Tachometers & Automotive Gauges

If the instructions for your tachometer are not available, most manufacturers use a green wire for the tachometer signal. The green wire connects either to the negative side of the ignition coil or to the tach wire connection on an aftermarket CD ignition box (which takes priority for the connection over the ignition coil to prevent damage to the gauge or ignition box).

Most tachometers are electrically designed, requiring a signal from either the ignition coil (negative side) or an aftermarket ignition box to provide the engine speed, or revolutions per minute (RPMs). The signal provided is converted into a calibrated electrical movement of the tachometer gauge needle on the display. The tachometer receives power from a 12-volt source that is on only when the ignition key is in the "on" position. If equipped, a light will be on when power is applied to its wiring, usually connected to the instrument panel lights for visibility in dark conditions.

When installing a tachometer, first determine the mounting options available and mock the tach into place. Many tachometers have a pedestal that allows them to be bolted or screwed in place such as on top of or under a dashboard instrument panel. Some tachs are designed to use a clamp to attach to the steering column between the steering wheel and instrument panel. Others are designed to mount directly into the dash through a pre-cut hole. It needs to be in a position to allow easy visibility while driving and allow a quick glance to give you the info you are looking for quickly. Once mounted, route and connect the wiring and then start the engine to test operation.

For most automotive tachometers and gauges (excluding ammeters), 14-gauge wire or larger diameter (lower gauge number) is required. For ammeters, 10-gauge wire or larger (lower gauge number) is required due to the much higher amperage that will go through it from the alternator. To determine the wire gauge you have (if it is not printed on the wire insulation), strip the insulation off the end of the wire and measure the bare wire strand diameter with a dial caliper. For example, 14-gauge will measure 0.064 inch and 10-gauge (for ammeters) will measure 0.102 inch. As long as the wire is at least that diameter, it will be safe for use. The more amperage that will go through a wire, the thicker it needs to be. If 10 or more amps are going through the wire, the longer the wire is, the thicker (or lower numerically gauge) it will need to be as more resistance builds up with longer wire, causing heat and in extreme cases, a fire.

For basic rpm gauge operation, most designs use 4 wires to allow it to work properly. The first is a ground. It is important to confirm that the ground wire connection is clean and tight as well as verify it is in contact with a metal part of the vehicle that is grounded to the negative battery terminal. If unsure that the ground is working properly, the tach can temporarily be connected directly to the negative terminal on the battery to check for a change in operation. The next wire is the power wire, which should be connected to a fuse box circuit that only powers on when the ignition key is in the "on" position. Using a multimeter, confirm and measure that 12 volts is reaching the tachometer. The third wire is the signal wire from the ignition coil or aftermarket ignition box. Check the connection to make sure it is clean and tight. The multimeter can be used with the continuity function, to ensure the signal is reaching the tachometer. The last wire powers the illumination. 12 volts directly to it will confirm the light works, otherwise the bulb may be bad or the ground may not be working properly. If all of the wires have been tested and confirmed properly connected and functioning, and the tachometer is not working, it will need to be repaired or replaced.

A JEGS expert is ready to answer your questions and help you find the rpm tachometer, automotive gauges set, or individual automotive gauges that you need for your hot rod, muscle car, street machine, or race vehicle.