How to Check or Test Polarity on a Car


Introduction: How to Check or Test Polarity on a Car

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Video tutorial on how to determine polarity when working with a vehicle’s electrical system. Electrical polarity is used to determine which pole is the positive or power and negative or ground. In many situations it’s important to determine which wires are positive or negative as this can affect how a circuit or component operates. If the polarity is wrong, meaning you have the positive and negative wires mixed up, whatever you connected may not work or it can cause permanent damage. Reverse polarity will not allow leds to work, cause a motor to turn in an opposite direction, etc. While you can refer to a vehicle’s wiring diagram to determine polarity, it’s always best to verify yourself using the proper equipment and I will demonstrate this using a test light, digital multimeter, and analog multimeter.


  • test light
  • digital multimeter
  • analog multimeter

Step 1: Removing Any Components or Required Trim

While I’m redoing the audio system in my truck, I’m having to sort through which wires do what. Typically black was a standard choice of color on North American or Asian vehicles for negative. European vehicles would use black as a positive wire instead and brown would be used as a negative wire color. But that has changed over the years due to the complexity of electrical systems.

Step 2: Using a Test Light

First using a test light, using the ground clamp, connect it to a known ground source. This can be a known ground wire, engine, frame, body, brackets, or whatever else that may be a main ground source on the vehicle. For this, I’m using the metal bracing in behind the dashboard. I know this is a good ground source as some of the wiring does have a main ground connection as you can see here.

Next is turning on the circuit if needed, then touch the light on the electrical connectors in the terminal. If you know what circuit is on, once that light illuminates, this is a power source. If it was a ground source, the light would not work. Two negative sources or two positive sources would not allow the light to work. Using a known ground source, as a process of elimination, the last touched source would be positive.

Step 3: Using a Digital Multimeter

Moving onto a digital multimeter. Make sure the test probes are in the proper location, then is setting the multimeter to the two-digit DC voltage setting. A vehicle’s electrical system is DC, meaning direct current. Here I’m using the trailer harness as an example, I already know the exposed terminal is ground. Touch the black probe to the ground terminal, then use the red probe which is used for positive, touch that on the other terminals. Once a reading is shown and voltage is present, you should have a positive value reading. With the probes being in the correct location on the meter, black would be negative, and red would be positive.

If you have the test probes mixed up on the test source, so red would be on negative and black would be on the power source, then the meter would give a negative readout.

Another process of elimination instead of only using the plug is finding a known generalized ground source just like using the test light. For this, I managed to get a good ground source from the license plate bolt on the bumper. This is a good method for reducing the chance of making an error within a connector.

Step 4: Using an Analog Multimeter

And finally is using an analog multimeter. Again the test probes will need to be in their correct positions. Set the meter to the DC voltage setting, in this case instead of multiple digit reading, each setting will max out a certain value on the needle swipe. Being a 12 voltage system, if we set it to 10, we would only see a maximum readout voltage of 10. Therefore you would never see anything above 10 such as current battery voltage or when testing an alternator’s output. So we’ll need to set this at 50 instead and use the appropriate readout on the needle swipe. Touching the black on the ground terminal and the red on the positive, you can see the voltage present. Again you can also have the black test lead on a known ground source like the license plate bolt such as previous.

Step 5: Testing Signal/Pulse Wires

Now for when the analog meter shows its strength is when testing a signal wire. This can be used for a pulse circuit such as a signal bulb, hall-effect sensor, etc. I have turned on the four-way flashers, again touching the black probe on the ground terminal and the red probe on the positive flasher terminal, here you can see the pulse on the analog meter.

Switching the meter over to the 10v setting, here you can see a more prominent needle movement. You can get a clean, noticeable movement.

If you had the probes reverse, the black probe on positive, and the red probe on negative, the needle will try to move in the opposite direction. Keep in mind, depending on the meter, this may cause damage such as bending the needle which is why it’s always good to connect to a known ground source with the correct probe.

If we were to use a digital multimeter for checking pulse signals, while a reading is shown, the values will jump around is this may not provide an accurate reading or work on something like a hall-effect sensor.

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