Jaguar XJ-S. Manual - part 35

 
 

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15-MINUTE TIMER -- DITCHING:  The 15-minute timer apparently causes really bad fuel economy during those 15 
minutes, especially aggravating for owners whose driving patterns happen to have them within those 15 minutes most 
of the time they are driving.  Craig Sawyers says, “Admittedly, the V12 is thirsty, but a certain amount of that is to do 
with the 15-minute timer on the vacuum advance.  Before that cuts in I get 14 (UK) mpg.  When it cuts out it comes 
back to 19-20.”  Mike Morrin says, “My weekday commute is only 3.5 miles each way, the thermal switch operates 
after about a mile and totally disconnecting the 15 minute circuit gave me a 2 MPG overall improvement.” 

If you’re sick of using so much fuel, disconnect the timer!  The car will run just fine with full vacuum advance when 
cold, it just won’t warm up as quickly.  Since the timer energizes the 3-way solenoid valve, the 2-way solenoid valve, 
and the supplemental air valve (see page 303) and de-energizes them when the 15 minutes are up, you can merely 
disconnect the wires from the timer or the coolant temp switch and the car will never lack vacuum advance again.  The 
easiest way to disable it is to just pull one of the wires on the thermal switch on the rear end of the right side coolant 
manifold.  This is obviously easy enough you can try it and see if you like it. 

Once you decide to make this change permanent, you can make things neater by removing all of those valves and 
plumbing up the vacuum advance system with the same routing that effectively existed after the 15 minutes had 
expired.  You can also trashcan the timer itself if you want, and replace the coolant sensor with a plug. 

The 2-way solenoid valve and the 3-way solenoid valve are involved in killing the vacuum advance at idle during cold 
starts, but the purpose of the supplemental air valve is to boost the idle RPM to compensate for the lack of vacuum 
advance.  Disabling the cold start solenoid valves theoretically eliminates the need for this idle boost (the Auxiliary Air 
Valve is intended to provide idle boost for cold starting), but if you find the car idling too slowly when cold you might 
consider leaving the supplemental air valve connected and disconnecting the solenoid valves only.  In this regard, this 
system may serve as a patch for a weak or deteriorating AAV (see page 263). 

Also note that, on later cars, the same supplemental air valve was used to boost the idle when the A/C compressor was 
engaged.  This is a really nice feature, and one might consider keeping the supplemental air valve in place for that 
purpose.  If your car is an earlier model lacking this A/C idle boost, you might want to wire it up!  See page 303. 

 

15-MINUTE TIMER -- MODIFICATION:  The only problem with ditching the 15-minute timer is how long the 
engine takes to warm up.  The vacuum advance makes the engine run efficiently, so it doesn’t generate as much waste 
heat.  Killing the vacuum makes the engine inefficient so it generates a lot of waste heat and warms up quickly.  
Richard Mansell says, “I disconnected mine recently and found that it seemed to take a lot longer to warm up.  It is now 
reconnected.  I may disconnect it again for the summer (if we actually get one this year).”  Awwww, little Ricky’s gotta 
have his heat! 

Craig Sawyers suggests a compromise:  “If you unplug the timer and link together the grey and pink/white wires, this 
connects the 45°C switch directly to the valves.  So the valves will operate until the engine reaches 45°C and then 
switch off.  I did this with my brother in law's XJS when his module blew.”  This is a really neat fix, since it means the 
car will get to 45°C in a hurry and then get good fuel economy from then on. 

Of course, 45°C isn’t all that hot, so the heater will still not be working as well as some might like.  So, there are other 
possible fixes.  The most obvious would be to replace the coolant temp switch with something with a higher cut-out 
temp, like 70°C.  It must be stressed that the switch really needs to cut out before the thermostat temp is reached; 
otherwise, there’s a chance that the idle vacuum advance will remain disabled indefinitely! 

Another idea would be to leave the timer in place but shorten that 15 minutes to something more reasonable.  Sawyers 
describes how to modify the OEM timer:  “The counter is a 14-bit binary counter.  The most significant bit is used to 
trigger the valves.  So you can select a delay of 7.5 minutes by reconnecting diode D1 internally to pin 2 instead of pin 
3 on the MC14020 timer chip.  Or if you connect to pin 1, you get 3.75 minutes. 

Daniel Lethiais elaborates:  “You will find inside this unit a small electronic plate with integrated circuits.  Near the 
HEF4020B is the diode D1.  If you want a delay around 4 minute you just have to cut the connexion beetween pin3 to 
D1 (with a cutter, knife...) and connect pin 1 to diode D1. It is very easy to do. 

 
 

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“Yesterday I have tried a connexion between Diode D1 and pin 15.  I have checked my unit outside the car with a 
separate power supply and an oscilloscope.  The timer is now around 2,5 mn.” 

 

Sawyers:  “You can get intermediate times by changing the oscillator frequency.  This is set by the values of either C3, 
or R6/R7.  The timing is in inverse proportion to the parallel combination of R6 and R7, and to C3.  So halving the 
value of either will double the time, etc.  For instance, changing R6 from 820k to 470k (leaving other components 
unchanged) will reduce the time from 15 minutes to 9 minutes.” 

Of course, you could just try to find a replacement timer instead of getting into the schematics. 

When considering shortening or eliminating the vacuum advance disable, one must consider its original intention.  
Unfortunately, there’s no telling what that was.  It looks like an emissions feature and in fact is part of the scheme 
called “B Emissions”, but the UK and Europe supposedly had no emissions requirements at all when these cars were 
built.  It could be argued that its purpose is to get the engine warm quickly to minimize wear, but one would have to 
remember why a cold engine has high wear: there’s too much fuel running through it.  Since this system clearly 
increases the amount of fuel running through it, it’s questionable whether it does any good in that regard.  All in all, the 
entire purpose of this system may be to make the heater work as quickly as possible.  That’s an expensive heater, 
especially since this thing provides 15 minutes of lousy fuel economy even in warm weather. 

 

VACUUM ADVANCE MODULE -- PRE-H.E.:  Mike O’Neill noticed that the vacuum advance module from an 
MGB will fit the pre-H.E. distributor.  However, John Nuttall found that the MGB advance unit -- as well as some units 
for Triumphs -- look similar and will fit, but provide differing amounts of advance at different vacuum levels.  So, if 
your engine is largely stock and you want to keep it that way, you probably should seek a Jaguar advance unit.  On the 
other hand, if your engine is not stock and you need to provide nonstandard amounts of vacuum advance, this provides 
some places to look.  Also, these units might provide materials for rebuilding your old vacuum advance module in 
much the same way Napoli used a Chevy part in the procedure described above. 

 

VACUUM RETARD:  According to the article “Advancing by Vacuum” by Roger Bywater in XJS Bulletin Issue #1, a 
vacuum retard module is typically connected to a throttle edge tapping on the inward-opening side of the butterfly.  
Hence, vacuum is applied at idle, but as the throttle is opened the edge of the butterfly passes over the port so it sees 
ambient air pressure.  The system therefore provides a few degrees of retard at idle, but does nothing at other throttle 
positions.  This helps make the engine idle better, since some engines have trouble igniting the charge at high vacuum; 
according to Bywater, the carburetted V12’s fit into this category.  By waiting until the piston is closer to TDC, the 
charge is more concentrated and compressed when the spark occurs.  Obviously, such a retard scheme would help 
emissions at idle by ensuring good ignition. 

Ian Macfarlane, regarding his 1985 XJ-S:  “In Australia the distributor has a retard connection on the vacuum module 
which is supposed to operate for about 15 min. after starting via a solenoid valve.  But the wire leading to the solenoid 
valve had been cut (as had the wire to the supplemental air valve).  Thus, even though the distributor timing was correct 
with the vacuum tubes disconnected, the engine ran at about 6 deg retarded under normal conditions. 

“Although this would not exactly apply to other emission systems it is another option for those with an overheating 
problem - if the vacuum system was not working properly it is feasible that overheating could occur due to retardation.” 

Having the vacuum retard system activated when it shouldn’t be is a valid concern, but this author isn’t convinced that 
cutting the wire to the solenoid valve will do it.  According to the schematics of this system, the solenoid valve 
controlling the vacuum to the retard side of the vacuum capsule is a “normally closed” type, meaning that cutting the 
wires should render it always closed -- no vacuum retard. 

Mark Jackson, who owns an ’87 Australian model, says, “I have been told (and have the proof on the car) that the local 
dealers got so fed up with people complaining about poor performance on start-up, that they just bypassed the 
electronic valve mounted under the RHS manifold by routing the vacuum pipes past it.” 

 
 

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CENTRIFUGAL ADVANCE - SEIZING:  At this point in this book, I used to report how the pre-Marelli Jaguar V12 
centrifugal advance was highly prone to seizing.  However, with more and more readers reporting back, I can say with 
relative certainty:  Unless your distributor has been overhauled, it is seized right now.  Period.  This has proven to be a 
100% failure rate, every time, every car, there simply is no doubt anymore.  And a seized advance unit results in low 
power, overheating and major engine damage if not corrected.  If you are not absolutely positive that your distributor 
has been overhauled already, it is imperative that you schedule an overhaul as soon as possible -- and drive the car very 
gently or not at all until it’s done. 

Folks, this is probably the single most important item in this book.  Centrifugal advance seizure may be the cause 
behind most of the problems the Jaguar V12 has -- overheating, dropped valve seats, etc. -- and the reason behind most 
owner dissatisfaction, low resale value, Chevy engine swaps, you name it.  Is there any way I can convince you, the 
XJ-S owner, to take action now?  If it’ll make you feel better, give me a call, I’ll tell you in person:  Overhaul the 
distributor NOW. 

Sad to say, reports are that this failure mode -- notorious as it is -- is not widely known among Jaguar dealers and 
mechanics.  Despite the frequency of charging customers $10K for a new engine, it has never occurred to many of 
these shops that the engine failures were preventable.  Some people have reported that their Jaguar dealer overhauled or 
replaced several unrelated components before finally figuring out that the centrifugal advance was seized.  Others never 
figured it out.  Perhaps their fancy diagnostic computers don’t blink “centrifugal advance seized”.  Perhaps they just 
like collecting those fees for new engines.  Whatever, keep in mind that just because you have had your car “checked 
out” by the dealer doesn’t mean you don’t have the problem.  It’s not even a certainty that the advance is OK if you 
specifically asked them to check it; some of these mechanics don’t even know how to check it.  For those who have 
mechanical abilities, it is highly recommended that you check your centrifugal advance yourself, or better yet simply go 
ahead and overhaul it.  If you aren’t the type to get your hands dirty, it probably would be money well spent to have an 
independent shop confirm the first shop’s conclusions. 

John Napoli confirms this:  “I know or deal with about a half-dozen repair shops.  Some are run by friends (no 
chicanery ever) and the others, including my local Jag dealer, are pure business relationships.  None of them ever heard 
anything about the chronic Jag problem of seized centrifugal weights.  Not even the Jag dealer, although maybe they do 
know and don’t tell.  I’ll go further and say that no Jag owner I’ve encountered has ever heard of it, either.  You get 
these strange looks when you bring it up (“If I don’t know, then it can’t hurt me.”).  I find it hard to believe, but it looks 
like a lot of the information regarding the idiosyncrasies of these cars never reaches the people who can use it most.  
Perhaps this is part of the reason why so many Jag owners have bad experiences with repair shops.” 

The usual indications of a seized advance mechanism are an XJ-S that lacks power at higher RPM and a nasty tendency 
to overheat.  In some cases, when the throttle is backed off suddenly there is a brief power surge before decelerating.  If 
your car doesn’t have the performance at higher RPM that it should, check the advance mechanism immediately; a 
distributor seizure is easily fixed, but warped blocks, dropped valve seats and burned pistons are much more expensive 
and are the inevitable outcome of continued operation. 

To confirm for yourself that your advance unit is seized, remove the distributor cap.  Attempt to turn the rotor 
counterclockwise.  If operating properly, you can easily turn it about 11° (H.E.) or about 13° (pre-H.E.) against a 
spring.  Note that this thing should feel really loose and rattly, and when you twist it and let go it should snap back with 
a metallic clink; if at all sticky or gummy-feeling when turned, time to overhaul.  The forces that move this thing are 
balanced against each other, so a very slight difference is supposed to make it move; while the springs may seem 
mighty strong when you’re fiddling with them, even a slight amount of drag or stickiness is enough to keep it from 
moving as it should.  It may be movable by hand but not as far as it should or not as freely as it should.  Unfortunately, 
in a book such as this it is difficult to convey adequately how to tell that an advance is gummy until a person has felt a 
properly-operating advance for themselves.  This might be an excellent topic for a monthly Jaguar club meeting -- to 
have someone who knows what the advance should feel like check every V12 that shows up.  Of course, if totally 
seized, the problem is obvious; you cannot move it at all except for backlash in the drivetrain. 

Another method of checking for a seized distributor is to compare the timing (with the vacuum line to the distributor 
disconnected and plugged) at 2000 RPM with the timing at idle (under 900 RPM).  The two readings should differ by at 

 
 

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least 12° (H.E.) or at least 22° (non-H.E.).  If they are the same or very close, the centrifugal advance is seized.  If the 
advance at 2000 RPM is more than a couple of degrees different when you come up to that RPM from idle than it is if 
you come down to that RPM from 4000, then the advance is sticky; overhaul it. 

 

OK, SO IT’S SEIZED -- NOW WHAT?  To prevent/correct a seized centrifugal advance mechanism, it’s necessary to 
overhaul the distributor, clean out all of the original lubricant, clean up the parts, and reassemble with a reliable 
lubricant.  Roger Bywater says, “Most of them probably run for years without the cap being lifted so by the time 
anyone does get round to it the spindle is likely to be running dry and getting built up with debris.  Adding lube at that 
stage may well loosen the debris and aggravate the situation so really it needs complete stripping to ensure that it will 
not jam at some future time.”  Bywater, having worked for Jaguar, can be expected to blame the seizures on poor 
maintenance -- and perhaps he’s correct.  I personally suspect the original lubricant used at assembly was lousy, based 
on what the stuff looked like when I got my distributor apart!  It really doesn’t matter what the cause; either way, the 
distributor has to come apart. 

Of course, you don’t have time for overhauling.  You will probably want to try the easy fix:  Remove the distributor cap 
and rotor, and put some penetrating oil down the center and see if you can work it loose.  If you are successful, it is 
recommended that you check its operation regularly -- or better yet, just go ahead and overhaul it when you get the 
chance.  The results of operation with a seized centrifugal advance are simply too costly to ignore. 

This is one place where getting a new part will probably not help.  There is little wear in the distributor, and an 
overhauled and properly lubricated used part can be expected to last the life of the car.  But a new distributor may have 
the same problem as the original distributor did.  It is recommended that even a brand new in-the-box distributor be 
overhauled prior to installation (to get that lousy lubricant outta there). 

The good news:  The reported incidence of centrifugal advances seizing after they have been overhauled is nil.  This 
may be because the original grease is the problem, and once it’s removed the problem goes away.  It may simply be 
because once the overhaul has been done the owner understands what’s going on and lubricates it regularly.  Whatever, 
once the distributor has been overhauled, there is apparently no reason to expect to ever have to do it again. 

Randy K. Wilson points out that the problems with seized centrifugal advances on pre-H.E. engines may actually be 
different in nature.  “From what I’ve seen the gummy advance problem is unique to the V12 H.E. distributor.  Earlier 
distributors also had a problem with seizing, but these galled from a total lack of lubrication.”  Of course, this could 
mean that the H.E. only gets hot enough inside the distributor to turn the lubricant to varnish while the pre-H.E. got hot 
enough to burn the lubricant away entirely.  Or, it could mean that Jaguar added grease to the assembly in response to 
the dry seizures, and the grease caused its own seizures. 

 

DISTRIBUTOR REMOVAL:  If the centrifugal advance is seized, or if you wish to prevent seizure in the future, the 
distributor should be removed and disassembled.  It’s not difficult, but it will help to read the following sections 
thoroughly before tackling the job. 

Note:  If you prefer, it is possible to rebuild the centrifugal advance mechanism with the distributor in place.  It’s easier 
to take it out and do it on the bench rather than leaning over the fender, though. 

Before removal, care must be taken to ensure the distributor can be reinstalled with the gears meshed correctly so the 
rotor faces the same direction as before.  If the crankshaft is not to be disturbed, this is very simple:  Note the position 
of the rotor prior to removal, so you can be sure it is in the same position when reinstalled.  It’s a gear mesh and one 
tooth off would be obvious, so just note the position well enough you can make sure you are on the same tooth when 
reassembled. 

Note:  If you have the older Lucas OPUS ignition system, you are advised to position the engine at TDC on cylinder 
1A, which is where the timing marks on the crank pulley line up and the rotor is pointing at the front left headlight.  If 
the rotor is pointing at the right side door, you need to turn the engine around one full turn.  The reason that this is 
important is that the OPUS has a wheel within the distributor that has three slotted holes that line up with the distributor 
mounting bolts in this position.  Even if you can get it off without moving to TDC on 1A (by busting up the plastic