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Turn Signal Problems and Diagnosis

Updated on August 25, 2017
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Turn signals can fail in many ways.
Turn signals can fail in many ways. | Source

Turn signal problems and diagnosis can get confusing. Specially when a system shares wires and connections with the parking, warning, and braking light systems.

Most common problem sources include bulbs, wires, connectors, fuses, flasher units and switches. So the average car owner can troubleshoot these faults with the use of a test light and, sometimes, a digital multimeter (DMM).

When dealing with any vehicle electrical circuit fault, it's a good idea to have the repair manual for the particular vehicle make and model you'll be working on. It helps you identify not only wires and components and their related connections, but how to access, troubleshoot and replace components, as necessary.

Furthermore, sometimes it is necessary to access the turn signal switch. You'll have to remove covers and, possibly, the steering wheel. If your vehicle has air bags, it is necessary to disable the passive restraint system (air bags) before working around the switch to prevent accidentally hitting an inertia sensor that may activate the air bags.

Always follow the instructions and warnings in the repair manual for your particular model to prevent an accident. Haynes manuals are popular and practical maintenance and troubleshooting guides. Get the one for your car make and model.

To make troubleshooting easier, this guide is divided into the most common types of turn signal system failures, troubleshooting procedures, and tips to guide you toward the potential fault much faster. Just look for the section heading that describes your particular problem.

Index
I. One of My Turn Signals Doesn't Work
Checking a Light Bulb Socket for Ground and Power
II. One Side of the Turn Signal Lights Doesn't Work
III. My Hazard Lights Don't Work But the Turn Signals Do
IV. Turn Lights Flash Too Fast or Slow
V. Turn Signal Lights Don't Work
VI. Turn Signals Don't Flash
VII. Turn Indicators on Dash Don't Flash
VIII. Turn Indicators Don't Work But Turn Signals Do
IX. Checking the Turn Signal Switch
X. Testing the Turn Signal Flasher
XI. Turn Lights Don't Turn Off
Check all turn lights when troubleshooting the turn signals.
Check all turn lights when troubleshooting the turn signals. | Source

I. One of My Turn Signals Doesn't Work

This is one of the most common faults in turn signal systems. Here are some steps you can take to troubleshoot the problem:

1. First, check the bulb. You'll need to gain access to the bulb through the engine compartment (front light), trunk (rear light) or by removing the lens itself. Access changes depending on model. If necessary, consult your vehicle repair manual.

2. Most likely, the turn signal bulb is a two-filament type. Make sure both filaments are in good condition, and that the bulb glass is not darkened. Otherwise, replace the bulb with another one of the same type and wattage.

3. Examine the bulb socket for corrosion or damage.

4. If necessary, check the socket ground and power connections. See the next section.

Light bulbs can wear out and stop working.
Light bulbs can wear out and stop working. | Source

Checking a Light Bulb Socket for Ground and Power

You can use a test light here:

1. After removing the bulb, clip the test light to the socket wall (ground) and touch each socket contact inside with the tip of the test light.

2. Have an assistant operate the turn signal. The test light should flash. Otherwise, there something wrong with the ground or power connections.

  • Connect your test light to a good chassis ground. A bolt or unpainted metal bracket will do.
  • Then touch the socket contacts with your test light tip and have your assistant operate the turn lights. The test light should flash. If it does, check the ground connection at the socket.
  • If the test light didn't flash, connect your test light to battery power and touch the socket wall (ground) with the tip of the test light. Your test light should illuminate. If it does, check and repair the power source connection at the socket.

One turn signal can fail in a turn signal system.
One turn signal can fail in a turn signal system. | Source

II. One Side of the Turn Signal Lights Doesn't Work

This is a variation of the previous problems. This time, though, the left or right side of the signals doesn't work. You may see the dash indicator lamps illuminating brightly, but the turn lights won't flash.

You may be dealing with bad bulbs, or a fault between that side of the circuit and the turn signal switch itself.

1. First, check the bulbs to see if they are still in good shape: No darkened areas or damaged filaments.

2. Then, check the sockets for corrosion, wear or damage.

3. Confirm the ground and power connections are working properly. Refer to the 'Checking a Light Bulb Socket for Ground and Power'.

4. Finally, check the part of the switch that operates that side of the turn signal using your test light or digital multimeter. Refer to the section 'Checking the Turn Signal Switch'. You may be dealing with an electrical open, or a switch failure.

Check that your hazard lights work when troubleshooting your turn signals.
Check that your hazard lights work when troubleshooting your turn signals. | Source

III. My Hazard Lights Don't Work But the Turn Signals Do

The hazard or emergency lights may share the same circuit with the turn signal circuit.

1. When your hazard lights don't illuminate but the turn signals work, usually you are dealing with a faulty hazard-lights flasher.

2. However, if the flasher unit is working fine, check the fuse.

3. Also, check for a bad connection at the turn signal switch or an open in the part of the circuit that connects the flasher unit to the turn signal switch. See the section 'Checking the Turn Signal Switch.'

4. Finally, check for an electrical open or a short circuit in the wiring that connects to the external lights.

IV. Turn Lights Flash Too Fast or Slow

A change in the flashing rate of the turn signal lights isn't common but it can happen.

  • If this occurred after replacing the flasher or one of the light bulbs, you may have installed the wrong type of flasher or bulb for your vehicle model.
  • Or one of your light bulbs may have burned out.

Other potential problems that may lead to a change in the flashing rate:

  • Poor ground or power connection at the lamps.
  • A loose connection at the turn signal switch.
  • If the lights are flashing faster than normal, make sure the alternator is not overcharging the battery.
  • If the lights are flashing slower than normal, the alternator may have an undercharge condition, or your battery may be undercharged.

The following video shows you how a burned-out bulb caused one of the indicator lights to flash faster.

V. Turn Signal Lights Don't Work

Another common problem occurs when none of the turn signal lights work.

* Normally, you are dealing with a blown fuse, a bad flasher unit, or a faulty turn signal switch. Locate the fuse using your car owner's manual or vehicle repair manual, or check the 'Checking a Light Bulb Socket for Ground and Power' section above, or the 'Testing the Turn Signal Flasher' further down.

Other circuit problems may also lead to this condition:

1. Check the bulbs condition.
2. Make a visual inspection of the bulb sockets - look for corrosion and damage.
3. Verify that the ground connection is working properly. See the section 'Checking a Light Bulb Socket for Ground and Power.'
4. Also, check for an open (most likely) or a short.

Make sure your turn signal indicators work as well.
Make sure your turn signal indicators work as well. | Source

VI. Turn Signals Don't Flash

If the emergency lights and turn signal lights illuminate but don't flash, check first for a burned-out bulb. This is the most common cause. If not, you may be dealing with a bad flasher unit or a bad turn signal switch. See the sections Testing the Turn Signal Flasher and Checking the Turn Signal Switch.

If necessary, make the following circuit checks:

1. Check for a blown circuit fuse.
2. Verify that circuit connections are not loose, corroded or unplugged.
3. Inspect the wiring harness or terminals for the rear section of the circuit.
4. Check for a bad connection between the turn signal switch and the flasher or between the flasher and the ignition switch.
5. Check the power side connection to the turn signal switch for a bad contact or open.
6. Finally, check for an open or short in the lamps circuit.

Identify the wires in your turn signal switch using your repair manual.
Identify the wires in your turn signal switch using your repair manual. | Source

VII. Turn Indicators on Dash Don't Flash

You may know there's something wrong with your turn signal lights because the indicator lights on the instrument panel illuminate but don't flash as usual.

1. First, check that your turn lights work properly. If they come on but don't flash, most likely the flasher unit is bad.

2. If one of the signal lights doesn't come on, check the bulb; check the bulb socket for corrosion or damage; check for a bad ground at the socket. See the 'Checking a Light Bulb Socket for Ground and Power'.

3. Check for an open in the circuit, between the light(s) that doesn't work and the turn signal switch.

Use your vehicle turn signals schematic when troubleshooting your turn signal lights.
Use your vehicle turn signals schematic when troubleshooting your turn signal lights. | Source

VIII. Turn Indicators Don't Work But Turn Signals Do

Another problem you may encounter is that your indicator lights don't work at all, but the outside turn lights work fine. Possible causes:

  • blown out indicator bulbs
  • corroded bulb sockets
  • a bad ground
  • a problem with the flasher unit

If necessary, consult your vehicle repair manual to gain access to the bulbs on the instrument panel and ground connection.

Although some models use a separate light for the left and right turn indicators, other models use a single light for both turns.

  • If your instrument panel has only one indicator light for both left and right, check the flasher unit. Some vehicle models combine the brake light, taillight and turn signal into one light bulb.
  • If you have replaced one of the lamps, make sure the bulb is installed properly.
  • If the two separate instrument panel indicator lights don't work but the outside turn signal lamps work, check the indicator light bulbs and flasher unit.

Check turn signal switch connections.
Check turn signal switch connections. | Source

IX. Checking the Turn Signal Switch

Troubleshooting the turn signal switch is pretty much straightforward. Usually, depending on your particular model, getting access to the switch electrical connector is the hardest part.

For this test, it is a good idea to consult the vehicle repair manual for your particular model. You need to identify wires and the best way to access the signal switch electrical connector. Also, if your model has air bags, you may need to disable the system to prevent the bags from accidentally inflating. If necessary, your manual will walk you through the process for your particular model.

1. Find the wiring diagram for the turn signals in your vehicle model using the repair manual.

2. Identify the wires carrying power from the flasher unit to the switch and the wires carrying power from the switch to the turn signal lights. Check the color designation for these wires and, if necessary, the terminal number on the electrical connector of the turn signal switch.

3. Gain access to the turn signal switch electrical connector. You may need to remove an under-dash panel or column cover to see the electrical connector.

4. Look for the wires that you identified in step 2.

5. Turn the ignition key to the On position, but do not start the engine.

6. On the switch connector, find the wire that supplies power to the turn signal switch.

7. Connect your test light to a good ground under the dashboard - a metal bracket or bolt will offer a good ground.

8. Operate the switch to turn the left turn signal light.

9. Back probe the wire that supplies power to the switch; the test light should flash. If the test light doesn't flash, there is a problem with the flasher unit, or an open or short between the switch and the battery power source.

10. Back probe the wire at the connector that supplies power to the left turn signal lights; the test light should flash. If the test light doesn't flash, the switch is bad. Replace the switch.

11. Back probe the wire that supplies power to the right turn signal lights; the test light should not light. If the test light illuminates, replace the switch.

12. Repeat the test, but this time operate the switch so the right turn signals light. The only difference in your results this time, is that your test light should flash when back probing the terminal that supplies power to the right turn signal lights, but not the left turn signal lights. Otherwise, you need to replace the switch.

The turn signal flasher may be lcoated under the dashboard or inside the engine compartment.
The turn signal flasher may be lcoated under the dashboard or inside the engine compartment. | Source

X. Testing the Turn Signal Flasher

NOTE: This guide deals with analog type flashers. If your system is equipped with a solid state unit, consult your vehicle repair manual for special instructions. Also, make sure to use a digital multimeter with at least 10 Mega-ohm of impedance to protect sensitive circuits in your vehicle as you troubleshoot electrical systems.

A good way to test the flasher unit is to use a known good unit, but that may not be practical. So the next best choice is to test the suspected flasher while installed in the vehicle. Your circuit connections may differ from the outline described here, but you'll find the next general steps helpful. If necessary, consult your repair manual.

1. First, locate the flasher; you may find the unit under the dashboard near the steering column, or near the battery in a junction box or 'power center'. Consult your vehicle repair manual, if necessary.

2. Locate the wiring diagram for the turn lamps in your vehicle repair manual.

3. Identify the colors of the wires that connect to the flasher unit. Identify the wire that supplies power to the flasher, and the wire going from the flasher to the turn signal switch to operate the turn signal lamps.

4. Connect your test light to ground.

5. Turn the ignition switch to the On position but do not start the engine.

6. Operate the turn signal switch in either direction (left or right).

7. Back probe the power wire that connects to the flasher unit. The test light should illuminate. Otherwise, your have a blown fuse or there's an open between the flasher unit and its power source (fuse box or battery).

8. Without operating the turn signal switch, back probe the wire going from the flasher unit to the turn signal switch. The test light shouldn't flash. Otherwise, the flasher is no good.

9. Now, operate the turn signal switch and back probe the wire going from the flasher unit to the turn signal switch. The test light should flash. Otherwise, the flasher is no good.

The next video shows you a common procedure when replacing a flasher unit on a GMC truck. Depending on your particular vehicle make and model, you may or not need to remove the dashborad trim. On some models, the flasher is readily accessible. Consult your vehicle repair manual.

XI. Turn Lights Don't Turn Off

This is a special turn signal failure. It has to do with the mechanical operation of the signal switch rather than an electrical system failure. For example, you signal traffic your intention to turn left or right, but after cornering, the turn signal continues to flash.

There are two reasons for this type of failure: Either the canceling finger inside the switch is broken, in which case you need to replace the switch; or the canceling cam/clockspring is broken, in which case you need to replace the cam or assembly, depending on your particular model.

In most cases, the repair is within the reach of the average car owner. Still, consult the repair manual for your particular vehicle make and model. Specially if your model has air bags. You'll need to disable the system for safety reasons. Also, your repair manual will tell you what specific tools you need, if any, and the procedure to follow.

Turn signals, and the system they are part of, can fail in many ways. When facing a problem, always check every light so that you know what part of the system or lights are failing. This will make troubleshooting, and fixing the problem, easier and faster. This guide is designed to help you make your repair faster, and get back on the road safer.

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