Tach pulse problems with wasted spark

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wrenchdad
Master MS/Extra'er
Posts: 614
Joined: Mon Feb 14, 2005 5:30 am
Location: Natchez, Mississippi

Tach pulse problems with wasted spark

Post by wrenchdad »

This is my setup
MSI Rev 3 board running 29q2 code using two wasted coil/ignitors twin packs from a 99 Corolla on a 4AGE 20v Silvertop in a 91 Corolla Sedan with a 5-speed. I have swapped out the stock dash with one that has a factory tach.

the problem

I am using the "tach output pin" to drive the tach using the 2N2222 transistor with two 1K resistors circuit. I am seeing a pulse out of the MS, looked at it with a scope, its a 1ms grounding pulse from a 12vdc standing signal. BUT this is still not driving the tach.
Not knowing what this tach needed, I rigged up the old 4AFE dizzy with its builtin coil/ignitor and the orginal ecu on the bench, unplugged the MS and hooked the old setup into the harness on the tach output wire,and turned it with a drill.
The tach worked, that told me; one, the tach works and two, yes, I am wired correctly. Ok, now looked at the stock setup with a scope at about 1000rpm, here is what I think is the problem, I am seeing about 7ms pulse and at 2500rpm about 6ms pulse. I am thinking that the shorter pulse at 2500rpm is just due just the dwell being less at higher engine speed.
The MS puts out right at 1ms pulse at these rpms. So it looks like I need a longer pulse width to drive this tach.

Any ideas on a simple answer to this?

thanks,
wrenchdad
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