Best place to get ignition signal for an ECU ?

[prebordao]

I'm working on a homemade ECU (please, don't laugh ) to control my soon to be ITB's.
I based my ignition signal circuit on a Megasquirt, so I'm following their recommendations regarding it.

I was thinking of hooking it up to the (-) coil connector, but the Megasquirt page refers that "MegaSquirt cannot handle a capacitive discharge ignition (CDI) output directly"...
This means that, having CDI, I can't hook it to the coil (-) ?

On the other hand, I have a Pertronix-like instead of points. I think it is a Hall effect device, so I guess I can use it as an input also...?

[ischmitz]

If you have an early CDI setup with a 3-pin Bosch or aftermarket CDI box the points inside the distributor are what trigger spark. This signal is also used as source for the tachometer (RPM). You will see six pulses per 720 degree of crank rotation.

Keep in mind the distributor has its own mechanical advance and thus the timing of the spark relative to the crank position varies with engine speed and load (intake vacuum). Variations can be about 40 degrees.

Let us know what your home-brew ECU is going to accomplish:

- fuel signals only?
- spark and fuel signals?

For fuel-only the timing of the fuel injection event is not super-critical. So a pulse that goes with the engine rotation such as the points signal may suffice.

If you want to control spark and fuel you need to get a signal that is locked to the crank position and then the ECU will calculate timing. There are aftermarket kits that that use a modified crank pulley and a VR or optical sensor to produce an engine speed and a reference signal. This way the ECU will calculate timing based on engine speed and load from a look-up table.

Regards,
Ingo

[prebordao]

Thanks Ingo.
I believe I have an early CDI (73), but I'll check the number of pins, anyhow. So, if I understand, for these early CDI's, I can still use the coil (-) to trigger the ECU ?

It will only control fuel, so an exact timing relative to crank is not critical. I just want to know the number of ignition events to help me calculate the right amount of fuel to inject.

I'm getting close to finish: ECU and software are done, I'm finishing the throttle linkages and electrical harnesses... But of course, I still have to teste the whole system together.

In a few weeks, I'll post more info.

[ischmitz]

No, you should NOT use the (-) of the coil. You can use the trigger input of the CDI box. With a 3-pin CDI the trigger input has an internal pull-up resistor against VBat inside the CDI box. The points or the Pertronix (points replacement) inside the distributor close a switch and pull the trigger line to GND to release a spark. This happens exactly 6 times per 720 degree of engine rotation.

Looking forward to some more info. Good luck,

Ingo