magnetic trigger coil driver load req'd for signal use?

[Mike R Brown]

clayb wrote:

> I took the easy way out and bought a Crane aftermarket crank trigger and
> used a Pertronix points replacement kit to make my distributor into a
> cam position sensor.
>
> I suspect the Autronics SMC is not getting it's trigger signals. It does
> not register any RPM while cranking, nor does it fire the ignition.
> Also, it does not turn the fuel pump back on when cranking begins.
>
> both triggers are built as ignition coil inductive drivers, and want to
> switch the low side of a 12V ignition coil to ground. I simply connected
> them to the signal inputs of the ECU, thinking ECU internal pull-ups
> would provide the 12V part of the square wave, while the coil drivers
> would switch to ground. I tried 9K external pullups, but still the ECU
> didn't seem to know it was cranking. It started up with no other errors,
> connected to all its sensors.
>
> With these big transistors, do I need a high current load to get a 12V
> square wave to signal the ECU? Has anyone else run into this before.
> Should I use small-value resistors and more current, like 20 - 40 mA?
> I'm going to try and borrow an O-scope tomorrow, so I can tell for sure.
>
> - Clay
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Clay,
    Are you sure that the original ECM used inductive pickups in the
distributor ?  If not no pull-up is going to give you the signal levels you are
looking for.  When I put a crank trigger on the '34 I ordered the hall effect
pickup from Crane.  This will give you either low level signal used by magnetic
inputs or a 12v square wave required by many ECMs.

Mike

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