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I purchased a used underdash wiring harness that did not have the connections for my tach. I'm trying to run just tach wires to get it working. I grounded the black wire from my tach and tried hooking up the red wire to the positive side of the coil but it shorts out my coil. I tried the negative side and same thing happens, shorts out my coil and the engine dies. I know I'm forgetting something fron shop class 101. Anyone know of an easy hook up for a tach?

Jim
 

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There are two basic tach setups. Current sensing, or in-line connection, and voltage sensing. The latter requires one lead from the negative side of the coil; the other is ground. The current sensing type plugs inline between the ignition switch and the coil. It sounds like you have the original Ford tach (in-line) and are attempting to hook it up as a voltage sensing tach.

http://clubs.hemmings.com/baymustang/platesmall.jpgLet me check your shorts! My multimeter is just a-waiting! Formerly known as Midlife in the old VMF.
King of the Old Farts *struts*
 
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