Topic: Help! burned out dyna S ignition units!!

[HB-1fan]

k7 old school chopper with the elctrics all stuffed into the little chrome box under the carbs. A fresh bottom and 836 kit, new dyna S and matching coils. Had it running and rode it to sort out the jets. I left the charger on it with the ignition "on" overnight and it has never been right since. I put in a second  Dyna S and while trying to start, it smoked it. If I trashed a coil with the overcharging would this burn the dyna S units?
 The hot wire for the bike comes from the starter solenoid to the only fuse(20a) then through the ignition(key switch). Should the fuse be before or after the switch?

Any help would be appreciated?

Rick

[Bodi]

The fuse is correct, it should be before any switches or other stuff. Leaving the Dyna on overnight may have killed one coil drive transistor, they probably are not designed to be on all the time and it just overheated. If I recall correctly the Martek instructions say to not leave ignition ON with the engine stopped for mong periods, can't recall such a warning for Dyna but it makes some sense.
The replacement burning out is odd. Are you certain it was connected properly? You can check the coils fairly easily, the primary (points side) coil should show about 5 ohms resistance (stock coil - aftermarket ones may be 3 ohm).
A coil will also heat up a lot if left powered - I don't know if it would fail or not. Take a look at the coils, serious overheating generally causes an ooze of black gunk from inside.
Check out Hondaman's ignition box before investing in a new Dyna.

[Triffecpa]

I burned up a couple of Dyna S units on my GS1000 race bike because of a faulty ground wire.  If your wiring is all enclosed in a box, you might want to double check for a short.

TR

[JLeather]

Hey man,
  First of all, if you aren't already a member, check out www.HondaChopper.com.  All about CB750 SOHC choppers.  There's a simplified wiring diagram there too in the FAQ pages for wiring a chopper without the unnecessary crap.

Now, as to your problem, I do know that jump-starting a CB750 can fry a DynaS sending unit.  The interesting thing about your symptoms, though, is the fact that it still ran on the first one.  Usually, in my experience, when the DYnaS dies it goes suddenly.  Almost like turning off a switch or running out of gas.  Have you tried putting the old module back in yet?  Also, Dyna had some trouble with a run of units about a year ago I believe.  It's possible your replacement was from that run?  If you've got 'em, I'd put points on it and see if it runs properly.  They're a bit more forgivving, and contrary to popular belief you can use the points with your Dyna coils if they are properly set.

[TwoTired]

Now, as to your problem, I do know that jump-starting a CB750 can fry a DynaS sending unit. 

Um, only if you also believe in Voodoo.

It runs on 12V. If the jump unit is also 12V, the Dyna has no means to tell the difference.  Now, if the person connecting up leads reverses polarity, that's abuse it may no be able to withstand.

I'd be far more inclined to believe faulty grounds can be an issue.  If only I had a DynaS schematic, I could then say with more certainty if back EMF currents could find their way to damage components.

I'd like to hear more about the battery charger you left on it overnight, and if there is any chance you, even momentarily, connected it with reverse polarity. (Particularly while the battery was flat.)

Possible scenario:
Reverse connection while key switch on fries dynaS.  Failure mode is a shorted switching transistor.
Coil is then energized for an extended period, overheats and damages internal winding insulation.  This lowers the primary resistance.
New DynaS fails from coil with too low primary resistance creating excessive current in DynaS's switch transistor. poof.

Action: Measure coil primary resistance. Replace as necessary.  If new DynaS still does not function, then replace that, too.

Were I you, I would also make durn sure the DynaS unit has a solid return path to the battery, too.

Cheers,

[Helo229]

I don't know that this would be applicable or not, but I do a good deal of work with computers and other electronics and overheating is a constant issue. Considering the amount of juice that flows through your coils, would enclosing them inside a chrome box really be the best of ideas? Perhaps while they were enclosed heat built up fried the coils themselves? At least to the point that they'll no longer function under load?

[eldar]

When I got my dyna, I remember instructions about not leaving the ignition on for a long time as it can cause a coil to over saturate and damage itself with potential damage to the dyna as well.