I haven’t read Bywater’s treatise in quite some time but I recall that at least some of the rational behind the complicated vacuum advance machinations was to improve idle quality and general engine performance/driveability…and not related to exhaust emissions. And looking at the build various configurations bears this out. Many of the non-emission control configurations use the same gadgets and gizmos as the USA cars…but ‘triggered’ differently.
Years ago, in my XJS days, I was able to convert my USA-spec vacuum advance system to Australian-spec…as I had been told this was the optimal arrangement. The exact details are long forgotten but this was accomplished in a matter of minutes by merely switching a couple vacuum hoses and bypassing one gizmo…or something simple along those lines. The ‘improvement’, if there was one, could not be discerned from the driver’s seat. Interesting exercise, though :). In the end, after fussing and experimenting…out of curiosity as much as anything else…I ended up going right to ported vacuum for the advance system. Again, no change that I could feel in the seat of my pants. This implies, if nothing else, that the control system was (perhaps amazingly) functioning and not particularly detrimental. Precision measuring equipment might’ve revealed something that the seat-of-my-pants left undiscovered, though.
We’ve learned over the years that a great many HE v12s are running around with inoperative vacuum advance capsules and seized-up centrifugal advance mechanisms. Naturally, just returning the advance mechanisms to operational status is a huge improvement.
It’s quite fascinating if you’re in the right frame of mind. Frustrating if trying to trace and repair a fault. None of the Jaguar literature I’ve seen gives any sort of testing information for the myriad of gizmos. If you don’t know the intended, proper operating characteristics of xxx-widget how can you test it? You can perhaps determine if the widget is flat-out dead and inoperative…which is worth knowing… but a subtle fault might go undiagnosed
The Haynes service manual has a V12 HE motor with several vacuum options. The simplest of the vacuum options is the ser 3 “later” model vacuum system. Does this “later” mean that it’s ECU 16, I do not know? This is not the case.
The vacuum is now in line, and probably it is also a fully functional solution? I do not know why the “later” version is much simpler than any other system? If it also works for it, why not?
As far as I know the introduction of the later 16CU ECU does not specifically coincide with xxx-vacuum advance scheme. The easiest way to determine if you have the 16CU ECU is to simply look at the unit itself; it is clearly labeled “6CU” or “16CU”.
A simple test would be to apply vacuum (with a hand pump, for example) to the vacuum capsule with the engine running. If the engine responds you’ll at least know the vacuum capsule is fundamentally operational.
And/or check the advance at various with a timing light, first with the capsule connected, then again disconnected, to observe the difference.
Without knowing the exact intended advance curve is impossible to tell if the system is working exactly as intended but you can at least tell if it is working
I have checked and there is a car ECU 16, due to catalytic converters. The ECU 6 can be replaced by ECU 16, but ECU 16 can not be replaced by ECU 6. Websites will find a wiring diagram of these HE engines ECUs. Ecu 16 is a few pin more and is used in all where there are catalysts. The Ecu connector is 16 and 6 with each other.
Vacuum system includes a temperature sensor which overrides the vacuum delay timer when the engine temperature rises. The vacuum timer works only with a cold engine and when the coolant temperature is below 45 degrees Celsius.
In that engine, the temperature rises rapidly to 45 degrees Celsius, so the absence of vacuum timer may not be essential? Engine warming does not go away for more than 15 minutes and the temperature sensor will shut off the vacuum timer for a few minutes.
With a cold engine ignition is a few degrees a bit too early if vacuum timer is missing from the system. Ignition is then directly in the vacuum settings of the hot engine. Does not change the effect?
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Traditionally, the ECU deals with fuelling, Tuomo - the ignition system is a selfcontained, independent system dealing with ignition only…
The only overlap is that the ECU is ‘informed’ by the ignition system when the coil fires, but has no ignition inputs. With catalytic converters it is important to minimise raw fuel into the cats - and this is done by ECU’s regulation of fuelling. Some study of the difference in ECU connections may clarify if the ‘16’ has any connection with the ignition system beyond the trigger function of the injection…?
‘Vacuum timer’ and ‘vacuum delay(!) timer’ used interchangeably is bewildering - a vacuum delay valve was used to smooth out sudden changes in advance with pedal movement - it did not in itself eliminate vacuum…
The point of vacuum regulations is to modify the centrifugal advance according to engine load. A fundamental characteristic of any petrol engine is that the degree of cylinder fill, the amount of air entering the cylinder, varies with manifold vacuum. This influences compression pressures, which relates to engine knocking if the ign timing is too advanced…
Best engine performance is when ign timing is close to the ‘knocking’ point at all times. With ‘our’ primitive analogue controls we just ensure that the system does not move into knocking territory, initial setting must be low enough to ensure this. On modern system, antiknock sensors backs off ignition if knocking is detected - otherwise the high advance is maintained throughout.
To complicate matters; in some cases vacuum is used to retard ignition - in others to advance it. In both cases; if vacuum is shut off - timing falls back to centrifugal regulation. Which also may differ depending on distributor fitted - life ain’t easy…
One factor with catalytic converters is that they must be hot to do their job, and controlling fuelling is one way to heat cats - but I’m not sure how controlling ign timing can do that…?
The main point is still that ignition timing is material for best engine performance…
The beauty of the later, better 16CU unit is that it is a direct replacement for the 6CU; plug-n-play. All the pins will be the same as the 6CU counterpart.
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The ‘older’ 6CU has no influence on the ignition, Doug - does the extra pins on the newer16CU connect, when used with the appropriate model year, have any bearing on the ign timing…?
The ECU 16 cu enters the switch number 23 lambda sensors coaxial, and the coupling 6 and 7 lambda sensors. If I understand correctly, then these ignition timing is changed in such a way that the catalytic converters to clean exhaust gases more efficiently? I’m not sure.
Ecu 6 cu these connections are missing.
alright. I did not know what information lambdat gives to the ECU. Good knowledge. My friend has xj12 ser 3, and we are getting ignition timings at different speeds. That car works great and works as a good comparison point. My car had to change distribution vacuum and springs.
I did not find any new distributor springs for anything, so I had to put some other used springs. Now, we do not know if all adjustments are working properly? Check and compare those two car ignition values. Maybe the easiest way to find out where is the fault, if there are different values? Values without vacuum, and other values with vacuum coupled.
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The Lambda (O2) sensors detects oxygen in the exhaust downpipe, Tuomo - and fine tunes mixture to maintain Lambda=1: 14,7 grams of air to 1 gram of petrol.
The O2 sensor does not work until heated until heated, by exhaust gas or electric heating, to 350C - until then it sends no data to the ECU. Data is electric voltage; 0,8V with no oxygen present - 0,2V with oxygen detected. With no data from the O2 sensor - sensor cold, or when the Lambda is ordered ‘open loop’, the ECU uses basic mixture settings; adjustable at the ECU vacuum capsule…
The cat converters do not work until hot - but there is no feedback to the ECU on cat temps.
There is no feedback from the ECU to the ignition system - ignition system is a self contained, the ECU deals with mixture. Whether ‘timers’ in the ignition system is designed to alter ign timing to compensate for lack of precise information on engine, O2 sensor or cat temps is guesswork.
Certainly, on the original ‘European’ system; oil temp sensor was used to switch vacuum source with the engine hot - thus altering ignition timing dynamics. What was done with the US, or indeed later versions of the ign vacuum controls - is still vague…
I remember reading that when you first look at the Jaguar V12 engine, it is recalled that a big pile of hoses and tubes that have been applied under the bonnet with an oily spat has been poured onto the engine.
I got the same idea when I first looked at the engine. Now it has all the new seals, so there is no oil on the engine. All oil stays inside the engine.
