Tripps, since it's an after market tach it's getting its signal from 1 of the coils, if coil is breaking down underload than tach will also drop. Trace pickup wire back to coil and make sure all connections are good. If so either move pickup to another coil and see if still does it ( bad tach) or replace coil.
 
my 2 cents
instead of the coil stopping the tach it might be that the tach is causing the coil probs.
if the tach is shorted then the coil or crank sensor will be shorted.
off the top of my head i would assume that tach to be hooked up to one of the coil grounds or to the crank sensor. my best guess would be that they would tap into a wire at the ecu.
if u can find that wire and disconnect it might tell if the tach is the prob. also if u can check to see if any of the coil wires goes to ground with ign on.
 
****, I'm good.:rolleyes:
I can't believe all that dicking around yesterday on the front coil, 2 bolts, 15 minutes and the front coil was in and out, done, the hardest part was lining up the bolts through the steel plate, coil, and into the threaded boss below, different setup than the other two coils.
There's a very specific procedure I follow exactly when reassembling.

1 Lower tank and bolt down.
2 Realize that I left the speedometer cable under the tank.
3 Unbolttank and fish out cable.
4 Bolt tank down, connect speedometer and bolt down speedometer.
5 See plastic side cover on ground that has to go on before tank.
6 Unbolt and unhook speedometer.
7 Unbolt and raise tank.
8 Install side cover.
9 Bolt tank back down and reinstall speedometer.

If you carefully follow this procedure every time, it will become ingrained habit and you won't even have to think about it.
 
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