MSD Streetfire Ignition Upgrade. Tach no go?

[Arlo Cooper]

Does anyone have any idea why my tachometer would stop working with a MSD Streetfire Multi Spark module upgrade? I have been over the wiring a few times. Have pulled the tach out and routed another wire straight from the back of the tach to the grey wire on the module, still no go. I have a pertronix ignitor in my dizzy can I take a tach feed from the dizzy terminals? I have seen everyone else with these just wire straight up and not have a problem or need an adapter. 

 

Any info would be highly appreciated.

 

Arlo. 

[Mike87]

probably need one of these

 

Tach Adapter

[AustrianVespaGuy]

The module puts out a 12V square wave which is useless for the stock 2002 tachometer, which whats to see a high flyback voltage spike from the coil.  I would think that you can still leave the tach connected to the negative side of the coil like it was originally and it should still work just fine. . . What coil are you using in this setup?

[visionaut]

MSD needs a tach adapter like Mike said above...

[AustrianVespaGuy]

15 minutes ago, visionaut said:

MSD needs a tach adapter like Mike said above...

As a matter of interested does anyone know why leaving the tach hooked to the negative of the coil does NOT work in this case?  Does the MSD control the positive instead of negative side of the coil or something?

[visionaut]

MSD Wiring instructions for Tach Adapter...

 

[jimk]

There also is/was another mofel 8920.

This one failed on me and took out the tach.  So I made a tach adapter from instructions on the Adaptronic site msg board using a coil from a std relay and a couple of resistors/transistors/diodes.  It basically utilizes the back emf that happens from a coil and triggers the coil with the transistor from the square wave signal from the MSD tack signal (or ECU if you have EFI)

I gutted the MSD adapter box and installed the new adapter in it.

 

MSD 8920 Tach Adapter.pdf

Edited August 30, 2018 by jimk

[Simeon]

5 hours ago, AustrianVespaGuy said:

As a matter of interested does anyone know why leaving the tach hooked to the negative of the coil does NOT work in this case?  Does the MSD control the positive instead of negative side of the coil or something?

 

In a manner of speaking but that’s not why. With an MSD the coil is used as a transformer which is momentarily applied at the point of ignition with a primary voltage in the region of 400V to give a secondary voltage of x0,000V (depending upon the turn ratio in your coil). Since it doesn’t work with a collapsing inductive field anymore, there is no flyback voltage to drive the tacho. 

[Dudeland]

15 hours ago, AustrianVespaGuy said:

The module puts out a 12V square wave which is useless for the stock 2002 tachometer, which whats to see a high flyback voltage spike from the coil.  I would think that you can still leave the tach connected to the negative side of the coil like it was originally and it should still work just fine. . . What coil are you using in this setup?

Dude you just blew my mind! 

[AustrianVespaGuy]

13 hours ago, Simeon said:

it doesn’t work with a collapsing inductive field anymore

The key principal I was missing, thank you @Simeon! I somehow never realized CDI changed the operating principal of the coil to a transformer! Makes complete sense now that I think about it; horray learned something today!  And apologies to the OP for the poor advice; sorry!

[Arlo Cooper]

Thank you all for that I learned alot from all that information. More that the other CDI threads. 

I have ordered an adapter. Thanks again.