# MegaSquirt Tach Output with MSD 6a?



## xr4tic (Dec 10, 2001)

I just installed an MSD 6A, and found out that I no longer have a working tach because it doesn't work off the coil neg post anymore.
Searching says I need the MSD 8910 Tach Adapter, but I'm wondering if anyone has tried running the stock tach off a MS output, based off one of the circuits in the msextra manual - http://www.msextra.com/manuals...hoout


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## need_a_VR6 (May 19, 1999)

*Re: MegaSquirt Tach Output with MSD 6a? (xr4tic)*

I have tried, but not with full success. It was on a Mk1 cab with the MS tach circuit driving the stock tach, using a normal relay coil to pullup to 12v. Bench tested perfect with a Autometer tach and worked in the car for awhile. Then out of nowhere it would fry the processor, vbatt would read 0 and the car would run like dookie. Replace processor, repeat. 
Now this was on 029Q which was *just* before they changed to a 50% duty tach output. It might work with newer code versions, just beware that you might run into a similar problem. Another possible fix would be a current limiting resistor (1k would probably do it) in series with the relay coil.


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## xr4tic (Dec 10, 2001)

hmmm, sounds like I'm just better off spending $30 for the adapter and be done with it.


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## need_a_VR6 (May 19, 1999)

*Re: (xr4tic)*

A new processor is only $18 and that's only a maybe







You could be the first person to get it to work on a VW think of the fame!


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## xr4tic (Dec 10, 2001)

Is it true the MK3 tachs run off a 5V square wave?
If so, maybe this is the motivation I need to do the MK3 dash swap I've been putting off....


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## need_a_VR6 (May 19, 1999)

*Re: (xr4tic)*

Yes MK3 tach runs off 5v square, I've used the MS tach circuit with a 4.7k pullup successfully many times.


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## xr4tic (Dec 10, 2001)

cool, so this circuit:








with a 4.7k resistor where it says 1K - 10K?
If I get bored, I may take a look at the tach in the mk2 cluster, may be a way to trigger it off of a lower voltage.


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## need_a_VR6 (May 19, 1999)

*Re: (xr4tic)*

Yep, exactly. You can always try a 2k pot in series with at least 200ohms and try and go way low on the resistance, and see where that gets you.


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## steve12345 (Jan 8, 2006)

*Re: (need_a_VR6)*

I'm running a mk2 tach off of msd6a's. I'll have to go look at what I did. I _think_ that I just put diodes on the tach output pins of the 6a boxes and tied them together after the diode and sent it to the tach. I'll go look.


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## steve12345 (Jan 8, 2006)

*Re: (steve12345)*

Yup. With one MSD 6a you should be able to run the tach out connector right to your MK2 tach lead. On my boxes, it's the recessed spade connector on the end of the box. I have two for my wasted spark config.
hth
http://****************.com/smile/emthup.gif


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## xr4tic (Dec 10, 2001)

I'll give it a shot, all the posts I've read say it wont work, that it needs the MSD Tach adapter


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## xr4tic (Dec 10, 2001)

Didn't work, of course, I may have fried the tach circuit, I did have it hooked up to the coil with the MSD unit for a short time


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## steve12345 (Jan 8, 2006)

I thought the tach adapter was for square wave tachs like MK3s?


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## xr4tic (Dec 10, 2001)

from what I've read, the tach adapter has a small coil built into it, to simulate the voltage spike that an ignition coil produces on the negative terminal.
I think I've found the schematics and tach driver datasheet for the stock tach, there may be a way to convert it for use with a 12V or 5V signal.


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## steve12345 (Jan 8, 2006)

*Re: (xr4tic)*

Should work on a MK2 if you connect to the tach output connector.
Here's my rig


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## oilpangasket (Dec 5, 2007)

*Re: (need_a_VR6)*


_Quote, originally posted by *need_a_VR6* »_Yes MK3 tach runs off 5v square, I've used the MS tach circuit with a 4.7k pullup successfully many times.

what pin on the white connector on the back of the cluster is for the tach input? and what seems to be the best setting for the 2 trim pots on the megasquirt tach output circuit?


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## [email protected] (Sep 8, 2008)

*Re: (steve12345)*

MSD's tach out signal is a square wave; the tach adapter is for things that need a high voltage pulse instead.


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## oilpangasket (Dec 5, 2007)

*Re: ([email protected])*


_Quote, originally posted by *[email protected]* »_MSD's tach out signal is a square wave; the tach adapter is for things that need a high voltage pulse instead.
Quote, originally posted by need_a_VR6 »
Yes MK3 tach runs off 5v square, I've used the MS tach circuit with a 4.7k pullup successfully many times.
what pin on the white connector on the back of the cluster is for the tach input? and what seems to be the best setting for the 2 trim pots on the megasquirt tach output circuit?



just trying to figure out where I wire the megasquirt tach output to to make the mk3 cluster recieve it. I have read pin 22 on the ecu.


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