Megasquirt 2 HEI tach input

[electro1989]

New ms2 having issue with rpm jumping. Have HEI mod with opto tied to 5 volts with 390 ohm resistor. RPM jumps from 1000 to 2500-6000.
I have scope of tach input and output of Opto to MIcro. Looks like it not switch correctly. This is not a GM module it a aftermarket module. Could this be the reason that the signal has a lot of noise. I am using a shielded cable to Tach signal. Set for falling edge.

[Matt Cramer]

Is this a computer controlled HEI module, or are you triggering off the coil negative?

[electro1989]

Triggering of of GM HEI tach signal . Its mechanical/vaccum unit. So I am not controling the ignition, that the next step after I get fuel only working good.

[Matt Cramer]

Don't use the HEI mod for this - it is only for the computer controlled HEI. Instead, set it up for coil negative triggering.

[electro1989]

I am using a 8 pin gm hei module. Coming off the tach output. So it would be best to go of the negative coil instead with hardware set up for coil?

[Matt Cramer]

By the tach output, do you mean the wire that drove the factory tach (which connects to the coil negative) or the "R" pin of the module?

[electro1989]

Factory tach output.

[Matt Cramer]

Use the R pin instead.

[electro1989]

Switch to neg coil. Set up board for coil input. Megasquirt
operating great now.