Sharing a GM crank signal

[BaldTurboFreak]

Greetings!

I'm setting up a Microsquirt to control 2ndary injectors on a Gm 2.0T (direct injected) engine.
The Motor has what GM calls their 58x wheel (aka a 60-2) and uses a hall effect trigger to read it. When I scope the signal I get a beautiul, clean 5v squarewave.
I was tryin gto use optoin+ on the crank signal wire. My trouble arises when I ground out the opto- it cuts the voltage down so far that it stalls the motor.
I tried using a 10k pot to trim the current down, but couldnt get enough to trigger the uS.
I tried usign the Vr+ input instead and coudn't get it to trigger.

So I tried pulling the digital signal from one of the coil inputs (self excited like ls-1 coils) and I can trigger just fine off of it. The only trouble is the rpm is 25% of what it should be. Tentatively I plan on using rectifier diodes to just pull all 4 coil TTL's into the optoin+ input.

But the whole 58x wheel thing is a stick in my craw. Anyone haveany ideas? I think a pullup circuit may interfere with the factory PCM.
Maybe a seperate transistorbox to trigger a 12v wave to the opto?

[racingmini_mtl]

A transistor and 2 resistors (one pull up to 12V and one current limit) should work. Or you could use the opto as a transistor and connect the input to the base of the output. You'd then just connect the signal to optoin+ and leave optoin- unconnected. You might need an additional inline resistor to limit the current more than the on-board 690 Ohm.

Jean