The megasquirt is OLD technology when it comes to ignition. It was originally designed to be a fuel-only computer that was triggered by mechanical points. Lots of the first questions on the megasquirt mailing list and modifications to the original circuitry have to do with triggering cleanly from a noisy mechanical points system. Megasquirt triggers when mechanical points open and close.
The voltage of a set of open points is +12, closed is about +8 (graph 1).
The inductive trigger in your distributor puts out about +3 volts, and switches to -3 as the tip of the stator (the X shaped thingy on the distributor shaft) passes the sensor (graph 2).
Your existing distributor pickup will never trigger megasquirt directly.
You need to use something to give megasquirt a +12 volt signal to trigger with (graph 3), instead of the +3/-3 that the inductive pickup gives out.
I use an old Allison/Crane optical ignition system that has a u-shaped optical sensor with a LED on one side of the U, and an optical sensor on the other side of the U. There is a disk with 4 slots that fits over my distributor shaft so that as a slot passes the sensor, light is allowed from the LED to the Sensor, thereby triggering the control unit to make a spark. Usually there would be a coil connected to the control unit to make a spark, but in my case, I replaced the coil with a dummy resistor of about 100 ohms, because i don't need a spark just yet. This is only a megasquirt-triggering signal. The REAL spark happens later. On the yellow output wire of the control box, I get a nice +12 volt signal that goes to +0 volts every time a slot passes by the sensor in the distributor... Perfect signal to trigger Megasquirt.
So I hooked it up. The Yellow output wire from the Allison/Crane ignition box connects to the Megasquirt tach input, pin 24. Once Megasquirt has been triggered, it calculates the proper delay for the spark, then at the proper time, the spark output of the Megasquirt (used to be the fast idle solenoid control on pin 30) goes to the white input wire of the MSD (MSD 6AL, MSD 6BTM.. whatever). The electronic ignition inputs to the MSD are not used. The MSD then pulses the REAL ignition coil and makes multiple sparks, which are routed thru the distributor to the proper cylinder.
1>What optical system? Allison, Crane, Luminition - there are a few other brands. This is 25 year old technology. Don't pay more than $25-$40 for one. I see them pop up on EBAY once in a while. Hookup is 3 wires, Red, Black and the output is yellow. Plus the optical sensor and 4-slot disk (for 4-cylinders).
2>Can I use something else? - maybe. Toyota igniters are too darn smart. If you remove the coil and substitute a resistor, the igniter shuts off after a few seconds. I haven't found a Toyota system 'dumb' enough to be fooled. You could actually run with 2 coils, 1 just wasting spark. I did for a little while.
3>My tach won't work now, even on the tach output of the MSD box. - Right! Same thing, the tach triggers the same way as the megasquirt. The Toyota engineers never re-designed the tach after the cars went from points to EFI. There is an output wire on the Toyota igniter that has the +12 - +8 volt point-style signal to run the tach. But you can't use the Toyota igniter because it won't run without a coil. The tach output of the MSD box is 0 volts, and goes up to about +8 when a spark occurs. Usually this isn't quite enough voltage to trigger the original Toyota tach. Connect the tach wire to the output of the optical control box. Actually I didn' think of that when I put my car together, so I made a circuit that 'floats' the 0-8 volt MSD tach output signal up to 8-16 volts so that it can be counted.
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