Coil Pack Tach Support ?

[acarlson]

I have a question on supporting old tachs. The Megajolt documentation describes a schematic for splicing a coil pack into a single signal for a tach. AutosSport Labs sells a small circuit board which does the same thing. My question is "Why are the schematics different ?"

In the image below, the left side is the MegaJolt schematic. The right side is the Tach board schematic. In the left side we have only 1n4004 diodes and a zener diode. The tach board on the right side has resistors across the diodes and no zener diode. Why are they different and what's the purpose of the 100K resistors ??

[garrycol]

I cannot answer your question re the resistors but I built the tach driver as per the right hand circuit diagram and it works great.

[brentp]

We found that the resistors across the diodes provide a bit of a leakage current which seems to make certain older tachometers happy.