A tachometer keeps you from destroying your engine by displaying the revolutions-per-minute of the crankshaft to keep you from over-revving the engine. Over-revving a Chevrolet 305 engine can cause the main bearings of the engine to spin and cut off oil to vital internal parts. Correctly wiring the tachometer will allow the tach to display engine RPM without draining the vehicle battery or causing a surge in the vehicle's electrical system.
Disconnect the ground cable from the vehicle battery's negative battery post.
Remove 1/2 inch of insulation from the end of each wire protruding from the back of the tachometer with a pair of wire stripper/crimpers.
Cut the wiring running to the light control switch of the vehicle with a pair of wire cutters. Remove 1/2 inch of insulation from each end of the cut wire. Slide one end of the cut light control switch wire and the end of the white wire into one end of an electrical crimp connector. Crimp the end of the connector containing the two wires with the wire stripper/crimper. Slide the loose end of the light switch wire into the open end of the connector. Crimp the second end of the crimp connector to complete the connection.
Repeat the process to connect the red wire from the tachometer to a 12-volt power supply coming from the vehicle's fuse box, the black wire to an engine ground wire and the green wire to the tach terminal located on the side of the distributor cap.
Connect the ground cable to the negative battery post. Close the hood. Turn on the vehicle. Check the crimped connections if the tach does not function correctly.
