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Vehicle Lighting System Quick Diagnosis

Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.

Are you experiencing vehicle lighting system problems?

Are you experiencing vehicle lighting system problems?

Most vehicle lighting system failures stem from simple problems like a burnt-out bulb, loose wire, or corroded connection. There are other causes that are not as common, such as short circuits or a failure in the charging system.

This simple guide is a quick reference to help you solve a lighting problem in your car in a practical way, without going into too much detail. In many cases, this is all you need to resolve an issue with a lighting circuit in your car.

However, if you need something more in-depth, you can use this other guide: Diagnosing Problems With Your Car Lights. Also, you will see several titles in the Resources section at the end of this article where you will find more help with electrical circuits.

Depending on your particular lighting issue, you may have to locate parts, identify wires, take voltage measurements, and perhaps figure out the best way to access connectors and other parts of a circuit. For this, it is a good idea to have the repair manual for your vehicle make and model.

You can get a relatively inexpensive copy through Amazon.

Haynes manuals include:

  • Step-by-step procedures
  • Pictures
  • Component locations
  • Diagnostics
  • Electrical diagrams
  • Maintenance schedule
  • Oil and other fluid specifications
  • Torque specifications

You can use your manual for vehicle maintenance, repair projects, and troubleshooting. So in a short time, you will recover your small investment.

In This Article

  1. Simple Diagnostic Procedure for Electrical Problems
  2. VIDEO: How to Locate a Short Circuit
  3. One of My Lights Is Dim
  4. One of My Lights Doesn't Work
  5. All the Lights in the Circuit are Dim
  6. None of the Lights in the Circuit Work
  7. Why is My Light Blinking?
  8. One of the Low or High Headlights Does Not Work
  9. None of My Low or High Beams Work
  10. My Turn Signals Flash Too Slow
  11. Turn Signal Lights on One Side Don't Work
  12. My Interior Lights Don't Work
  13. My Interior Lights Stay On
  14. My Brake Lights Don't Work
  15. My Hazard Lights Are Not Working
  16. My Hazard Lights Flash Too Fast
  17. Resources

1. Simple Diagnostic Procedure for Electrical Problems

Depending on your particular case, start at any of these three points in the circuit.

  • Whenever you are dealing with a single faulty light bulb, start your diagnostic at that part of the circuit.
  • If you are dealing with a fault affecting only one side of a circuit—for example, the turning lights on the left don't work—concentrate on that branch of the circuit.
  • When none of the components in a circuit work, start at the voltage source of the circuit. There could be a bad fuse, breaker, or connection to battery positive; or a bad ground affecting the circuit.

You can apply the following diagnostic procedure using a voltmeter when diagnosing a circuit.

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Sometimes, it is necessary to confirm if a bulb or connection is receiving voltage. For this, use your digital voltmeter.

  1. First, do a visual inspection:
    • Make sure the bulb is good.
    • Check that the connections to the bulb socket are in good condition and free of corrosion; otherwise, make the necessary repairs.
    • Make sure the electrical contacts on the socket are clean.
  2. Remove the bulb from the socket.
  3. Set your voltmeter to 12 Volts DC (direct current).
  4. Power up the electrical circuit you are testing.
  5. Measure the positive side of the circuit:

    • Connect the red lead of the voltmeter to the voltage contact of the receptacle.
    • Connect the black lead of the voltmeter to the negative post of the battery.

    Usually, negative or ground is the metal surface the socket rests against in the assembly; other times, it is a separate wire, usually black. If you need to identify wires, refer to the corresponding electrical diagram in your repair manual.

  6. Measure voltage in the socket's ground connection:

    • Connect the black lead of the voltmeter to the socket's ground contact.
    • Connect the red lead of the voltmeter to the positive post of the battery.

    If necessary, find the corresponding electrical diagram in your repair manual. Identify the positive and negative contact on the receptacle using the diagram.

  7. If you are working on a 12 volt circuit, your voltmeter should read 12 volts or pretty close to it. Check your repair manual.
    • If you read less than 12 volts (which could happen if the bulb is dim) or 0 volts (the bulb doesn't come on), that part of the circuit is bad.
    • If necessary, use the electrical diagram in your repair manual to trace the voltage to battery or ground, as the case may be, in that part of the circuit.
    • Without moving your voltmeter connection to the battery, trace the voltage along the corresponding side of the circuit by moving the other voltmeter's lead along the positive or ground circuit, depending on what part of the circuit you are troubleshooting.
    • Check the voltage on both sides of electrical connectors and switches present in the side of the circuit you are troubleshooting.
    • The fault will be between the point in the circuit where you can read 12 volts and the previous point of less than 12 or 0 volts.
    • You will have found the faulty part of the circuit when your voltmeter reads close to the required 12 volts and the previous point with insufficient voltage.

When it comes to circuits, most often than not, you'll find a trouble spot at a connection, switch, splice or poor ground due to a loose, damaged or corroded connection or part.

2. How to Locate a Short Circuit

If you think you have a short circuit but can't seem to find the source of the problem, the following video shows you a relatively easy process.

You'll need a tester, which you can make yourself using a 12 volt bulb, a suitable receptacle, wires, and a connector that can be used in a fuse receptacle as shown in the video.

You also need to have the electrical diagram for the circuit you want to diagnose, so that you know how the circuit receives power, and how it is distributed. This information helps you isolate the short circuit source.

Once you understand the logic of this process, troubleshooting becomes easier. The issue now is locating and accessing connectors that you need to disconnect to isolate the problem.

A 12V circuit tester is also an excellent tool to check for opens, shorts, and grounds in a vehicle electrical circuit. However, make sure to buy one safe for your car's computer.

3. One of My Lights Is Dim

This means that the affected bulb is not receiving enough voltage. The most common problems behind a dim light include:

  • Electrical resistance on the positive side of the circuit.
  • Electrical resistance in the ground of the circuit.
  • Gain access to the socket.
  • Check the connections and the positive and ground wires that connect to the socket. Connections must be tight and free of corrosion.
  • Inspect the electrical contacts in the socket.
  • Make sure the electrical wires are not damaged.
  • Test for voltage, and ground if necessary, on that part of the circuit using your voltmeter.
vehicle-lighting-system-quick-diagnosis

4. One of My Lights Doesn't Work

In this case, the bulb has burned out or there is a fault in that part of the electrical circuit.

  1. First, check that the bulb, if necessary: On your multimeter, use the Continuity feature or the lowest Ohms setting.

    Infinite resistance, or no beeping sound, means the filament in the bulb is broken. A beeping sound or low resistant on your reading means the filament is good.

  2. Check that voltage is reaching the socket and that ground for the socket is working as well.
  3. If there is no voltage or ground, trace that side of the circuit to find the fault.

Depending on the model of your vehicle, you may have to remove one or more parts to access the socket and connections.

5. All the Lights in the Circuit are Dim

This problem is similar to a dim light bulb problem. In this case, however, the entire circuit is being affected.

In your vehicle repair manual, locate on the electrical diagram the part of the circuit that supplies voltage to all the lights on the circuit, and use the diagnostic procedure outlined in Section 1.

You may be dealing with a loose, damaged, or corroded wire, switch, or connector.

6. None of the Lights in the Circuit Work

First, focus on the components that could affect the entire circuit.

  • Check the fuse. If the fuse is blown, there may be a short circuit.
  • Check that the circuit breaker is working.
  • A connector or switch on the power or ground side might be damaged.

If necessary, find the portion of the circuit that powers all the lights and test for voltage and ground using the diagnostic procedure outlined in Section 1.

7. Why is My Light Blinking?

Whether one or more bulbs on the circuit are blinking, the two most common reasons behind this failure include:

  • A loose connector or wire.
  • A short circuit that is activating a circuit breaker.

8. One of the Low or High Headlights Does Not Work

Usually, the problem lies in:

  • A bad light bulb.
  • High electrical resistance in the voltage or ground circuit corresponding to the affected light.

9. None of My Low or High Beams Work

Check those points in the circuit that may affect both high or low beams, depending on the case:

  • First, check the circuit fuse for the high or low beams, depending on the case.
  • Check the dimmer switch.
  • Check if there is an open circuit on the voltage or ground side.
  • Examine both bulbs. If both bulbs are burned out, there is a good chance that there is an overload in the car's charging system.

In the Resources section at the end of this article, you'll find a link to an article that will help you diagnose your alternator voltage regulator.

10. My Turn Signals Flash Too Slow

Usually, slow turn signals are caused by:

  • ​​A faulty turn signal flasher.
  • Too much resistance in the power or ground side of the circuit. Inspect the sockets.
  • If you just changed one of the turn signal bulbs, make sure you are using the correct one for your application.

If necessary, consult your repair manual.

vehicle-lighting-system-quick-diagnosis

11. Turn Signal Lights on One Side Don't Work

In this case, the problem may be:

  • A bad bulb on that side of the circuit.
  • Too much resistance in the voltage or ground in that part of the circuit. Check the sockets.
  • If you replaced one of the bulbs, make sure you are using the correct bulb for your application. If necessary, consult the repair manual.
  • Check the turn signal switch.

In order to identify the circuit wires and connections, consult the electrical diagram in your vehicle manual.

12. My Interior Lights Don't Work

If you have problems with the interior lights:

  • Check if there are any burned out bulbs.
  • Check if the circuit fuse is blown.
  • Inspect for any loose or damaged interior light switch wires or connections on the door frame or dash, depending on your car model.

13. My Interior Lights Stay On

Check the following items:

  • The light switch on the door frame could be stuck.
  • The light switch on the door frame could have a short circuit.
  • Check the operation of the dashboard switch for the interior lights, depending on the car model.
vehicle-lighting-system-quick-diagnosis

14. My Brake Lights Don't Work

The most common source of trouble for brake lights that fail to work, include:

  • A failed brake light switch.
  • A blown circuit fuse.

However, other possibilities could be:

  • Your turn signal switch is faulty (depending on your particular car model).
  • The light bulbs are burned out.
  • There is a bad ground connection in the circuit.
  • The power cable is loose or disconnected.

See the Resources section at the end of this article for help checking your brake lights. Also, consult your repair manual.

15. My Hazard Lights Are Not Working

There are several faults that can interrupt the proper operation of the hazard lights, for example:

  • The hazard lights flasher could be faulty.
  • The circuit fuse may be blown.
  • The hazard lights circuit is open.
  • The hazard light switch is faulty.

16. My Hazard Lights Flash Too Fast

Usually, when the hazard lights flash too fast, it is due to:

  • A short in the front or rear of the circuit.
  • Use of a replacement bulb that is not correct for the application.
  • Use of a replacement flasher that is not correct for the application.

17. Resources

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2022 Dan Ferrell

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