The stupid tach in my car is driving me nuts. At idle with the choke on the tach sits right at 1k rpm and the with choke off it sits at 0rpm. When you rev it it sweeps normally with engine speed but I know the reading is off. On the highway at cruise it doesnt really matter what vehicle speed you are at the tach sits around 2k rpm. When you accelerate hard or decelerate the tach moves with engine speed. I just replaced both coils with MSD blaster coils and heavily cleaned the contact terminal on the coil for the tach pickup along with the yellow/green pickup wire for the tach signal and its still doing the same thing. Where do I go next? I was going to check the signal wire for continuity and resistance. I have heard some people have to replace the gauge cluster to fix the problem. Any suggestions are greatly appreciated.

 

Anyone?

 

Have you checked your trailing igniters.

 

What about it? The tach picks up signal off of the coil not the ignitor.

 

...& the coil is driven by the ignitor...

Sounds like you're picking up a DC bias that shouldn't be there. Definitely could be the ignitor, or could be a failing tach. Tach is essentially an electromechanical pulse counter; if it's not getting a zero-biased signal, it will shift higher.

Could also be a loose connection that only "makes" when the car is experiencing G's.

If you had access to an o-scope you could check the signal going to the tach... but most people don't have one.

The fact that it matters whether the choke is out or not, leads me to think that you have a wiring fault. Shouldn't matter to the tach either way if the choke is pulled, unless engine speed is actually changing.

 

which wire from the tach needs to tie into the trailing?

 

Yeah the engine speed is higher when the choke is on but from what I have seen the car should idle at about 2k with the choke on and about 1k with the choke off correct? I do have an osiliscope somewhere, Ill have to dig it out.