Learning, Wiring,

What is the 12v Accessory Wire on a Car Stereo?

Let’s dive into what a 12v accessory wire is on a car stereo.

Most car stereo systems have three cables while some have two. The red and yellow cables are mandatory while the orange and third one (with white stripes), is optional.

The 12v accessory wire is the power that helps to turn the stereo on/off and it only works when the car is running. 

You can use a multimeter and/or test light to identify the cable which is usually red. The process will however vary slightly depending on the type of car stereo.

Locate the 12v accessory cable in your car stereo by following these steps:

  • Get a multimeter and set it to 12V.
  • Take out the stereo and expose the wires.
  • Turn the ignition on and test the three cables, note the one that registers 12V.
  • Turn off the ignition – the 12 volts should disappear.
  • Switch on the ignition again and if the 12 volts value reappears, then the cable is the 12v accessory wire. 

Let’s go into more details below.

The 12-Volt Accessory Cable on a Car Stereo

The car stereo has three cables; red, black, and yellow. The red wire is the 12v accessory cable. The cable is particularly utilized by the stereo to trigger itself. But for it (accessory wire) to turn the stereo on, the ignition key should be in the accessory position.

How Does the 12v Accessory Wire Turn on The Stereo?

Turning on The Stereo:

The red wire threads from the stereo to the ignition switch accessory cables. So, if you switch on the ignition, the red wire is activated which then turns on the stereo. , the 12v accessory cable must always be connected to power a stereo system.

How the Stereo Goes Off:

When the key is withdrawn from the ignition port, the 12v accessory cable loses power automatically. Then, the wire is deactivated. It enters the low-power mode turning off the stereo – since it is the activated red cable that powers the stereo.

How to Locate the 12v Accessory Cable

The straightforward way of identifying the 12v accessory wire is by checking the color – the red wire. But let’s learn another technique of doing the same for cars with OEM and aftermath stereos.

1. Vehicles with Factory/OEM Stereo

Step 1. Set the multimeter to 12V – you may alternatively use a test light. Secure the ground wire of the multimeter to the car’s dashboard.

initial reading of a Cen-Tech digital multimeter at 0v

Step 2. Remove the radio to expose the three wires.

removing the radio to expose the three wires

Step 3. Turn the car’s ignition to ACC and test all the wires. Mark the wire that registers 12V and withdraw the ignition key. This action should take away the readings.

Fluke multimeter at 000.1 reading

Step 4. Again, turn the ignition to ACC and note the readings. If the wire (that registered 12V) displays the 12V again, then that is the 12v accessory cable. 

turning on the car ignition

The red cable works only when the ignition is on and is deactivated when the ignition is off.

2. Vehicles with Aftermarket Stereo System

For aftermath car stereos, you can perform the red wire identification test at the fuse box or on the ignition harness. Just like in the OEM stereos, you may use the test light. (1)

Step 1. Set the multimeter to 12V and secure the ground part of the tester in the dash.

Step 2. Switch on the ignition and examine all the cables in the fuse box or the ignition harness. Note the wire that displays 12V on the multimeter monitor/screen.

Fluke multimeter at 0.000v reading

Step 3. Similarly (as in the OEM stereos), turn the ignition off and observe the readings. 12 volts should disappear.

Step 4. Deactivate the wire by turning off the ignition. Note the readings. If the voltage value returns, then that is the 12v accessory wire.

What Cables Power the Car Stereo?

Conventionally, the stereo system is powered by two or three cables. It depends on the type of your stereo.

Let’s discuss each of the three wires.

The 12v Constant Wire – Yellow Wire

This cable is tasked with keeping the memory alive. It also powers the clock, pre-sets, etcetera. And it is always hot because whether the car is off or on, it will work. That explains why it retains its 12 volts of power. (2)

The 12v Accessory Cable – Red Wire

This cable is marked ACC.

As aforementioned, it functions to electrify the stereo system – to turn it on. And it works only when the car is running or when the ignition is on.

When the ignition is off, it gets deactivated and in turn, switches off the stereo.

The Dimmer Wire – Orange with White Stripes

This cable is optional for stereo systems. It is also called the illumination wire.

It helps to power both the headlights and the dash lights. It can either diminish or increase the brightness of the lights.

Take a look at some of our related articles below.

(1) OEM – https://www.investopedia.com/terms/o/oem.asp
(2) keeping the memory alive – https://www.fatherly.com/love-money/keep-a-loved-ones-memory-alive

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About Sam Orlovsky

AvatarCertifications: B.E.E.
Education: University Of Denver - Electric Engineering
Lives In: Denver Colorado

Electrical engineering is my passion, and I’ve been in the industry for over 20 years. This gives me a unique ability to give you expert home improvement and DIY recommendations. I’m not only an electrician, but I also like machinery and anything to do with carpentry. One of my career paths started as a general handyman, so I also have a lot of experience with home improvement I love to share.

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