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How to Wire Voltmeter Gauges on a Car

by Don Bowman

Electricity is a valubale commodity, especially in modern automobiles. There was a time when all an electrical system really had to do was run an ignition system, headlights, tail lights, and maybe a radio or AC blower. Now, drivetrains are shot through with electron-driven devices; fuel pumps, interior accessories, lighting and computers all run on electricity now. Ironically, real voltmeters are one of those things that seem to have disappeared even as electrical devices have exponentially proliferated. Installing an aftermarket gauge is fairly simple as such things go, but you will have to spend some time looking for the right wires to run it.

1

Find a mounting location for the voltmeter; under the dash usually works. Mount the voltmeter with the screws provided. On the back of the voltmeter, there are two connections -- one is positive and the other negative. Take careful note of which is which. Hook them up backward, and the voltmeter will work backwards, indicating a discharge when in fact it is charging.

2

Use 16-gauge wire for the hookup if the wire was not provided with the voltmeter kit.

3

Remove the bottom of the dash to gain access to the harness. The harness can be seen coming out of the steering column and going into a rectangular connector close to the column. Probe the connector for a wire that has power with the key off and does not drop to 0 when the engine is started. The large, 10-gauge yellow wire is the starter and will not have any power except when the starter is engaged. Never mess with a yellow wire with a bold black stripe around it. Mechanics call it the yellow jacket wire because, because it's part of the air bag system.

4

Confirm that you have a good constant-on power wire. Look for a grounded accessory wire. Usually, the brown wire is the accessory wire and the green or blue wire will be the main power to the ignition. Any of these will work, however every manufacturer has its own colors for the different circuits. It is perfectly safe to test these circuits. If you can't find a good ground wire, or don't wish to tap into a ground wire, you can ground the voltmeter to the metal chassis with a self-tapping sheet-metal screw.

5

Attach the voltmeter negative to the ground wire with a wire tap, or to the metal chassis using a screw and drill. Connect your tester to the ground, and start probing for a "Switched" power wire that only has power when you turn the ignition key to the "on" position. You should find several going up behind the dashboard and into the steering column. Once you've found a switched power source, turn the key off and cut the wire about 4 inches up from the connector (the cut will be between the connector and the steering column). Attach the wire coming out of the connector to the positive terminal of the voltmeter and the end going to the steering column to the negative side.

Tips

  • The voltmeter connection can be made at the battery positive and negative if desired.
  • Use either butt connectors or the commonly supplied wire taps to connect the voltmeter wires to the wiring harness. Butt connectors are stronger and more reliable, but wire taps are faster and don't require cutting the original wire.
  • Aftermarket voltmeters can come with a number of options, not least of which being a back-light for night running. These lights are often stand-alone, requiring their own ground and power supply. The smart thing with these is to ground them to the chassis, and tap into the dashboard light power wire. You'll find it behind the interior light dimmer switch. You can connect it to a regular switched power source, but the light will stay on whenever the car's running, and won't brighten or dim when you adjust the dashboard lights.

Items you will need

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).

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