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High rpm tacho problems, 940 with wasted spark

Sjeng

Sjeng

Member
Joined
Sep 3, 2017
I am running wasted spark ignition on my 940.

Using the miata (mitsu j702t) power stage which also has a tach output.

The tach output of the power stage is low voltage, and my instrument cluster requires a high voltage. So I used this circuit to generate a high voltage signal, as suggested in the wasted spark thread:

HiVTacho.GIF


I used a simple relay as a coil, and wired the power stage tach output to the 1K resistor.

It works okay, I have a tach signal. However, when the rpms increase to over 5000 then the tacho starts to bounce and it will not show the increasing engine speeds, but keeps oscillating around 5000.

Any ideas what could be the problem here?

Love to hear any suggestions.
 
What transistor specifically are you using? It may not be able to switch at a high enough frequency to output faster than 5k
 
Hi, I am using the 2N5551 like suggested in the circuit drawing.

Could it have to do with the output signal voltage of the miata power stage, which requires me to use another resistor than 1K ohm?
 
If you don't get it working, you could always put a gutted relay where the original coil was. That's what I did for a while.

I just wired the j702t tach output to the ground wire for the coil, and it worked for me. :e-shrug:
 
supreme940 said:
If you don't get it working, you could always put a gutted relay where the original coil was. That's what I did for a while.
Click to expand...

Okay it works now. Tach ground to relay coil and the ignition power wire conected to the other end of the coil. Doing nothing with the J702 tach out. :)
 
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