Tacho output causes misfire

[2swe]

Hi,
I have built an tacho output circuit with relay coil as per manual, since then I see a lot of misfire, AFR reading jumps between 14 and full lean..

tachometer works well with this circuit.
when I disconnect the 12v supply fom relay coil, engine runs smooth, without misfire.

the 2N5551 is groundet at Proto area ground, same as TC4427 driving my COPs ,isn't it problem ?
should I ground 2N5551 somewhere else, possibly move it outside the MS box ?

thanx for any input..


its MS1 HR11 and VW golf Mk1 1978

[Matt Cramer]

What if you use a 1K resistor instead of a relay coil?

[prof315]

It won't work. Old school factory VW (VDO) tachometers that trigger off of coil- need a BIG voltage spike to operate. Moving the transistor ground out of the MS would be worth trying IMO.

Jeff

[2swe]

I have tried 1-10K potentiometer today, does not work oscilloscope sees nice square wave, but the tachometer is dead..

[DaveEFI]

Never occurred to me to try mounting this unit inside MS. When I used it, I mounted it close to the tach.

[2swe]

so I moved the transistor into relay case.
there is only 1K resistor between JS0 and SPR2, the rest of circuit is out the MS box.
oscilloscope sees square wave with big spike on leading edge, engine runs fine, no misfire, but tachometr is still dead.

I found there is a resistor parallel to the relay coil, I hope it will start working without this resistor.

[2swe]

solved thank you all.