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Installing a Tachometer to a Coil

by Jack Hathcoat

A tachometer is a fun accessory to add to a performance car that is not difficult to install. Originally developed for steam locomotives, they are now used in the aircraft and automotive industry. A tachometer is designed to gauge the engine RPM, or revolutions per minute. For example, the crankshaft completes 3,000 revolutions in one minute when the tachometer needle points to "3" on the face of the gauge. The danger limits are indicated by the red-colored portion of the gauge, and it is unsafe to operate the engine at these high limits.

Mount the tachometer in a convenient location, one that is easy to read. On back of the unit are three wires: a red, a black, and, usually, a green wire. Route these three wires under the dashboard.

Strip 1/4-inch of insulation off the wires, using a wire-stripping tool. Install a terminal on the red wire, and crimp it tightly in place with a crimp tool. Install a fuse tap onto a fuse that has power with the key on, such as the radio fuse. Crimp another connector to the black wire, and install it on bare metal, under the dash, such as the steering column mount. Use a wrench to loosen a nut or sheet-metal screw that is located on bare metal, and install the wire.

Run the remaining wire through the firewall, underneath the dash, and into the engine compartment. Locate an opening with a rubber grommet installed through which the wire can pass. Route the wire, so that it does not chafe against metal and short.

Attach the wire to the negative side of the engine ignition coil. Crimp a connector that will fit onto the coil terminal, and secure it, tightly, with a wrench. Start the car, and test the tachometer operation.

Items you will need

About the Author

Jack Hathcoat has been a technical writer since 1974. His work includes instruction manuals, lesson plans, technical brochures and service bulletins for the U.S. military, aerospace industries and research companies. Hathcoat is an accredited technical instructor through Kent State University and certified in automotive service excellence.

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