Originally Posted by flemsmith
I was looking at the Dyna III ignition module I bought some time ago, and it says it needs to work with coils that have at least 5 ohms primary resistance. I'm pretty partial to the ones I have already, and they're 3 ohms. Wonder if the output amps really can't drive 3 ohms? Since they sell what looks like the same unit for BMW's that are rated for 3 ohm, I'm wondering if they really have a weaker output drive, or just a more conservative instruction sheet? I'm almost inclined to try them anyway, and just not go far from home for the first 100 mi or so in case they go belly-up. But, on the other hand, I'll probably worry about them too much to relax. If anyone has real world knowledge, I'd be interested.
Sorry, no real world experience; but here's some theory:
According to the wiring diagrams in the instructions, the BMW coils are wired in series with a crossover wire connecting one coil to the other. This makes the total resistance that the driver sees double the resistance of each coil, so 6 ohms. The Guzzi diagram has a separate output driving each coil, so it's totally reasonable that they'd say the coils need to be 5 ohms minimum for the Guzzi setup since the BMW setup calls for coils with 3 ohms minimum.
I suggest either switching coils or using ballast resistors to lower the current to what dynatek recommends. 2 ohm resistors with a minimum power rating of 15 watts should be right. A pair of 1 ohm 10W resistors in series for each coil would be easier to find if you're buying parts locally.