electronic ignition question

fester · Post by **fester** » Fri Oct 12, 2007 10:08 am

Guys I just need to clarify the workings of the digiplex ignition a bit more. This is for my 89SS.

I want, for various reasons, to use a late model injected tacho on my 900 SS. I am looking at designing a wiring circuit to correctly trigger the late tacho as they normally get a trigger from the ECU.

I have ascertained from one source that the late tacho requires a 12 volt signal so that makes it a little simpler but the late bikes generate 2 signals per 360 degree crank rotation.

I am assuming the early bikes with the digiplex only get one trigger signal per rotation, can someone confirm. I am expecting I need to get a wiring circuit that multiplies the signal but I need to know if I am multiplying 1 x 4 or 2 x 2.

If anyone's got any clues on a circuit too that wouldbe handy.

Tamburinifan · Post by **Tamburinifan** » Mon Oct 15, 2007 12:02 pm

1 per rev.

dave906 · Post by **dave906** » Mon Oct 29, 2007 1:14 pm

Fester
The Digiplex setup on my '90 906 Paso has a single reluctor pickup. There are four points on the flywheel where the pickup generates a pulse for the ignition box. Starting at TDC for the horizontal cylinder there are actually two ridges on the flywheel (so a double pulse is generated), the first is at 0/360 degrees, the second about 5 degrees further round. There are three more single ridges at 90, 180, 270 around the rest of flywheel.
The double pulse at H-TDC is so the ignition has a reference. By design, it also knows that V-TDC is 270 degrees after H-TDC. It does not know if the engine is on the compression stroke or the exhaust stroke so it generates a spark on both strokes for both cylinders.
So the answer is you will have five pulses per revolution if your ignition setup is the same.

cheers
Dave

Tamburinifan · Post by **Tamburinifan** » Thu Nov 01, 2007 1:50 am

I am absolutely no expert on 906 ign system, But;

there are actually two ridges on the flywheel (so a double pulse is generated), the first is at 0/360 degrees, the second about 5 degrees further round.

Any chance you`ve mixed up ign pulse & ign pionts f idle & full advance?
At idle, ign at 5 degrees point gives signal & after about 3k RPM, it`s the full advance point ( 35 degrees) that gives signal.

Anyway, five ign signals/impulses seem strange...

dave906 · Post by **dave906** » Mon Nov 12, 2007 12:26 pm

No I don't believe so Gert.
The second ridge is as close to the TDC ridge as it can reasonably be within the geometry of the flywheel and pickup.
I believe the ignition uses this double pulse as a reference for TDC-H, from where it can compute the rpm, advance and TDC-V updating each rev.
I have looked at the ignition reluctor pulse train on an oscilloscope and the relationship of the pulses does not change with rpm in degrees.

There is a double pickup ignition which was used on silimar age 900SS', and I believe this can be transplanted to the 906 by replacing the flywheel and the reluctor pickup mounting plate. This places two pickups in the flywheel housing spaced by 90 degrees - one for TDC-Vert, one for -TDC-Horiz. However there are also two ignition boxes, one for each cyl and they function independently. Again they fire a spark on both the compression and exhaust strokes as they can not know which is which.

Dave

Tamburinifan · Post by **Tamburinifan** » Tue Nov 13, 2007 11:48 pm

I don`t have enough knowledge about this to argue so hopefully you are right.