curious tach behavior- 4kqT

[a4kcstq at comcast.net]

I'm using a MAC-10 ECU in an MC-turbo converted 4kq.  When I use the tach output wire from the ECU (pin #7, violet/black), my tach tops out at around 2600 RPM.  When the engine goes above that, the needle drops to zero.  When the engine goes below 2600, the needle begins reading normally again.
When I use the original red/ylw/blk wire from the ignition coil to signal the tach, it operates normally.  I'm using the full ignition system from the MC engine.  Any thoughts or BTDT?  What do other 4kqt installers use to drive the tach?  Maybe the oil pressure warning controller is interfering?  It's also connected to the same red/ylw/blk wire from the coil.
Seems like it would be more correct to get the tach signal from the ECU, but the coil seems to work.
-Peter