Help for Newbies on Spark Timing

[David Papworth]

> later. In the meantime, anyone know if you can tie the tach signal from
the
> negative side of the coil directly into the ECM distributor reference pin?
I
> would do it myself and see what happens, but I don't want to fry an ECM
> unnecessarily. I would look at Ludis's schematics, but I don't know enough
> about the electronics side of things to tell if it would fry.  JW

The circuits in the 727 or 165 ECM appear to be trying to amplify and
condition
the low level hall-effect signals from the distributor.

Don't try to tie directly to the negative side of the coil -- a pure
12V to 0V signal would work OK, even though the circuit is designed
to work with much less. The problem is that the primary side of the coil
has inductive spikes to the tune of hundreds of volts. Neither ECM has
the kind of diode/resistor/capacitor snubber network that I would trust to
protect the receiving op-amp from damage. There is no way to
determine from only the schematics whether U13 can tolerate
these spikes with its own internal ESD protection.

Something like an MSD ignition box inserted between the lead from the
distributor
and the coil would suitably isolate you from the coil primary -- you could
then use
the distributor output to drive the ECM and the MSD box to drive the coil

Alternatively, it is possible to design a reasonable snubber circuit
to convert the coil primary into a "safe" signal to drive into the ECM.
This is basically what happens inside an electronic tach.

Something like the following might be a reasonable start:
the diodes clamp anything above +12 or below ground,
the capacitor shunts any high-frequency energy that remains
due to non-zero turn-on time of the diodes. The 10K ohm
resistor acts to isolate the circuit from interfering with the coil, and
to attenuate excess energy.

(from coil) ----| 10K ohm |----+---------------------- (to ECM)
                                               |
            +12V----|<|---------- +---------|<|----------- GND
                                               |
                                               +--------| |------------- GND
                                                            .01 uf

Any circuit that uses a distributor with its own advance mechanism to
drive the ECM is going to cause the fuel injection events to advance
along with the timing -- something that doesn't happen normally.
This probably won't make a lot of practical difference in a batch-fire
system but is a little "strange".




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