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Old 04-01-2015, 07:18 PM   #1
blaine.hale OP
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Quick Dyna 3 diagnosis question

1978 Moto Guzzi 850 LeMans, not that it matters.

The ignition is at least 10+ years old when I got it. I got decent spark initially, then it kind of got a bit week. Yesterday, spark gave out all together. No spark at all. I went around and checked all grounds. The ignition unit, as we all know, grounds on the negative terminal to the battery which grounds to the frame of the majority of bikes.
No ground issues, new battery.

I then pull the multi meter and start checking for 12v at the coils (switched power.) Turn the ignition on and I get nothing at the coils...weird! I pull all cables from the coils and leave JUST the switched power from the main harness (removing the ignition from the equation) and I get 12v power to the coils. I then start hooking the ignition unit up to the coils one cable at a time. Narrowed it down to 1 red cable that's killing the 12v and it's coming from the dyna III ignition unit. There's something like 4-6 red cables out of that box and it's one of the reds to the coils that's killing the 12v.

I reached out to Dyna but figured I'd get everyone's opinion here as well. Think it's a dead dyna unit? I can't think of anything else it could be. It all clearly pointed to that one wire coming from that unit completely killing the switch power 12v at my coils.
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Old 04-01-2015, 09:24 PM   #2
Tim McKittrick
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My Lemans III came to me with no spark and a Dyna 3 ignition. The ground from the ignition box is supposed to hook directly to the engine and not to the battery- if you look through the online PDF of the origonal instruction manual they are very adamant about this. Mine was hooked to the battery, and had apparently been run like that for many years- but the PO had encountered random misfiring and the occasional dropped cylinder. I moved the ground to the engine as specified and suddenly everything was just Jake: big fat blue sparks.

Why this makes a difference I don't know- it wouldn't seem to matter given that the battery cable hooks right to the motor as well, but there you are. If your controller is grounded to the battery you might want to experiment with moving it to the engine before you go much further.
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Old 04-02-2015, 04:37 AM   #3
blaine.hale OP
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That is incredibly weird...I will give that a go tonight, thank you! Is there a particularly good spot on the motor you picked to ground on?
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Old 04-02-2015, 09:20 AM   #4
Tim McKittrick
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I routed the ground lead to one of the valve cover bolts. My LM had SP lowers on it so the lead was protected and not easily visible.
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Old 04-02-2015, 09:32 AM   #5
Tim McKittrick
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Here are the installation instructions:

http://www.motomeccaspares.com/produ...structions.pdf

Note both the diagram and instruction step #3

If anyone has an explanation as to why this makes such an important difference I'd love to hear it!
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Old 04-02-2015, 09:56 AM   #6
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Resistance. Especially on older stuff corrosion,dirt ect. build up between engine mounting bolts and frame increasing resistance. Modern electronics are very sensitive to it and the less the better. Dyna stuff particularly,dunno why but improper coil impedence on Dyna S can fry the unit in short order. To the point they recommend using only their coils.
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Old 04-02-2015, 10:16 AM   #7
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This is all good stuff. I'm super diligent about cleaning connectors, and have done so on this bike. I did all my volt readings after having cleaned all switched power and groundings to the ignition.
After I do that and still see something wrong, it's usually either a bad ground or bad unit. Looks like I need to try different grounding before I give up on the dyna
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Old 04-02-2015, 01:08 PM   #8
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Dyna will repair (if possible) their stuff. Or at least they used to.
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Old 04-02-2015, 02:50 PM   #9
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Well, I hooked everything back up. I was hoping to come back and say "holy cow that worked!" but nope :(
I grounded out on one of the valve cover bolts and still have the same issue. I get 12v switched power to the coils until I hook up one wire from the ignition unit to the coils. That kills the voltage down to around .30-.25v
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