Xj 12 ser 3 ignition. eu-version

(tuomo jylha) #1

Hey. What kind of ignition values ​​in the v12 HE motor should the vacuum be disconnected and the negative pressure coupled? XJ12 ser 3. Eu version. It would be good to know the values ​​at different revolutions to check the correct functioning of the ignition system. Is there any kind of ignition advance map in which it could check?

(Frank Andersen) #2

I assume that you are referring to the change of vacuum source on the ‘European’ ign control system…?

At no point during driving is the vacuum actually disconnected from the distributor - instead vacuum source is switched from throttle to manifold vacuum with the engine hot. An oil temp sensor operates a three-way valve for the switch-over - simultaneously also opens a supplementary valve to allow air from the air filter, bypassing the throttle. The latter function has no bearing on ign advance…

The characteristics of the throttle vacuum is that it is about ‘0’ with the throttle closed - otherwise varying with throttle opening, basically relating to air speed through the throttle. Manifold vacuum is high in idle, but varies, lowers, with engine load - basically relating to engine rpms versus throttle position.

In either case, the centrifugal regulator is increases advance with engine rpms - superimposed on the vacuum regulation. This makes the advance map very complicated, and as far as I know there is no complete mapping readily available.

To some extent, the centrifugal can be function tested by reading advance variations at various rpms - with vacuum disconnected. Similarly the vacuum source can be assessed with a vacuum gauge connected to the hose at the dist - but the combined effect on advance is load related, and not amenable to stationary testing. However, measuring idle advance with and without vacuum connected gives some indication. But whether a map exist for correlation…I don’t know…

Principally, with the advance adjusted to 24 deg BTDC AT 3000 rpms, vacuum disconnected engine hot, and all components functioning as they should - ign timing should be as good as it gets…?

xj6 85 Sov Europe (UK/NZ)

(tuomo jylha) #3


Thanks you for the information. This car has broken this timer. Clock
switch and temperature sensor have been skipped. The vacuum system is
put on the eu model. Vacuum system is now continuously open. The mean
forward power rises steadily at 750 rpm / 6, 3000 rpm / 20, 5500 rpm /
26, without vacuum. Is there information about how much and almost
exactly the vacuum regulator increases the amount of the advance? It
would be important to know that the advance does not rise too high so
that the pistons do not fall and the engine breaks down. Too late a
delay reduces power and only increases fuel consumption.
I’ve heard that the clock switch degradation is a typical problem in
these cars, and it is often skipped.
Distributor springs also stretch, especially the thicker spring.
Vacuum tubes have been replaced.

Car is Daimler double six, year 1987. The same car as xj12.

Tuomo Jylha. Finland

Lainaus Frank Andersen noreply@jag-lovers.com:

(tuomo jylha) #4

Hey. Thank you for the information. This car has a clock switch
broken. Clock switch and temperature sensor is now skipped. The
centrifugal controller raises the advance at 750rpm / 6, 3000rpm / 20,
5500rpm / 26, without vacuum.
Do you know how much vacuum should raise the advance?
Too large advance burns the Pistons, and can break the engine. Too
late the advance does not give the engine power, and only increases
fuel consumption.
I’ve heard that these cars have The typical fault that the clock
switch degradation, and it is the skip.
Vacuum tubes are new.

It would be good to have the engine work properly. The car is old, and
it can have many of the consumed and broken components in that complex
ignition system.
The car is only for summer car and for minor use.
The car is a Daimler double six in 1987 but the same as xj12.

Tuomo Jylha. Finland.

Lainaus Frank Andersen noreply@jag-lovers.com:

(Frank Andersen) #5

The ‘clock switch’ is not mentioned, Tuomo - do you mean the changeover valve controlling the vacuum source…?

The reason for the vacuum switchover when the engine is hot reputedly relates to difficulties with smooth idle and best engine performance throughout the rev and load range. Without vacuum, advance only relies on centrifugal advance which is ‘safe’ against engine damage, but will definitely impair performance and fuel economy.

Some improvement may be expected if the dist were connected to ported vacuum - it is better than no vacuum at all. Ie connect the vacuum hose from the distributor to the vacuum port at the throttle body.

The initial advance setting is 18 deg BTDC at 3000 rpms (instead of the present 20). The vacuum advance varies and is not specified - nominally; at manifold vacuum (18" Hg) it is typically 6 -10 deg, but tends to taper of towards highest rpms. However; ported vacuum is around ‘0’ with the throttle closed - but in both cases the vacuum varies with engine load and pedal position. The centrifugal advance is as you list - gradually increasing with rpms.

You may of course measure vacuum variations with a vacuum gauge against rpms (unloaded) at the two sources, and compare that with advance changes, just to get some idea? The important thing is to avoid excessive advance - which will show up as engine ‘pinking’. Sounds a bit like peas dropping onto a tin roof, but may be difficult to hear on a V12. However, it usually occurs only during heavy loads - and if heard; just back off the pedal and lower the timing…

Ideally the 3-way vacuum switch should be reinstated for best results, of course - if you still have it available; check its function. The failure may be the oil temp sensor rather than switch itself…

Try driving with the vacuum connection to the throttle spigot first - verifying that there is only one spigot at the throttle body. On the group of 4 spigots at the manifold there should be three connections; to the AC system, the automatic gearbox and the fuel pressure regulator. The fourth spigot should then be sealed off…

xj6 85 Sov Europe (UK/NZ)

(Doug Dwyer) #6

I suspect ‘clock switch’ is the 45 second timer which operates a vacuum solenoid valve which, in turn, vents the vacuum advance signal to atmosphere after engine start. Used only on car with catalytic converters…I think.

I admire anyone who will suss out and verify the correct operation of the various vacuum advance schemes. There are several versions. It requires testing of each switch and valve as any single failure might render the entire system inoperative

Some owners, faced with an inoperative system or other motivations, simply run a vacuum hose from the advance capsule to ported vacuum and call it good…and live happily ever after.


(tuomo jylha) #7

I did a test, and put the vacuum diagram in the car (later model). I measured the ignition advance with that system. The vacuum is now connected to the vacuum:

-750 rpm / 6 degrees
-3000rpm/ 22 degrees
-5500rpm/ about 32 degrees

Without vacuum:
-750rpm/ 6 degrees
-3000rpm/18 degrees
-5500rpm/ about 22 degrees

Yes, the car has catalysts. I meant a vacuum delay timer with a timer (15 min timer). The magnetic valve system is not currently in use. The setting is directly in the settings of the heat engine.

(tuomo jylha) #8

I did a test, and put the vacuum diagram in the car (later model). I
measured the ignition advance with that system. The vacuum is now
connected to the vacuum:

-750 rpm / 6 degrees
-3000rpm/ 22 degrees
-5500rpm/ about 32 degrees

Without vacuum:
-750rpm/ 6 degrees
-3000rpm/18 degrees
-5500rpm/ about 22 degrees

Yes, the car has catalysts. I meant a vacuum delay timer with a timer
(15 min timer). The magnetic valve system is not currently in use. The
setting is directly in the settings of the heat engine.

Lainaus Doug Dwyer noreply@jag-lovers.com:

(tuomo jylha) #9

With that setting, the engine sound sounds good and does not include knocking sound. The engine responds quickly to the accelerator pedal. Test drive and fuel consumption tell the rest, and whether it’s right?

For some reason my answer comes two times ???

(Doug Dwyer) #10


Test drive and fuel consumption will, at minimum, tell you if you’re satisfied with the results. What is right may or may not be the same thing :slight_smile:

Somewhere in the archives, here or elsewhere, I think the true intended advance curve can be found…with some searching. The results of your testing look good to me but I can’t say that they are actually correct.


(Frank Andersen) #11

As Doug implies, the variants of the dist vacuum is seemingly beyond reckoning, Tuomo. And the use of either vacuum advance and vacuum retard complicates matters…

All systems work very well with their respective components intact. But with items missing or faulty; the engine will deviate from perfect, one way or another - which may or may not be very noticeable.

Over time; some POs likely have bastardized the systems - and the original setups, which seems to vary in detail, cannot be easily reconstructed. Basically; every engine type has a specific ‘ideal’ timing for each combination of rpms and loads. The analogue centrifugal and vacuum advance mechanisms only approximates the ideal timing…

And particularly for the V12 HE engines, the high compression requires some ‘safety’ features to ensure the ignition advance at no time becomes harmful. At the same time also ensuring the advance was high enough to ensure good engine performance and economy…

On the ‘European’ advance version; the dist uses ported vacuum until warmed up, then switches to manifold vacuum - reflecting the difference in some characteristic of a hot engine. This is likely to improve economy. With your vacuum connection, ported vacuum only you are running with ‘cold’ engine advance - which may affect economy…

The ‘US’ set-up uses two coordinated vacuum ports with calibrated dist vacuum dumping to ensure against knocking - and also restricts the advance at high rpms compared to lower rpms. Both to ensure against knocking (at high rpms) while maintaining higher advance a lower rpms for economy.

Both systems were updated as stricter emission controls were introduced. I don’t know the connections fo the clock timer, and what it actually does in relation to the advance. Only testing of the device can reveal how it functions - and the result for the advance…?

To satisfy your curiosity an vacuum influence; you can measure the actual vacuum readings against advance changes at the relevant rpms…? I’m a bit suspicious as to the 10 deg vacuum advance at 5500, it seems contrary to HE practice - a bit too high…? But as you seldom use such rpms it may be immaterial…?

xj6 85 Sov Europe (UK/NZ)

(tuomo jylha) #12

I just knew that putting vacuum tube gas throat butterfly and air cleaner, vacuum is not so great at higher engine speeds. The second ignition vacuum unit can then be switched off. Then it works according to the “traditional” vacuum system. This will add some fuel consumption, but it is safer for the engine. Let’s see how it works around? If it does not work properly, I will put a new vacuum delay timer.

There is a type fault in this v12 HE engine, this ignition vacuum system. At least one component is usually broken and the ignition timing is incorrect. Vacuum Timer dealy, 3-Way Valve, 2-Way Valve, Pump Tank, Hose Leak, Distributor Vacuum, …, …

(Frank Andersen) #13

I don’t know what the vacuum delay timer actually does, Tuomo - it can either switch sources or cut the vacuum, leaving only centrifugal advance…?

An inline delay valve was fitted to the dist vacuum line - to slow down dist reaction to sudden vacuum changes. The main point of vacuum regulation is control of the vacuum - not eliminating it…

That said; some experimentation with advance settings is not banned. But usually confined to the initial setting to compensate for engine wear and tear - trusting the system to spread the changes properly throughout…

The ign advance system is complicated due to the characteristics of the V12 HE engine. The relative importance of individual component failures is guesswork, but basically; it will only compromise engine performance and economy - not engine safety.

However, one factor should be considered; too little advance generally causes engines to run hotter…

xj6 85 Sov Europe (UK/NZ)

(Aristides Balanos) #14

The vacuum delay timer kills the vacuum advance for 45 seconds, so the engine will warm faster.
At the same time, it activates the Extra Air valve to boost idle.

In my car I’ve disabled the vacuum kill option, but kept the idle boost.


(Doug Dwyer) #15

The ROM …at least the one I have…is a bit more specific, mentioning faster “light off” of the cat converters is the reason for the 45-second timer.

A 15 minute timer has been mentioned. I know some markets used this…but I’m unable to (easily) find reference to it at the moment.

This all gets rather tricky. There are market and year-by-year variations that are not mentioned in the ROMs (at least not the editions I have) so digging thru ‘supplements’ and technical service bulletins is required, sometimes fruitful, sometimes not…but always time consuming. This is complicated by Jaguar being less than forthcoming on the specifics of V12s installed in Series III cars…as opposed to the much more common XJS cars.

XJS tech info can typically be used but be prepared for some subtle differences …the purposes for which may require some speculation.


(tuomo jylha) #16

I think that it does not make a big difference in putting that XJS or XJ12 vacuum system in that car? The engine is the same, ECU (16) is the same and the fuel injection is the same.
The EU version of the compression engine is larger and the fuel octane is different from that of other countries. This must be taken into account when setting up the ignition advance. If it works in those (EU) regulations what I have to connect, so good. Does the vacuum delay timer affect only the cold engine and it changes the vacuum engine vacuum system? The AJ6 Engineer website provides a good explanation of that and how it should work. Hope I understood its vacuum function correctly?

I’m doing a car for the exhaust gas analysis and it shows how the engine burns the fuel engine at different speeds.
If the car feels running properly, the engine temperature does not grow too high and fuel consumption does not grow much bigger than it should be, then everything is probably fine?

(Frank Andersen) #17

With vacuum disconnected by a time or whatever; Tuomo - advance relies on centrifugal only and is lower than good, but the engine heats marginally faster.

As Doug infers; lots of the ‘extra’ items relates to emission control and catalytic converter protection - they are of no benefit for engine performance, and unlikely to improve fuel economy. In principle, the initial ‘European’ set-up did not include cats or emission control. It simply catered for good engine performance and economy…

The US market was always important to Jaguar, and they bent over backwards to meet ever more stringent emission regulations. Detrimental to engine performance, but they kept trying. The original ‘low’ compression V12 engines was very thirsty, even by American standards, and redesigned heads for higher compression was one solution - but only radical engine design would do. So the V12 was discontinued - and the AJ6/16 introduced, including better ign advance control…

Doug’s endeavours on ign advance controls are very much appreciated - an uphill struggle in the bewildering maze of Jaguar’s machinations. But any factory installed set-up would work - it’s bastardizing due to single component failures, and resulting PO actions, that makes it difficult to reconstruct the originals…

Run your car with ported vacuum; while originally intended for an engine not fully warmed up - it will serve. The Jaguar prided themselves in a perfectly starting and running engine at all engine temps - and eventual anomalies should be observed over time…

A comprehensive exhaust gas analysis is very telltale, particularly if done on a dyno. But fuel consumtion/economy is depending on more than ign advance; mixture is adjustable, and should be done in conjunction with the analysis - and according to spec procedures. Even then, driving style has a major influence on economy; if you have the driving computer you have a real time feedback. The computer is very precise - and comparing readings under similar driving conditions with different settings may be illuminating…?

While of course observing engine’s well being continuously - you’ll be just fine…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

(tuomo jylha) #18

I bought this car for the summer a few years ago when I think the “old school style” of the Jaguar / Daimler is more stylish bringing my new Jaguar.
There were many minor faults in the car when I bought it. Now, those other bugs have been fixed. Ignition vacuum has been the only problem. Let’s hope it’s okay with that simple vacuum system?

This car is meant to make a “gentleman” summer car and a time machine to the past. I do not look, and I do not want to do perfectism. But that it works and is neat, that’s enough.

If you bought a pre-finished and perfectly restored car, would it have been too easy? Hobby

(Doug Dwyer) #19

The timer begins a new countdown whenever the engine is started, regardless of coolant temperature.


(Doug Dwyer) #20

I regard the Series III as “The most modern of all antique automobiles” :slight_smile: