So, I built the circuit described here:
http://megasquirt.sourceforge.net/extra/tachopin.html

But it didn't make my tach work. I have 94 wiring and dash.

I played with it on my little proto board (the kind where you have two power and ground rails, and it just has a bunch of holes you can stick components into to test circuits), and with 12v connected to pin 3 via a 1k resistor, and pin 1 connected to ground, there was only 9v across pins one and 3. When I put +5 to pin 2 via 1k resistor, voltage dropped basically to 0 across pins 1 and 3. So if I understand properly, this should be generating a 0 to 9v square wave at pin 3, which is what I connected to the tachometer wire that goes to the diagnostic connector, the tachometer, and would normally attach to the coil (but I have a 2000 coil).

Anyone know how to make this work?

-Mike

 

You know your tach need to be fed by a negative coil signal to work properly. If you have multi coil (coilpack), you need to build this:

 

Nah, I figured out the problem today. I messed up the circuit. I connected the DB37 to the wrong side of the resistor on the 12v side of the 2n2222. So basically, I was just always sending the tach 12v. Once I switched the output wire to the correct end of the resistor, the tach worked fine.

-Mike

 

sly75
thanks for the diagram; that makes so much sense.

COPS are in my future.

 

No problem dude, all diagrams are available through the different manual that msefi.com share