. 

The various emission control diagrams available invariably show the controls for the air pump on the same diagram, 
primarily because some of those vacuum control devices used for cold starting also affect air pump operation.  Don’t 
get confused; other than the fact that both systems are vacuum-controlled, they really have little to do with each other.  
The descriptions above only apply to the vacuum advance system. 

 

VACUUM REGULATOR CHECKING:  Stephen Tyler says his vacuum regulator was bad; “At the top of the unit is a 
metal disc & in that disc is a small hole.  If you apply a vacuum to the input side of the unit, you cannot obtain a 
vacuum out if the tell tale hole is sucking in air.  The hole is for an indication if the internal diaphram is damaged.  I 
removed the metal top & found that the diaphram had a hole in it.  There are probably many units like this, considering 
the age of these cars.”  This has grave implications for the fuel economy of a lot of cars out there.   

 

15-MINUTE TIMER:  The 15-minute timer is a feature of the “B Emission” H.E. cars, which means UK/Europe spec, 
82-88 or so.  If the coolant temperature is below 38°C when the car is started, the timer is engaged.  This disables the 
vacuum advance at idle and runs the part-throttle vacuum advance through a delay valve for 15 minutes.  If the coolant 
temperature is above 45°C when the engine is started, the coolant temp switch is open and the timer is not engaged.  
38°-45°C is the “dead band” of this switch -- whether it’s closed or open depends on whether the engine was cooling 
down or warming up when it entered this band. 

 

15-MINUTE TIMER -- LOCATION:  Everybody wants to know what this thing looks like and where it is.  Craig 
Sawyers reports: “It is a black box with ventilation slots, 2"x1.5"x1.5".  It has a single fixing hole at one end, and an 
edge connector at the other. 

“Location in UK cars is: 

ƒ  XJ12 -- under the driver's side (rhs) underscuttle, up high.  It is more or less behind the speedo, and a fight to find 

and remove. 

ƒ  XJ-S -- under the passenger side (lhs) underscuttle, much lower and easy to get to.” 

Steve S says, “the US 45 sec one would be located to the right of the passenger side small fuse box under the scuttle.  
Nothing is fitted in this space in the UK spec.  The UK 15 min job is to the left of the passenger side small fuse box but 
located on a different plate to the small fuse box set closer to the firewall (bulkhead); US spec cars have something to 
do with the seat belts located where the 15 min timer goes.” 

 

15-MINUTE TIMER -- REPAIR:  The timer itself is reportedly unreliable.  Tony Bryant:  “My '85 NZ Spec (= RHD 
Euro) had this 15-minute timer in the LH side of the LH footwell.  Cheap and nasty piece of trash.  It's a pertronix unit, 
based on a CD4020 cmos 14bit counter, and a CD4069(?) hex inverter with one of the inputs floating(!).  Floating input 
pins on CMOS are just begging for a blown up chip.  I replaced it with the 74HC equivalent, and tied the unused input 
to ground - before I ditched the whole system.”