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1988 Paso 750 with Digiplex.
I think I may have a coil going bad. After the bike gets warmed up one of the cylinders randomly looses spark (of course it doesn't do it in the shop where I can find it). I've already replaced the plugs & wires, trigger and cleaned and checked all of the connections. Also, the Digiplex unit is new. Rather than just ordering up a set of used ones from eBay has anybody ever tried to replace them with another product? I tried to find the specs in the manual but was unable to find anything to help me cross reference them. I'd really like to find something new instead of having to use recycled parts that will potentially go bad again.

Dynatek green(3ohm) or Black(5ohm) will do nicely. You can get either at Bevel Heven as well as the brackets to mount them

Thanks Higgy. I was looking at that kit but it looks like it's not a simple install and it's a bit expensive for my budget. I was hoping to find a coil that's more of a "plug n play" scenario.

Also - Do the coil mounts ground to the frame? I assume not since you want them to discharge at the plug?

Ignition Coils actually have two sets of windings of wire. And it is important to have them well grounded. The first set of windings are the ones connected to the Digiplex box. They make a large magnetic field when energized, and induce that field on the Ignition side of the coil. To fire the coil, the winding is set to ground in the Digiplex box. The grounding of the winding collapses the magnetic field, which generates the big pulse in the high tension winding. So, both windings of wire are connected to ground in the ignition coil, and that ground must be good. With no ground, you will get no voltage.

For coils on a budget, get some stock coils off of e-bay, but from a newer model. 3-5 ohms is what you want, as the originals were 4 ohms, but nobody does that anymore.

Cheers, Phil

I wonder if converting to stick coils might work - so many bikes have them now they are readily available at Breakers

Also - Do the coil mounts ground to the frame?

On a Dyna coil they don`t. There`s a + and - connector but the core sticking out doesn`t require an extra ground connection. Dynatec confirmed that when I had asked a couple of years ago.
I doubt the original coils are different.

G.

PS: you could try these but like any other coil it requires modifying the connections http://www.ebay.com/itm/Vtech-Angle-Coi ... 47&vxp=mtr

I got my second set of Dyna's on ebay for 35 bucks.

not an alternative but i bought new stock coils from italspares for around 40$(AU) each about a year ago.
prob,still has some

Duc750 wrote:I wonder if converting to stick coils might work - so many bikes have them now they are readily available at Breakers

What I meant was why not? Is there any reason not to use stick coils. I've just done a bmw boxer with a set I bought from a breakers for 60 gbp I sold tyhe two I din't need on fleabay for 40 gbp so the whole thing only cost me 20 gbp.

Ok they're not dyna coils but stick coils do seem to get around a lot of the traditional issues with HT lead losses etc

All coils will work. You just need to match the impedance, which stock was 4 ohms. Most coils are 3 or 5 ohms, which also work, as previously mentioned.

The benefits of Dyna coils is that they put out a very strong spark, but admittedly, there are many alternatives of high output coils. I have an aftermaket set from a Harley that work just fine for 5 years now.

You can be the first on the list to put coils from a new bike right on the spark plug. Eliminates the HT lead and spark plug cap all together. Search Ducati 999 / 749. I have no idea what the impedance is.

Edit: I just realized what you meant my Stick Coils. Not the old fashioned round barrels, but the new ones that attach right onto the spark plugs. Same as what I was thinking.

Cheers, Phil

I'm not so sure the new stick coils will work with the old tech digiplex be careful

higgy wrote:I'm not so sure the new stick coils will work with the old tech digiplex be careful

That was my "might" !

Hey everybody, thanks for the input. I really appreciate all of the suggestions.

I've used COP's (Coil On Plug) for a couple of Porsche engines that I've built but I think they were quite high impedance. Also, I think they trigger differently. It would take a bit of research to see if there is a type that would work. It's a great idea especially since they weight a whole lot less and with the open head design of the Paso should have no problem with staying cool. It's the interface with the Digiplex that would be tough. I know Fiat used the Digiplex system. Maybe I'll poke around on some of the Fiat boards to see if anybody is using COP's with their systems. It's a good project for the future. For now, I went ahead and bought a used pair of coils off of eBay ($35 shipped) just to solve the problem for now.

Generally regarding power output and requirements; put together from a book and info that once was on the Silent Hektik website

Typical 4 cycle engines have their lowest fuel consumption and lowest required ignition power (30-40mJ) at their maximum torque. At low revolutions where the carburation is impeded the specific consumption is increased and the required spark energy is raised drastically. Depending on combustion chamber, exhaust and ignition timing it can triple = 90-120mJ. In the range of max. power or under accelleration the need of fuel and spark energy increases, too.

Under laboratory condition it requires 0,2mJ to light up a perfect air/fuel mixture. Richer or leaner mixtures require 3mJ. The energy to light up the mixture is only a fraction of the actual required ignition energy. Common ignition systems require energies of over 15mJ for their high power discharge. To grant a certain spark duration and cover losses i.e. by dirty spark plugs etc additional energy is required which sums up to at least 30-50mJ. This corresponds to a coil that buffers ca. 60-120mJ.

Silent Hektik did use one of their own coils (53mJ power output, 17kV) and tested it on a transistor and a contact ignition. The contact ignition would have a loss of 0,2V and reduce the spark power output to 51mJ while the transistor ignition would have a loss of 1,8V and reduce the coils output to 39mJ.

An original old Ducati coil has an output of about 46mJ, 16kV. With the typical losses the coils output is not fully sufficiant for a Ducati engine. Swapping to coils with about double energy output (60-80mJ) solves the problem. This is why you do feel a difference in idle and throttle response after a coil upgrade.
A Dynacoil green puts out 109mJ, 32kV btw.

from the 1098 service manual:

The coils used on this model are known as “cigar” coils. They are similar in appearance to a spark plug cap. They transform the low voltage from the ECU into high voltage for the spark plugs. To test the coils, proceed as follows:
Measure the resistance between the two pins (1-15). The resistance should be 650 mΩ±50; if not, replace the coil.

I doubt you could use some even if you did find some with 3 ohms simply because of their length.
The spark plugs seem to sit quite low in the testastretta cylinder heads. On a Pantah engine those kind of coils would stick out quite a bit and I`d be afraid that specially the front one could vibrate off.

G.