un be known to me i had an issue with the positive lead from alternator being roasted from the heat of headers. took outfuse link and alternator. i believe i was seeing voltage dropping for past year. never gave it much thought. when i added Accel Super Coil last year it made tach nonfunctional above 4000 rpms. so i disconnected filter on tach and i was able to read tach up to 5800 rpms+. now with new alternator and wire harness replaced where melted and battery up to charge the problem with tach has reappeared.
suggestions?? dont think i want a 5" Super Comp tach??

 

heres where i had my tach on my old camaro... and loved it.

 

I'd say the new harness is letting noise back through now. Try hooking up the filter again and see if that helps. I've had the same thing happen, only it was due to a flaky iginition module. I wounder if your module can keep up with the big bad coil?

 

module is matched to coil. kit. both Accel. disconnecting filter fixed it last time around? i wil reconnect. wont hurt.