[EDIT: this is in response to the OP by carbmon, regarding the use of resistor plugs / caps, just to clear the confusion for those who read this post and did not understand why I posted this]
Resistor plugs vs resistor caps vs suppression wire.
Ducati electronica make some really good electronics packages. They also make some really shite wiring harnesses. Like any manufacturer, they need to provide a reliable package, but costs are an important factor. So somewhere, a decision is made regarding cost vs performance vs reliability and a compromise is struck. Enter the dark days of ducati v-twins (not the lucas electrics days, but not forgetting the Prince of Darkness either..)
One of the problems we encounter on an almost daily basis with Mercedes-Benz vehicles, is the owners who do 'home' tuning on them.
Partly because they want to be seen as a mechanically minded person, and also because many want to save a few $$ on servicing.
The problem is, for the most part, they do not use the service manuals, or OE spec parts.
A classic example of this is most '81-92 models that have electronic ignition (EZL) units, come into the workshop and have poor running, or wont start or idle, to name a few scenarios..
Inevitably, opening the bonnet (particularly on the v8 engined models) reveals a set of 8 super chunky silicone ignition wires and resistor plugs bought from the local auto parts store.
This causes 2 problems.
1. the distributor cap burns out, as the EZL unit pushes more voltage to the coil primary than a normal High energy ignition system. the Bosch coils used have a huge secondary winding which puts out about 20kVa more than an MSD type setup - it's a DANGEROUS spark, not just a healthy blue one. It can cause defibrilation, if not tested correctly.
2. use of resistor wiring & plugs causes the coil to overheat, creates what's known as 'back voltage' and the EZL blows. That lot just cost the owner big biccies. a coil is around $150. an EZL is 10 times that (and up to 50 times that for some models) Aieek! Now you know why you don't want to own a car without a good service history from a dealer or specialist independent…
What has that got to do with this thread you may ask. well it's simple. You use resistor plugs, or resistor caps. and never the twain shall meet. (There is a caveat to this, which I will explain later)
Early engines used a magneto, solid copper HT wires, bakelite plug caps and a spark plug which was essentially a steel conductor insulated with porcelain.
Things changed a lot over the next 80 odd years - technology developed during the wars, lead to subsequent use in civilian production engines. One of these was a 'resistor cap' which fitted onto the end of the HT lead, and the reason for it's use was to control the spark, to stop plugs from burning electrodes and the copper HT wire conductors.
These were common on aircraft, which needed reliable spark at high altitude, where a/f ratios and atmospheric pressure cause all sorts of issues.
Also remember that magnetos which were used to create the potential difference are powerful beasties and generate that high voltage required to create a spark directly from the crankshaft or camshaft in the engine… more rpm, more kVa, bigger hotter spark and no real way of controlling combustion temps and spark except by that old chestnut 'swapping out plugs' - a very common theme of the 30's, 40's 50's & 60's. A hot plug to start and a cold one to run.
When the kettering ignition system was put into production vehicles, things like dwell angle and advance were far more easily configurable, and coils were able to be sucessfully wound to suit specific applications. This meant that the spark plug wires had a known length, and thus the resistance was known, and an appropriate specification plug, along with a specific coil would ensure RELIABLE spark and for quite a long period. Problem was the points inside the distributor… instead of being a low voltage trigger, the secondary voltage jumped the gap. Still, it was only one thing to replace instead of several. The design would evolve.
Move forward to the 1970's and the transistor era. diodes rule OK. Some clever fart at Robert Bosch Gmbh developed a transistorized ignition, along with an electronic fuel injection system (which we all know as bosch D-jetronic, the precursor to all modern motronic injections systems used on all efi vehicles today) and that ignition system was essentially a kettering system, with all of the distributor guts ripped out and replaced with a set of trigger points. The very first incarnations still retained the mechanical contact points however - a slightly ingenious design, whereby if the transistor package failed, you simply rewired the transistor feed to the coil primary, and you had the prehistoric points in the kettering system to get you home…. that's german engineering for you….
But things get more and more complex, and the average Joe, who doesn't have an engineering degree, much less understand what transistors are (remember, joe grew up with valve radios and crystal sets) decides that when his fancy car with all this electronic gadgetry wont idle, chews fuel like there's no tomorrow and fouled plugs and is now unable to start…
*lightbulb moment*
I'll just go down to the parts store and buy me some leads, plugs and a dizzy cap… maybe a rotor button.
So Mr ABC auto parts gives him some cheapass leads, an aftermarket dizzy cap and a rotor button that looks the same as his old one, except the brass bit on the end is shiny and it says 'made in PRC' on the side. Yep, All good!
What the dickhead did not do, is read the service manual for his car. So instead of buying the OEM style Bosch or Beru ignition lead with the solid wire core and 5KΩ resistor cap which is what the owners manual and service manual specifically states, and the bosch non-resistor plug that matches the leads, he takes a lead down to the parts guy and the parts guy gets some Super Cheap / Best Buy / Autozone branded silicone suppression leads, to replace the old grey wire with a metal cap thingy on it - 'youre leads are shot' because the agent has no f-ing idea what a resistor cap is, doesn't carry MB spec parts and reckons the silicone leads are better. he says they wont cause interference with the radio, and thats why the resistance is in the leads...
(see where this is going?)
What could possibly go wrong?!
So Joe Average goes home, determined to fix the car and impress the missus with his repair skills. he fits everything. the new stuff works! Joe cleans himself up, and goes inside for dinner.
THe car runs fine for about a month. then the missus complains it's 'stalling at the lights' so joe goes and gives the car an italian tune up, and all appears well.
a week later same problem. Joe pulls plugs. theyre black and sooty and some are wet with fuel.
"damnation!" after all he just replaced the leads, the plugs the dizzy cap and rotor with all new parts!
Now, this might not make sense to those of you with GM or Ford or Chrysler vehicles that are over 20 years old, but European cars, the MB's, BMW's, Audis, Renaults, Fiats, Alfas, Peugeots & Citröens have used the resistor cap leads for over 40 years.
Beru & Bosch, who make the parts, did so, to control the voltage at the spark plug, and the best place to put resistance in a high tension lead is as close as possible to the plug - from a troubleshooting perspective and from a reliability perspective. having a solid conductor all the way to the cap gives you the ability to simply change a cap, and not the whole lead. no more throwing copper. just replace a ceramic plug cap (unscrew) and put a new one on, and away you go.
And you know if a lead is crook… if it doesn't have the exact 1kΩ or 5kΩ resistance, the cap needs changing. simple.
Now, insofar as running resistor plugs with resistor caps…. It's not necessary, provided the total circuit resistance is not LESS than specification. Sure, running a resistor plug and a resistor cap makes the coil work REALLY hard and Will give you a weaker spark, most plug manufacturers nowadays have stopped making non resistor plugs, with the exception being racing plugs or special use aviation plugs.
So what happens if the plug you needed was an old KLG or Bosch w9dc or some brand that does not even exist any more??
Well, despite the many pro/con arguments about the use of resistor plugs with resistor caps, the manufacturer comes out winning. Bosch, who stopped making non-resistor plugs then issued a service bulletin stating that no safety issues exist when using the resistor plugs in lieu of the standard plug. What they did not disclose is that the electronic units they manufactured were only designed to cope with a certain amount of resistance. Some are 1kΩ others 5kΩ. Motorists started having to replace plugs more often, because the cars started misfiring and fouling plugs, and eventually, tried other non oem parts (which is where the silicone leads come in).
One thing to note about resistor caps versus resistor leads:
Resistor caps are fixed resistance. the HT wire is a solid /multistrand copper core wire with absolute minimal resistance per metre. Silicone resistive leads, have a rated resistance of about 3kΩ per metre (depending on manufacturer). Now, as an example, not many v8 engines have metre long leads, and some require considerably shorter leads - in effect this means resistance is not consistent across all leads / plugs and this creates a variation in the spark, load on the coil, and can even affect ignition timing. Not good. In fact, as soon as we put a set of standard Beru or Bosch leads on a car, 9/10 times the problem is solved, without having to replace a distributor cap or rotor. So if your engine came with resistor caps, you should use them, and not the silicone type leads. It's a classic example of 'who's right' and the OE manufacturer knows best (in most cases)
Bosch and NGK still make some 'special application' plugs for older vehicles, but the original reason for making the resistor plugs in the first place was for RF suppression. The secondary reasoning was the Bosch HEI (reluctor coil) system developed for GM vehicles. Given there are so many variables in different manufacturers systems, a plug manufacturer has to make some concessions with regards to manufacture, and if a more 'generic' resistor plug can be used over a non-resistor plug and the manufacturing process is cheaper due to greater volume and demand, which way do you think they will go?
Unfortunately, Ducati, like Mercedes-Benz, used technology for their application which is (was) perfectly reliable in it's day. The fact that it was european meant it came with a stigma that it was 'complex and expensive' a complete falsehood in reality, and one that still to this day, costs people more money than it should because the people providing service and advice to owners of 'eurotrash' do not: 1. read service manuals or specification; 2. do not understand what they are talking about; 3.try to sell inferior parts which are not up to OE specification or design.
So if the bike is being modified, and the specification from the part manufacturer says you need 1kΩ of resistance, then so be it. If a resistor plug cap fails, it is the same as a broken lead. No spark. Infinite resistance. If a coil output cannot handle a dud lead or plug, then it shouldn't be used in the application. And a coil driver should be able to function without failure on a closed or open HT circuit. If it doesn't, then the design is not very tolerant, or something is very wrong with the wiring.
