Ign Advance on MR-2

[Les Newell]

The ouput of this type of sensor is very dependent on engine speed. At lower
revs it will be a sharp positive spike as the tooth leading edge passes the
sensor and a sharp negative spike as the trailing edge passes the sensor. As
the speed increases the waveform becomes more square and at very high speed
the waveform will approximate the shape of the tooth. The wave shape is also
dependent on the load on the sensor.
The output voltage is also highly dependent on speed. Open circuit voltage
can be anything from a few millivolts at cranking speed to tens of volts at
high revs. The only way to make sense of these messy signals is to detect
the zero crossings as they will always be at the leading and trailing edges
of the tooth.

What are you trying to do? Are you trying to replace the distributor because
you don't have room for it or are you trying to replace the ECU?

If you are replacing the distributor it may be easier to make up toothed
wheels that can be attached to the camshaft and relocate the sensors. The
toothed wheels could even be replaced by blobs of weld on the cam pulley
(assuming it is belt drive). Obviously you will still have to make an
electronic distributor but this should not be difficult. One way would be to
use a set-reset latch driven by two crank position sensors, one for piston 1
at about 10-30 deg ATDC and one for piston 2 at 10-30 deg ATDC. The latch
would then drive substantial PNP transistors, switching the high sides of
the coils (the ECU drives the low side). One coil would then feed pots 1 and
4 and the other pots 2 and 3, using the wasted spark system.

Les


> For all fo you who have replied and tried to help me, Thank you.  I'm not
> trying to be difficult, but I have to really understand how this system
> works.
>
> My dad and I are considering an engine modification which will necessitate
> converting to a distributor-less ignition, or at least fooling the ECU
into
> thinking it has a distributor....
>
> We're thinking that as the tooth approaches the magnet, it induces a
current
> which gets larger as the tooth approaches and smaller as it passes,
forming
> (perhapse) a sin wave pattern.
>
> Now, if the ECU fires a spark plug when this current exceeds a fixed
> voltage, we could advance/retard the ignition timing by adding a bias
> voltage into this equation.  The fact that the pickup only has two wires
> coming out of it implies that all of this math is being done inside the
ECU
> and not by biasing the pickup itself, right?  These details are important
to
> us when we try to build a circuit which emulates this behaviour.
>
> Any insight you may have would be most welcome.
>
> Thanx,
> Mike Diehl.
>
> -----Original Message-----
> From: Chris Conlon
> To: diy_efi at diy-efi.org
> Sent: 8/11/2001 12:46 PM
> Subject: Re: Ign Advance on MR-2
>
> At 12:42 AM 8/11/01 -0600, Diehl, Jeffrey wrote:
>
> >other with 1.  The single-toothed wheel is probably used to run the
> tach and
> >is slightly offset from the cooresponding tooth on the 4-tooth wheel.
>
> The tach should be run via a signal from the igniter. On an '87 it
> probably just takes a signal from the low voltage side of the coil.
>
> >There are no vacuum lines, so it is not doing vacuum advance.  The only
> >moving part is the shaft on which the two wheels move.
> >
> >So, my question is: How does this mechanism manage to advance the
> spark?
>
> That ECU uses an AFM. It doesn't measure MAP directly, but can figure
> out load, based on RPM and the AFM signal. It then uses the load value
> to adjust the spark timing. The ECU controls all aspects of spark
> timing... except dwell, which is probably controlled by the igniter.
>
> Actually that ECU is old enough that the Ne signal from the dizzy
> (the one with more teeth) may go to the igniter, which then feeds a
> copy of the signal to the ECU. (Operation is pretty similar to the
> other setup, though, from the ECU's point of view.) Just a quirk to
> watch out for when you're tracing down wires.
>
>    Chris C.
>
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