To begin with you must determine the location where you want to place the tach. Many tachs can be mounted on the steering column, however due to the size of mine I chose to mount it on the dash. All hardware was included with the tach. All I had to have was basic hand tools. A screwdriver, pliers, side cuts, wire strippers, and a digital multi-meter for 12 volt dc. And Black electrical tape.
Once you decide on the location READ the instructions carefully.
On my Tach : 4 wire hook up.
I ran the wires under the Instrument Housing that has the switches in it. I routed them towards the fuse box. After doing that I ran 2 wires through the firewall. I had to use extra wire to make them longer. Therefore I didn’t hook the wires up to the tach until after I ran the wires through the firewall and connected them to their destinations. One wire went to the Negative side of the battery(right to the terminal). The other wire went to the Negative side of the ignition coil. I had to unplug the coil wire harness plug from the coil and look down inside of the socket to find the + and – signs. I then used a wire splice that was included with the tach and spliced in to the negative wire of the coil. I then hooked up the tach to these two wires. My tach has a light in it so Next I went to the Headlamp switch and removed it leaving the wires plugged into it. This can be done by looking in behind the Lamp toggle switchs. You will see two small tabs with a small screwdriver release them while gently pulling on the switches. I have driving lamps so I have two switches. One may be a dummy switch on some models. Next you use a 12 volt dc meter and locate the dimmer switch wire. I turned on the parking lamps so that the dash lights would be on hooked up my meter and turned the dimmer switch while watching the readings. I tested each wire on the back of the headlamp switch until I found the wire that had voltage go up and down with the dimmer switch. This was the one I spliced into to supply power to the light. That way it could be dimmed. Last I connected the 12 volt power supply to the tach. I tapped into a fuse that turned off and on with the ignition key. You will want to check the fuse sockets one side is hot and turns off and the other side will be dead, connect to the dead side so if the tach shorted it will blow the fuse and lose power. Always do this last for your own protection! After all other wires are connected and taped. Double check all connections! Make sure you did everything correct then start the car. Your new tach should work great.
This is intended to give you an Ideal of what to do. Always refer to instructions included with your New Tach.