Doug,
My Series III 1990 V12 Vanden Plas (5.3L HE V12 w/ Lucas ignition) is equipped with only one coil (DAC6093) as shown in the attached picture. This is correct for my car, but I am not sure if it is correct for the earlier cars. It runs perfectly fine with just one coil to well above 3,000 RPM as it should.
Paul
Right, the 6093 became the official, correct replacement for the dual coils; as reflected in TSBs and parts catalogs.
I was never clear if the single 6093 coil was used on the production line as the cars were being built. Was it?
Both my 1988 V12s had the two coil system, FWIW. And both eventually got the DAC6093!
Cheers
DD
Cheers
DD
These amps do get hot, some more than others.
The intake manifold acts like a big heat sink.
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My wiring diagrams show a different amplifier with the two-coil set-up, Doug - not the AB14…
Of course the ignition module itself might still be the same, in a different ‘box’ - but with two coils wired in parallel; much higher current will go through the ign amplifier. Which creates more heat - and heat failure is a sometimes amp fault…?
Can’t see the reason for two coils - must be something subtle…
Frank
xj6 85 Sov Europe (UK/NZ)
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Of course it should, Paul - it’s meant to run the V12 at 5000+ rpms…
With mechanical points the points bouncing was a problem, absent with electronics, but even so; vintage V12, managed with one coil - unless dual plugs were used…
30 000 sparks a minute is no great shakes…
Frank
xj6 85 Sov Europe (UK/NZ)
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That is Zener diode, Rasmus…
It’s primary purpose is to block the battery voltage from going to ground - but at some specified higher voltage it ‘breaks down’ and let the high voltage go to ground without damage to the ignition module…
Ie; measured with a multimeter, which usually use 9V, it should show ‘break’ from ‘+’ to ‘-’, but whatever in the opposite direction. A failed diode may cause weak or no sparking - but won’t cut and restore ignition ignition…
Frank
xj6 85 Sov Europe (UK/NZ)
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Frank, I think you might be looking at information regarding Pre-HE engines. I’ve come across tech info from Jaguar that doesn’t clearly delineate the different systems.
It isn’t subtle when one of the coils fail, that’s for sure
Discovering the “why?” of Jaguar designs can be entertaining. But, as legend has it, Jaguar couldn’t or wouldn’t source a single coil that would keep up. I think it came down to saturation time.
Years after the HE was introduced they did come up with the DAC6093 coil to replace the dual coil arrangement. (This reminds me of Churchill’s remark about Americans: “Americans will always do the right thing…once all other possibilities have been exhausted” )
Cheers
DD
I wouldn’t mind a picture of his two coils, Doug - and their connections…
The ‘why’ of Jaguar is indeed entertaining, but they seldom did anything without a reason - however obscure. No doubt the electrotechnical explanation of the dual coil set-up is perfectly clear, once explained…
Since failure of one coil in the two-coil set would likely disable the ignition - one is not a spare for the other? But I really wonder if the same ign amp was used in both two- and single coil set-up…?
Frank
xj 85 Sov Europe (UK/NZ)
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Same amplifier, although there was a part number design…presumably coinciding with some sort of revision…circa 1986. But that pre-dates the introduction of the replacement DAC6093 single coil.
The secondary coil is mounted behind the grille. There is a stand-alone sub-harness connecting the “+” and “-” posts of the two coils. The HT tower of the secondary coil is unused and filled with some sort of potting compound.
If the secondary coil fails a very gentle driver may never realize it, as the engine will run fine below 3000-3500 rpm.
If the primary coil fails, well, that’s a different matter. You’ll know right away
Cheers
DD
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Identical coils otherwise, Doug…?
I’ve just got an itch - I can’t understand the function of the secondary coil’s influence on the primary…
frank
xj6 85 Sov Europe (UK/NZ)
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COILS: The Lucas Constant Energy ignition uses two conventional ignition coils wired in parallel. The high-tension lead of the secondary coil is sealed off, and only the lead from the main coil is connected to the distributor. Between firings, energy is built up in both coils. When the 12V supply is broken (“the points open” in the lingo of the pre-electronic age), the energy stored in the secondary coil cannot escape through the high tension lead because it is sealed off, so the energy comes back through the 12V leads instead. The primary coil then not only has to release the energy it has stored itself, but also the energy coming back from the secondary coil. These two energies add to produce a powerful output at the high tension lead on the primary coil.
The secondary coil, located in front of the radiator, is not a spare or a backup; it is designed into the system for producing a good spark. If either coil goes bad, the performance will suffer. The secondary coil is not special, however, and can be replaced with a conventional coil provided the high tension connection is covered so that it cannot arc to ground.
Since mid-1989, XJ-S’s have gone to a Marelli ignition system that also uses two coils. However, these two coils are totally separate; each one fires only six cylinders.
According to Alan Jenks, “Jaguar now recommends replacing both coils with a single “solid” (not oil filled) coil (#DAC 6093) that fits in place of the main coil. The aux coil and wiring is removed.” Roger Bywater says, “The best coil to use on Lucas HE V12s is DAC 6093 (Ducelier coil - 0.62 ohms primary); only one needed and works well in place of earlier twin coil set up. If you can find another coil with primary winding resistance of 0.5 - 0.6 ohms maybe it is worth a try. If the resistance is any more than that it will not be able to build up enough coil energy to fire a spark at the higher end of the rev range when the coil “on time” is very short (about 1.4 milliseconds at 6000 revs). It might also struggle around the peak torque point. Although the V12 constant energy ignition module is fairly tolerant (it runs OK with the blanked second coil removed - albeit with a loss of spark energy) I have encountered some that behave very oddly if the coil is not the correct load match. The DAC 6093 might be a bit expensive but it does the job…”
He adds that the original parallel coil arrangement was “conceived when coils of sufficiently low resistance were not available.”
John Goodman defends the earlier parallel coil setup: “The only difference (between the two coils) is the HT socket is sealed on the aux coil, and is no longer stocked by Lucas. The primary coil is still available from a Lucas distributor and is cheaper than the universal coil Jaguar sells to replace the two V12 coils. I am experiencing coil/ignition related problems on my car and have tried a few of these Ducelier coils (original equip on the XJR-S). I am convinced it is the coil causing intermittent starting and cutting out problems, thinking of resorting to the early twin Lucas coil system which never gave any trouble on previous cars.”
Jim Isbell: “By the way, if you measure the primary, be careful because when you remove the meter the secondary sparks and it can really make you jump; also, when the field collapses you can feel the bite on the primary as well.”
From http://www.jag-lovers.org/books/xj-s/05-Lucas.html - I hope this helps… a little!
Thank you, David - so far…
In a earlier post; Rasmus stated that primary resistance on primary coil was 0,5 ohms - while primary resistance on secondary coil was 1,1 ohm. Provided correct measurement was taken correctly, no wires connected on either coil ‘+’, it implies either different coils are used - or one coil is ‘iffy’…
In either case; could that cause ignition failure at higher revs under load - or the uneven idle? His symptoms, apart from a possible different cause of the uneven idle, is that ignition fails while cruising and pedal is applied…
Frank
xj6 85 Sov Europe (UK/NZ)
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Hello again!
I have some good news and some not so good news.
As stated earlier I ordered an AcDelco amp which arrived today, end of good news.
The temperature on the amp seems to get high, around 80c, but it never dies.
I have a new issue however, now the RPM meter is bouncing/vibrating. Sometimes it jumps only about 50rpm, but when cruising it bounces withing like a 500 rpm range. I also believe the meter isn’t showing the correct RPM:s, I doubt the RPMs should be 3500 when cruising at 90km/h.
The engine also sometimes sounds like it’s pinging in the 100km/h region, which could be due to the newly fixed vacuum advance not switching properly between ported and manifold vacuum. I also suspect there might be some exhaust crackles on highway speeds but it’s really hard to tell.
I’ll double check the coil resistances, however the problem still persists when the second coil is unplugged.
My main theory is still the amp overheating.Could a incorrect pickup gap cause this? I’m not sure whether the gap was set correctly when I put the distributor back in.
Any ideas on what the second white bullet connector wire goes to from the amp (male from amp)?
I found this on the AJ6 engineering site, it states that the two wires from the resistor pack are the wires to the ‘EFI trigger’ and to the ‘Tacho’, but the tachometer in my car runs of Coil +.
It also states that the amp has to limit coil current, and therefore gets very hot, considering the problem still persists when the secondary coil is unplugged, could the cause be a faulty primary coil?
Any ideas on what may cause overheating of the amp?
Kind regards!
Rasmus,
though I don’t have much to say about the 12 cylinder engine your low voltage strikes me and I’d clarify - and if necessary fix this issue first. It looks like your dynamo is not charging at all. At 3500 RPM you should see around 14 V.
Apart from that I do encounter a similar phenomenon on my car - looks as if the tach needle was kind of unstable around the 12 o’clock position. It might be an innocent malfunction.
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
That’s because you’re looking at the part the article is referring to the Lucas OPUS ignition while your car has the Lucas CEI ignition. Two different beasts
Edit:
Ooops. Never mind. I got confused myself !
Cheers
DD
In that hand drawn illustration I’m not sure what Bywater is referring to with “resistor block”.
I’ll try to find a better diagram.
On my car the tach runs off of a white/slate/blue wire coming from the amplifier.
If yours is attached to the coil something is amiss…especially if attached to the “+” post!
Cheers
DD
That’s very good Rasmus!
The amp does get hot, that’s why it’s mounted on the manifold and the need of a heat-sink.
Make sure everything is connected as per the diagram Doug posted and report back.
Yes indeed… Very good oil pressure though!
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In high gear you should read some 2400 rpms, Rasmus, give or take - and steady of course. Likely it is a tacho fault, of course - but a correctly working tacho is a diagnostic tool; to tell you the converter is slipping or you are in a lower gear. Either you trust the tacho - or you don’t…
The tacho is powered from the ign switch (white circuit) through a fuse to green wire. The ign amplifier controls tacho grounding - making the reading. Ie, the ign amp controls both the coil and the tacho - influencing both. But Jochen makes a relevant remark; the voltage gauge shows unnaturally low. The low reading itself should not influence the tacho - but an iffy alternator may cause ‘noise’ in the white circuit. Which may interfere with the tacho, and/or the ign amplifier - and to some degree with the ignition itself…?
However, the low voltage reading might be caused by a low or faulty battery. Have you checked battery voltage with the multimeter, with and without the engine running - and with what readings? A faulty alternator is a fault in itself, and should be addressed - and may have some bearing?
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Pinking sounds a bit like frozen peas dropping on a tin roof. With the US set-up, using the 4-vacuum hoses dump valve, and with 3 vacuum hoses connected to the throttle housing a connection error is a possibility? Have you checked the vacuum ports on the throttle body for respective vacuums - identifying the the one delivering manifold vacuum. Which has manifold vacuum in idle - while the ‘ported’ vacuum shows ‘0’ in idle…
In any case; pinking is very bad for the engine…
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Did you actually get the same primary resistance on both coils - with coil positive wires disconnected on both? As the set-up uses two identical coils - if they differ it may have some repercussions? As an aside; since the secondary windings on the secondary coil is redundant; in some ‘two-coil’ set-ups; a ‘coil’ without the high tension outlet is used as a secondary…
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Negative. The pick-up coil gap only relates to the signal to the ign amplifier - which is either strong enough to trigger the amplifier, or it is not. However; the gap of the pick-up should be minimum 0,2 mm (0,008") - and maximum 0,35mm (0,014’0. You should of course recheck it if in doubt - out of limit, it may cause erratic triggering, influencing both the tacho and the ignition. Also; the ‘tag’ on the reluctor should be parallel to the coil ‘dent’ - both ‘vertical’…
The module invariably gets hot, and needs a good heat sink - as Aristides points out. The module is carrying the coil current - which with two coils is twice the current for the single coil. We normally assess EI as either ‘break’, no current - or ‘make’ with no resistance causing heat. This is not - there is ‘resistance’, friction if you like - resulting in heat. And electronics are sensitive to heat…
Frank
xj 6 85 Sov Europe (UK/NZ)
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This was the source of my confusion, Doug. On the AB14 as I know it; there are only two wires from the distributor - and two wires out. To the coil ‘+’ (for amplier power) and coil ‘-’ (for coil ground). While in my diagram of the two coil set-up indeed showed separate connections on the amplifier to the ECU and tacho. Lading me to believe that the two-coil set up used a different amplifier. Which of course might still use the same ign module within - tolerating twice the current…
With ‘my’ AB14 the machinations of the module would be transferred to the ECU and tacho - with coil ‘-’ as an interconnecting point. Separate wires on an amplifier could just transfer the same signal to the other users - which in both cases have the same requirements…?
Frank
xj6 85 Sov Europe (UK/NZ)
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