New tach jumps and needle sticks. Steady at idle then when revved seems low and erratic needle movement. Called prosport they said the tach requires a 12v squarwave signal and I might need a msd converter. I have a 1990 tbi 350. From what I read the hei 8 pin module and coil tach out is square... So is the tach out on the coil of my truck square wave?

 

I'm going to put a filter on tomorrow and see if that helps but it's all for nothing if the "tach out" on the coil is not square wave. Anybody know? This it was it says in the fine print on the tach I bought.
"*Note: this is an electronic tachometer and thus requires a square wave electronic signal. This Tachometer is designed for use with 1/2/3/4/5/6/7/8/9 cylinder engines."

 

Yes, the tach output is a 12 volt square wave. However, since it comes right off the coil primary it also contains the inductive spike and ringing associated with the coil firing a plug.

A tach filter will help in that regard.

RBob.

 

When u say filter is that referring to a 10k ohm 1\2w resister inline on the tach output on the coil?

 

Put in the filter and nothing! Same prob....BUT threw on a spare coil and boom. Fixed. Hard to believe the coil can run the truck smooth as butter and have the tach side bad... thank you for the help!

 

A tach filter is usually two resistors and two capacitors. Set up as a low pass.

Good thing the coil fixed it, the old coil was on the way out...

RBob.