I found a 1970 Cougar carcass with a tach dash. I took the tach and have jerry-rigged it to my '70 Mustang's instrument cluster. Looks good with the white face overlay. I've grounded the back of the tach to the car's chassis and have run one of the two leads to my ignition switch for power and the other to the negative side of my coil. The engine fires up and the tach registers but the car runs extremely rough. As soon as I disconnect the wire from the coil, she runs smoothly again. I seem to remember reading something about this (perhaps about an 8K ohm resistor?) in one of my Mustang Monthlies but which one? Does anyone have the scoop on this? If so, what part do I ask for when I go to UAP, Canadian Tire, Radio Shack or wherever? Please give me as much detail as you can because, a mechanic or an electrician I definitely am not.
Thanks in advance, Richard