OEM Tach off base

[deedartgts]

I bought and installed a rallye instrument cluster for my 84 Dodge Rampage 2.2 to replace my standard idiot light cluster.  I changed out the sending units for the temp/oil so they work as they should. All works well except the tach. It reads about 1000 rpm's too much and the needle flutters or fish-tails at times. I changed out the coil from an 87 Turismo 2.2 and still no go. Thinking it might be the gauge itself, I swapped the one from the 87, which worked fine, into mine and it still does the same thing. I checked all the grounds and connections. Now I'm thinking it has to be the damn computer. So, I swap the one from the 87 (poor thing) and it still does the same thing. I have the FSM for this car/trk and do have all the wiring as shown in the book.
What else could it be? What am I missing here?
Thanks guys for any help with this.

[Jim_Lusk]

Where does the tach pick up its signal? Where does it get its voltage? How is the ground? You have three connections to a tach and in your case one or more is bad. I'm betting ground.

[deedartgts]

Thanks for responding Jim. The tach gets it's signal from the neg. coil post. The ground is integral with the printed circuit board. I checked all the ground connections in the engine bay and the two under the dash. There is a small remote circuit board dedicated to the tach, mounted on back of the printed circuit board on back of the instrument cluster. I had to add a wire from the factory circuit board plug-in to the bulkhead connector to connect to the already installed wire to the neg. coil post. I used an 18 ga. wire, however, the FSM calls for an 20 ga. wire. Could the 18g wire be too big and causing too much resistance?

[Jim_Lusk]

The larger the wire (the smaller the number) the less resistance, so that's not the problem. Make sure every connection is good (especially anywhere you crimped anything).