Hi I need help getting my 7” dash tach going. This car has a 350 tuned port from an 89 trans am, I pulled tach signal off of the correct white wire on the coil plug.

According to the Holley dash i/o instruction i originally connected to the RPM input and got a very erratic tach reading. I just made some changes connecting the tach lead to pin B26, and yes i have the ground lead connected to pin 19 and I get no tach reading now.

Is this a coil negative tach signal or should i treat this as a rpm input and reconnect to I/O 3 or 4 which support RPM input which was giving me an erratic signal. Ive read some threads talking about a tach sensor filter. Im frustrated lol. Any help appreciated

 

You say "erratic", describe in more detail.

 

At idle the tach bounce between 2-4k just touching the throttle will show 6k and it bounces all over when cruising.

 

MSD in the system?

 

No mad box just standard gm ignition for a 89 tuned port.