Analog tach signalling

[smalcol]

Hi all,
I'm trying to troubleshoot my tach, I'm running MS3X which works fine. It's a BMW M62B44 V8 engine in a 1971 BMW Bavaria which originally had a carbureted I6 engine. I have the MS3 tach signal running to a Dakota digital SGI-8 unit that converts it from the V8 pulse signal to the I6 signal the tach expects. The device has been set-up per instructions and the status lights operate as expected.(Blinking green with engine running) The SGI-8 has two outputs, a logic level 5V and "high voltage" signal. When scoping the signal from each output, the logic level is 5V, the "high voltage" is around 12V. I get no movement from the tach when connected to this "high voltage" output, which did work fine before the engine swap. It's a VDO tach, I've got it removed and it moves smoothly, don't see any signs of failure on the board or components.

Is anyone using the SGI-8 device with an analog tach?

Could I need an additional "booster" circuit to spike the voltage above 12V? I hacked something together based on spare parts and the design below, but didn't have the exact components outlined. (Didn't work)

http://www.msextra.com/doc/ms1extra/MS_ ... VTacho.GIF

[DaveEFI]

Early tachs almost always use the high voltage flyback pulse form the coil negative. The circuit you looked at is designed to replicate that from a low voltage pulse. it usually works.