Coil on Plug tach drive

Hello,
I am trying to find a circuit to drive an Auto Meter Elite Tachometer
from the positive wire to the 4 x coil on plugs, which needs to be converted from positive signal to negative signal for the green signal wire of the tach.
I read on here somewhere that I could use a relay coil pin 85 to positive coil feed and pin86 as negative tach signal. I dont understand how that would work as it still is seeing constant power.
I also saw the circuit for Convert a Positive Output to a Negative Output would that work with solid state relay and diode on signal ?


Yes I could buy a auto meter tach adapter 9117 but by the time it gets to Australia it costs about $200.
This is going in 72'ford mk1 escort with a 07 duratec 2.0L
Regards Paul.

There's a NEG signal at the single coil (-) terminal also possibly marked CB I had a 73 and it was frankly easier to put a GT instrument panel in.
The CB or (-) is actually 3-5 VAC and most aftermarket and at that time OEM tachos get driven directly off that wire.
Out of interest no road vehicle back then had "coil on plug" the first to introduce that was Saab late 80's.
Your vehicle would have a single coil, distributor and 4 HT leads going directly to the plug.
It would be good if you can print the wire list most tachos will have 4 leads, IGN +, signal (as above to coil), illumination and ground (earth).

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

I thought it was obvious this is transplant motor with custom ECU custom wire harness. Can't use neg terminal of coil, as the ecu drives the circuit to ground. and the resolution of 1 coil is too slow.
Just because you can read the signal on volt meter switched to AC doesnt actually make it VAC.
My tachometer is strictly 3 wires pos, neg and signal.
No original engine or interior wiring left.
Transplant engine is a Duratec 2.0L out of a Ford Focus '07.

Regards Paul

Go for an injector wire, 4 of them will be common coloured any of the other colours will give you the correct tach signal.
So your tach must illuminate when the ignition is on?
If you're still not sure use a DEI 454t available in Oz from any Viper alarm dealer.
I've done loads of remote starts on Euro Focus and that Swansea build engine was used in both Focus and Mondeo, that's where I took my tach signal.
The trigger wire needs to see 3-5VAC though it's not strictly speaking an AC signal, it oscillates and it's read more easily on VAC.
And I apologise I didn't read your OP last line.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

The autometer 9117 senses current spikes on the positive lead to the coils and it can be used for your application. The ECU you have controls the ground side. So four diodes, 1N4004 or equivalent, connected to each coil negative should work. Banded end to coil negative, non-banded ends connected together for your tachometer signal (green). May need to use a resistor between red and green on tach, start with 10K ohms.

Thanks Howie and Ween.
Yep permanently on but brightness is adjustable.
I was hoping someone would have a similar circuit to the autometer9117, without buying one, Like I said too expensive atm with the aussie dollar.
Think I might try the 4 x diode sounds like it will work without inducing noise on the circuit.
Cheers Paul

If I understand the theory correctly, the negative side of the coil will remain negative until opened by the pcm, making it a positive pulse. The collapsing of the primary field is what causes the secondary to fire.

Your idea of using a DC solid state relay may still work (I wouldn't use a mechanical relay for this) without the tach adapter. Connect your 4 diodes to the negative side of the coils with the stripe side away (non-striped side to the coils). The 4 striped sides will go to the positive side of the SSR trigger/input, while grounding the other side.

Now on the load/output side of your SSR, connect the green wire from your tach to the positive terminal and ground the negative side.

How it works is, when one of your coils fires, the negative lead becomes positive for an instant. This positive triggers the SSR, momentarily connecting the green tach wire to ground.

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My ex once told me I have a perfect face for radio.

I've used the diode method on chrysler 5.7/6.1 engine swaps, where there isn't a discrete output on the factory pcm for a tach.

thanks everyone.
I think I am going to try the SSR + diodes first. if that doesnt work going for the 4xDiodes on their own.
Mulitmeter shows the tacho signal(green wire) as half voltage of positive (red wire) when grounded. Not sure if that changes anything.
Going to be a while before testing this I post back to what works.

Thanks Paul

That M/meter showing is what you're after.


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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.