V12 Marelli Ignition

The Marelli ignition in the later models of the V12 has an identified problem/failure mechanism of the unique distributor cap. I have just experienced this problem and it is scarey. The distributor cap can develop an open circuit in the right hand (B bank)secondary circuit. This causes the ignition spark to not arrive at the 6 spark plugs on B bank. It does not shut off the fuel injector signal. If the car is in a cruising mode with the catallytic converters heated, this situation may be catastrophic and cause an engine compartment fire. The unburnt gasoline passing through the B bank cylinders arrives at the very hot catalytic convertor on the right hand side of the engine which adds to the heat with active combustion and very soon the cherry hot glowing convertor will ignite anything in its surroundings. In my case I detected the smoke and immediately shut down the engine. An insulated cable had already started to burn. The presence of a fire extinguisher saved the day. This is not a good situation!! It is applicable to catalytic equipped V12s with the Marelli ignition. I recoommend the following to protect your car and yourself:
1. Change out the distributor cap and rotor at least every 50K miles.
2. Do not use anything except an ignition cap that is a Jaguar original part. (my cap was Jaguar original but I understand that aftermarket brands fail a lot more often than OEM).
3. If your local regulations (state) permits it, get rid of the catalytic converters on both sides ASAP. Replace them with European down pipes from the manifold. If you opt to gut them, make sure that all of the material is removed from the converter body.
What exasperates this situation is that my failure mode turned out the be the inner contact ring of the cap burning out. This is yet another mode since I have also heard that the cap can develop an internal separation between the coil lead junction on top and the inner ring on the bottom, good grief!! If you like your V12 in its current unburnt condition, pay heed. Jack

John;

pull the rotor and carefully check the inside of the opening that fits on the top of the distributor shaft. I had a similar incident a few years ago and the Kirby Palm book refered me to the rotor. The rotors arc through the plastic until the plastic turns to ash and then shorts out causing what happened to you.

The book recommended applying silicone to the inside of the hole in a new rotor before installation. This apparently solves the problem because it has not happened to me since.

John...sounds like you had a lot of concerned friends....(that's a good thing)...hope all is better and WELCOME BACK ONLIME.

Mike, That is the second failure mechanism that could effect A bank. If I hadnt read the Kirby Palm epic, I wouldnt have shut the engine down so quickly. The treatment with silicone will not stop the first from happening. The brass ring conductor feeding B bank in the distrubutor cap fails and looks like it burned. I cant think of a way to prevent this except change out the cap at a practicle time before failure, not drive the car or get rid of the catalytics. I am getting Euro down pipes and an intermediate pipe from Coventry West. This is a very serious situation and could effect either A or B bank because of the numerous potential failure mechanisms. Actually a coil or any failure resulting in single bank ignition could cause six cylinder mode. While at cruising speed on the interstate you cannot feel the loss of performance due to the power and smoothness of the V12. Its attributes are at the base of its liability. Jack

Why not keep the Cats, and just convert back to the "old school" Lucas ignition system? It is an easy conversion, and there are plenty of donor cars to get the parts from. The Lucas system also allows you to "fine tune" the ignition advance curve and timing so you can increase performance and economy.(Not necessarily at the same moment). In steady state cruising at decent speeds, I regularly get 25 MPG in my 89 coupe. Doing this requires a certain sensitivity to the engines operating condition, but you seem skilled enough to do it. Roger Bywater has stated that the HE engines are sensitive to timing variations, but on my car, with a carefully monitored state of tune, I have achieved some good results.

Jesse, You have hit yet another solution to the problem that I think has great merit. I was tempted to do this with a 92 XJS that I could not coax it into running. I was on the verge of going back to a Lucas HEI system of the mid 80s but I solved the problem. It should be an easy conversion but I cant help but believe that the Marelli computer tuning is superior to the old vacuum/spring weight advance. Since Alabama is lax on emmissions checkups, I have gone to the Euro solution which gets rid of the chemical/fire mess. I think that a Marelli without catalytic back pressure should be great. Your idea of retrofitting an HEI ignition has great merit. Jack

The only problem I have with the Marelli (except the obvious failures) is that the timing maps are fixed. I don't remember how many maps there are, but whatever little trained electrons are in the box, that is all you get. Since the engineers have to "tune" for the lowest common denominator as far as maintainence, weather conditions etc, I suspect that the timing curves are a little "timid". I have wondered if the Euro spec ECUs have a little more agressive maps. That might be a nice addition to your euro solution. I am a bit of a tinkerer, so I like to play around with it, to see just how good I can get it. I figure, since every engine has slightly different requirements, I can truly optimize what I have. In other words, it is just fun!. Happy Motoring

Jesse

John, for those retaining the Marelli system, how often do you recommend replacing the distributor cap and or rotor?

Was the cap involved an aftermarket cap? or was it an original?

Mike, Good question that I have dodged!! My cap and rotor were the originals that came with the car new. They lasted 15 years and 100K miles of mostly highway driving. Upon inspection, the brass conductor inside and on the top of the cap that fed B bank was totally trashed. In addition the rotor was still functional but very badly burned from arcing as Palm has discussed. The rotor damage was starting to ground the spark to A bank. It had to be hours from complete failure. I think that 100K miles is over the outer limit. In this regard Kirby Palms admirable book cites failures that have occured within 50k miles or less. Due to the catastropic nature of the multiple failure potential, I would give it a safety factor and change the cap AND rotor in less than 50K miles. When changing the cap and rotor I would also recommend changing spark plugs and wire set as well if they were not replaced previously. Jesse, if you are interested in optimizing timing curves for specific conditions AJ6 Engineereing in Great Britian who did the Marelli adaptation and maps for Jaguar will also provide custom tailored ECUs for the XJS. A chap by the name of Bywater is an excellent and knowledgable contact there. That might be a high end solution but our V12s are high end cars. Jack

Mike and Jesse, I have revised my post regarding the interval for changing the Marelli caps and rotors. Please revisit the previous post. Jack

Jack;

I had attempted to email a .pdf of my post on the low power high fuel consumption to the email address listed at the AJ6 website, but never heard back. I was interested in what type of diagnostic equipment that they sold might have helped troubleshoot my problem. What address do you recommend be used when emailing Mr. Bywater?

Mike, I initially started to use E mail but found that it didnt get a very high priority. I then switched to phone calls to the United Kingdom since it is a business expense. I have the appropriate overseas number for Bywater but it is at the shop. I will have to retrieve it if you want to go that way. Jack.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Why not keep the Cats, and just convert back to the "old school" Lucas ignition system?[/QUOTE]

As far as I can tell from scanning this discussion, nobody has yet suggested the <i>best</i> solution to the Marelli cap/rotor issues: Leave the Marelli system in place but use a Lucas distributor with it. You can remove the pickup from inside the Lucas distributor, since it won't be used. You can remove the vacuum advance capsule and lock the vacuum advance. It's a good idea to keep the centrifugal advance mechanism functional, since it helps align the rotor with the cap at the various rpms.

The Marelli has two coils and the Lucas cap has only one coil terminal. Connect both coils to the single terminal using an MSD Automatic Coil Selector, which only costs a few bucks.

I recommend that *all* Marelli owners make this mod instead of buying one more rotor and cap. You can typically get a used Lucas distributor -- complete with usable cap and rotor -- for $75 or so. You'll need to replace the HT leads, since you'll need the set for the Lucas system rather than the Marelli system; some of the Marelli wires won't reach the posts on the Lucas cap. You'll also want to paste a firing order chart under the hood, since the HT leads need to be plugged into the cap the same way as the Lucas -- which is different than the Marelli.

Even with all this, you'll probably spend less on the distributor change than on the Marelli rotor and cap. And your worries about the rotor shorting to ground will be gone forever. And the Lucas rotor and cap last twice as long and cost less than half as much.