Micro sbc fuel on irratic rpm signal

[thatredgt]

Recently installed a micro on a sbc only running fuel. It has an hei distributor. Hooked opti + up to the tach spot on the hei for an rpm input and opti - to 12v switched. Got it fired up but the tach jumps all over the place. At a loss. Not sure what i need to change any help would be appreciated

[Matt Cramer]

Try adding a 20 to 24 volt zener on the input wire, banded end towards the tach terminal. This can help filter out noise from noisy ignitions.

[thatredgt]

Try adding a 20 to 24 volt zener on the input wire, banded end towards the tach terminal. This can help filter out noise from noisy ignitions.

Would this item work ? https://www.amazon.com/gp/aw/d/B00LWLM9 ... ref=plSrch

Or would this one work better? https://www.amazon.com/gp/aw/d/B00DJY23 ... ref=plSrch

[old guy]

You can find them a lot cheaper on E-Bay.

[thatredgt]

You can find them a lot cheaper on E-Bay.

I was looking at them 9n amazon because i can get them faster. However i found aome at a local electronic store. What watt rating should i be using?

[Matt Cramer]

1 watt is fine.

[thatredgt]

1 watt is fine.

Fried r20 today. Do i need the same thing or can i remove r20 and replace it with a normal resistor? And should i add another resistor in the harness to keep it from blowing? Or what?

[Matt Cramer]

Replacing R20 with a 1/2 watt through hole resistor (390 to 1K ohms) often lasts longer if you are using a hot ignition. The through hole resistors get somewhat better cooling.

[thatredgt]

Replacing R20 with a 1/2 watt through hole resistor (390 to 1K ohms) often lasts longer if you are using a hot ignition. The through hole resistors get somewhat better cooling.

Should i replace that and keep it wired the way i had it? 12v to opto - and diode in opto + going to tach signal on the hei?

[Matt Cramer]

Yes.

[thatredgt]

Yes.

Now i have a tach signal again. But it's back to being very irratic. I didnt touch the diode. Only replaced r20 with a 680ohm resistor

[thatredgt]

Started messing with crank signal noise filtering. It helped but i still get rpm spikes every once in awhile. Should i just keep turning the noise filtering up? Could i have fried the diode also when it fried r20?

[Matt Cramer]

The diode is easy to check - see if the resistance is the same way backwards and forwards. If it's failed short, you will get low resistance both ways. (And if it had failed open, you wouldn't be able to see RPM at all.)