# MS wasted spark tachometer question...



## msrwerks (Apr 21, 2008)

i am waiting on my MS to be delivered setup for wasted spark. i did not think to have the tachometer output installed in it when i ordered it. so... can someone point me in the right direction to getting my factory tachometer working with wasted spark? i am using this coil...


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## need_a_VR6 (May 19, 1999)

*Re: MS wasted spark tachometer question... (msrwerks)*

There are two solutions: use an MS tach drive circuit, or use a diode arrangement off of your current spark outputs. The first is better, but is a little harder to build, the second keeps you from opening the case. Which would you prefer?


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## msrwerks (Apr 21, 2008)

*Re: MS wasted spark tachometer question... (need_a_VR6)*

hi... thanks for your reply...
most definitely the 2nd solution. i would like to avoid opening the ms box if at all possible.


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## ABA Scirocco (May 30, 2001)

*Re: MS wasted spark tachometer question... (msrwerks)*

It's covered somewhere is the MS manuals. IIRC, the circuit's really simple and looks something like this.


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## msrwerks (Apr 21, 2008)

*Re: MS wasted spark tachometer question... (ABA Scirocco)*

thanks for the reply, but what exactly are the triangle symbols?


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## msrwerks (Apr 21, 2008)

*Re: MS wasted spark tachometer question... (msrwerks)*

i found this... will this work?
http://www.gadgetjq.com/tach_install.htm


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## ABA Scirocco (May 30, 2001)

*Re: MS wasted spark tachometer question... (msrwerks)*

Looks good to me.


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## L33t A2 (Aug 5, 2003)

*Re: MS wasted spark tachometer question... (msrwerks)*

those resistors twisted together like that are essentially negating the diodes with a bypass......


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## Prof315 (Jan 5, 2009)

*Re: MS wasted spark tachometer question... (need_a_VR6)*

Won't MS output a standard 12v square wave tach signal through the spare outputs? That's what I do with all of mine.


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## msrwerks (Apr 21, 2008)

*Re: MS wasted spark tachometer question... (L33t A2)*

do i wire the resister and diode inline with each other? 
the reason i am needing a solution to this is because i neglected to get this included with my megasquirt build and i really do not want to open the MS box.


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## need_a_VR6 (May 19, 1999)

*Re: MS wasted spark tachometer question... (msrwerks)*

Yes Prof, a square wave output of anywhere between 12 and 1v is possible using the MS tach circuit. In his install though, he needs a 'high voltage' tach circuit. I've used the MS to create them by using a relay coil to make an inductive spike, but it needs to be tuned to the tach in many cases. 
I've used the two diode method on a stock Mk2 tach with a VR6 swap. I had to add an additional 1k diode in series with the pair of 1n4004 diodes to keep it from jumping around at high rpm.


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## msrwerks (Apr 21, 2008)

*Re: MS wasted spark tachometer question... (need_a_VR6)*

would something like this work?
http://www.jegs.com/i/MSD/121/8913/10002/-1


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## Prof315 (Jan 5, 2009)

*Re: MS wasted spark tachometer question... (msrwerks)*

Yeah it works fine, I've got the Autometer version of one and it's for sale $65 shipped.


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## DTDub (Oct 21, 2003)

*Re: MS wasted spark tachometer question... (ABA Scirocco)*


_Quote, originally posted by *ABA Scirocco* »_It's covered somewhere is the MS manuals. IIRC, the circuit's really simple and looks something like this.









The diode orientation shown here is opposite of the orientation shown in the other link (http://www.gadgetjq.com/tach_install.htm). To me it seems like the orientation pictured here is correct, because the diodes will then allow flow from the (positive) tach to the grounding coils. If you flipped them like the link shows, I don't think it would work.
Has anyone build this and have it working?
Also, am I correct in understanding the resistors should go in series with the diode to filter the signal and not in parallel like the link shows?










_Modified by DTDub at 12:17 AM 11-23-2009_


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## ABA Scirocco (May 30, 2001)

*Re: MS wasted spark tachometer question... (DTDub)*

I just double checked wiring diagram of the stock coil and it looks like the tach gets current from the coil whenever the -ve of the coil is NOT grounded, so this diagram seems to be correct. The resistor is there, in parallel with the diode, to allow a slight bit of "leakage" passed the diode to smooth out any spikes but the resistance is high enough that the coils don't interfere with each other as a result.


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