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Old 05-08-2014, 07:57 PM   #1
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Default Tach Adapter for NA w/ Stock ECU and COP

So I'm going with a DIY of Fab9's COP setup using Denso pencil coils. They have no tachometer output, and neither does the Bosch igniter used in this setup. I read Joe's post here about using the coil trigger lines to generate a tach signal using two NPN transistors as in the spark inversing circuit here.

Does this look right? I'll be using three 1N4001 diodes, and three 1k resistors, as well as two 2N3904 transistors. The square wave inputs are the coil triggers, and the output will go to the tach. I want to use the stock ECU's tach 12v (Black/White) as the 12v source. I would tap into the coil triggers (my FSM disagrees with what most stuff on here says and says I have 90-93 wire colors) at the Brown/Yellow and Brown wires. It makes sense in my head, will it also make sense in reality?

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Tach Adapter for NA w/ Stock ECU and COP-tachadapter.png  

Last edited by TheScaryOne; 05-10-2014 at 03:58 AM. Reason: Clearer title
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Old 05-10-2014, 11:48 AM   #2
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Looks complete, you even have the pullups. I would be prepared to change R2 accordingly to pull the signal stronger.
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Old 05-10-2014, 12:17 PM   #3
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I don't think you need Q2 and R2 for a tach circuit. Polarity doesn't really matter does it? It seems like you're just counting pulses/second.

I don't think you need D3 either. The MS mods show an LED there, but that's just because it's already there as far as I can tell.

Finally, D1 and D2 might work to prevent crosstalk between the igniters, I don't know.

This is what I would build:

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Tach Adapter for NA w/ Stock ECU and COP-tachsignal_zps39a92316.png   Tach Adapter for NA w/ Stock ECU and COP-tachsignal_zps3c0eba25.png  

Last edited by DaveC; 05-10-2014 at 12:27 PM. Reason: forgot the base resistors
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Old 05-10-2014, 08:50 PM   #4
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I was wondering that about polarity myself. I couldn't really find a good post about what the tach signal is looking for or what the stock coils feed. I've got a friend with a scope, I think I might take a look at it to see whether or not I can just use the B/W tach wire without splicing and pull it to ground to make the tach work.
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