You want to hook up the green wire to the coil - side. The coil + side has a constant 12v until the negative side is grounded by the pcm. The negative side is an open circuit until it is grounded by the pcm. I've never hooked up a tach to the + side of the coil, but instead of just having a ground, you're connected to power and then it goes through the windings of the coil to ground. I imagine it may not hurt anything but try hooking it up to the - side of the coil.
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2005 Suzuki Hayabusa GSX1300-R
1980 Ford Thunderbird - 255 V8
ported heads, 5.0L ported stock headers, O.R. H-pipe and Flowmaster 2-chambers, dual roller timing chain
hi-po Mack Truck hood emblem 
1985 Mustang GT 5.0L T5, F-303, GT40p, headers, off-road h, flowmasters, MSD stuff, etc.
Sold 02/06/04 
1989 Mustang GT ET: 13.304@102.29 mph (5-24-03)
Sold - 1998 Mustang Cobra coupe, 1/4 mile - street tires: 13.843@103.41 (bone stock)
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