Auto Repair Help
Carl’s Corner – Auto Repair Help

Continued from Part 1


One of the most common issues with turn signals is short circuits between live contacts and ground, or other circuits. When this happens, you are likely to either see a brake or head light bulb flash along with the turn indicator bulb when the turn signals are turned on, or the turn signal bulb lighting up when the brakes are activated or the headlights are turned on.

While these types of short circuits do happen on cars and SUV’s, it is more common on pick-up trucks, and particularly on trucks that see a lot of off-road use since the rear light clusters and wiring on these vehicles are not as well protected as the wiring on other types of vehicles are. However, since the same issues that affect pick-up trucks affect cars and SUV’s as well, the simple diagnostic steps outlined here should resolve the issue nine times out of every ten, regardless of the vehicle type.

NOTE #1: Note that to diagnose inoperable turn signals you may need to have access to a repair manual that includes a wiring diagram for the affected application. If a repair manual is not available, a suitable wiring diagram can often be found free of charge with an online search. Note that you will also require a good quality digital multimeter; simple test lights are mostly useless since they cannot measure electrical resistances in circuits accurately.

NOTE #2: As with all car troubles, approaching the issue of defective turn signals in a logical manner saves time and prevents misdiagnoses, so start the process with asking yourself the following questions, starting with-

Are all the turn signal bulbs affected?

  • If all the turn signal bulbs are out, check the relevant fuse(s), but do NOT replace blown fuses until the short circuit that had caused the fuse to blow had been found and repaired.
  • If the fuse is intact but shows signs of overheating, do NOT replace the fuse until the abnormal resistance in one or more circuits is repaired.
  • If the fuse is intact and does not show signs of overheating, it is likely that the flasher relay is defective, but do not replace the relay until you have verified that the circuit that supplies the relay with power is intact, or not.
  • Refer to the wiring diagram to identify the wires feeding the turn signal relay with power, and verify the current in this wire with the ignition “ON” without the engine running. Note that on applications with solid-state relays, the input specified current may be less than battery voltage, so compare all obtained readings with the values stated in the manual before drawing any conclusions about the state of the wiring.
  • If the turn signal relay input current agrees with the value stated in the manual, measure the relay’s internal resistance, and replace it if this value deviates from the manufacturer’s specified value. Test the turn signals after the relay is replaced to verify that ALL the bulbs are flashing at the same rate. Also, turn on the hazard switch to verify that the turn signals work with the ignition in the “OFF” position.

NOTE: Some types of turn signal relay failures can cause all the turn signal bulbs to illuminate, but not to flash. While there are other causes of this, in the vast majority of cases a replacement of the turn signal relay with an OEM replacement part will resolve the issue.

Are only some turn signal bulbs affected?

If only some bulbs are affected, it is important to check if the affected bulb is only illuminating weakly, or if it is well and truly dead, since weak illumination can indicate a short circuit. Here is what to look for-

Are all the bulbs on only one side of the vehicle working?

If this is the case, suspect either a defective turn signal switch, or a defective turn signal relay. If replacing the turn signal relay does not fix the problem, replace the turn signal switch but be aware that removal of the steering wheel may be required, which could trigger the steering wheel (or other airbag(s)).

WARNING: If a defective turn signal switch is suspected, the better option is to refer the vehicle to the dealer or other competent repair shop for professional assistance, since triggering an airbag unexpectedly can cause serious personal injury.

Is the affected bulb not illuminating at all?

  • Defective bulbs do not always show physical evidence that they are indeed blown, so replace the bulb with the specified replacement bulb if it does not illuminate.
  • If the replacement bulb does not illuminate either, check the input current from the contact in the bulb holder to ground. Note that on most applications, the contact in the centre of the holder is live, while the surrounding metal body of the holder is grounded.
  • If the multimeter shows the correct current, the contact between the bulb and its holder is poor, so use long-nosed pliers to deform the edge of the bulb holder SLIGHTLY to improve contact between the bulb and its holder. If the bulb now illuminates and flashes at the correct rate, the problem on that turn signal is resolved.

Is the replacement bulb still not illuminating?

If the bulb does not illuminate and multimeter shows the correct current on the live contact (as per the steps above), test the ground circuit between the outer metal part of the bulb holder and a known live wire that does not feed a turn signal. If the bulb holder is grounded properly, the multimeter will show a constant current; if there is no ground, the multimeter will show an open circuit. Finding and repairing the open ground circuit will resolve the problem, provided that the bulb is good, and that there is proper contact between the bulb and its holder.

Is the affected turn signal bulb illuminating weakly?

  • If this is the case, remove the bulb and use the multimeter to verify that the specified input current is present on the live contact. If not, remove the entire light cluster from the vehicle and inspect the connector that connects the cluster to the main wiring harness for looseness, corrosion, or mechanical damage that could conceivably cause a loss of current. Remove all corrosion if possible, or replace the connector as required and test the turn signal.
  • If the problem persists but there is no corrosion of the connector, it is undamaged and fits securely, test the input current on the VEHICLE side of the connector. If the current is low, inspect the wiring all the way to the turn signal relay, and repair or replace wiring as required to repair the abnormal resistance or poor contact. Test the turn signal after repairs are complete to verify the repair.

Are the turn signals flashing too fast / too slowly?

Since turn signal relays generally depend on a resistance spike to act as a trigger to switch between “ON” and “”OFF” states to cause the bulbs to flash, an excessive resistance will cause the bulbs to flash to slowly, while an insufficient resistance will cause the bulbs to flash too fast.

Typical causes of turn signals flashing to slowly include-

  • Poor electrical contact(s) almost anywhere in the turn signal circuit on the affected side of the vehicle
  • Use of incorrectly rated bulbs
  • Short circuits between the turn signal circuit and other circuits or bulbs

Typical causes of turn signals flashing too fast include-

  • Loss of resistance, such as when one turn signal bulb on one side of the vehicle blows, or stops working due to for instance, loss of ground or contact with the bulb holder
  • Use of incorrectly rated bulbs

Fixing an incorrect flash rate usually involves verifying that all bulbs are rated for the vehicle, and finding/repairing short circuits and/or abnormal electrical resistances. Also, note that the resistance of the indicating light on the dashboard is included in the total resistance of the turn signal circuit, so if the light on the dashboard blows, the flash rate of the turn signals is also affected.

NOTE: Note that one or more of the causes listed above can also cause the turn signals to illuminate, but not to flash on some applications. Be aware however, that distinguishing between a defective turn signal relay and other issues that cause turn signals not to flash can be tricky, and it is recommended that if the steps in this guide do not resolve the issue, the better option would be to refer the vehicle to a competent repair facility for professional diagnosis and repair.

Are other bulbs illuminating along with the turn signal?

  • If this happens, the cause is (almost) always a short circuit in the light clusters’ connector between the turn signal circuit and the circuit that feeds the light bulb(s) that should not flash or illuminate. Remove the light cluster from the vehicle and inspect the connector that connects the cluster to the main wiring harness for looseness, corrosion, or mechanical damage that could conceivably cause a short circuit. Remove all corrosion if possible, or repair/replace the connector as required and test the turn signal.
  • If the problem persists despite having removed all visible corrosion, it is possible that the corrosion had penetrated the cluster to a point where it is beyond saving. However, before replacing the light cluster, it is important to test the brake, head light and other terminals in on the VEHICLE side of the cluster for the presence of a current.
  • If power is present on any terminal in the connector other than the turn signal, inspect the wiring harness that feeds the affected light cluster, and make repairs or replace wiring as required to fix the short circuit.
  • If there is no current on the brake light, head light, and reverse light circuits, replace the light cluster with an OEM replacement part to prevent premature fading of the cluster’s lens(es).

Are some turn signals illuminating at times even when the turn signal switch is “OFF”?

This can happen if there is a short circuit the connector of the affected light cluster, or in the wiring harness that feeds power to one or more light clusters. Depending on the site and severity of the short circuit, applying the brakes, turning on the headlights, or engaging reverse gear can illuminate not only turn signal bulb, but also all the bulbs in a light cluster. Note though that this type of short circuit will not always cause one or more fuses to blow, and the remedy sometimes involves replacing the relevant wiring harness, as opposed to attempting repairs, which is sometimes made difficult by the routing of this harness on many vehicles.

8 responses to “Diagnose The Turn Signals Dont Work Or Dont Work Properly – Part 2”

  1. stuart occulto says:

    very very in-debt 2007 ford with both arrows showing ,flashers ok replaced flasher relay fuse ok possible front bulbs ? confused 56 year old beginner that 07 is an f-150 xl thank-you for your support.

  2. Halfcack says:

    2018 Smart ED. Sounds picky but it is so aggravating. Right turn signal switch lever has a snap instead of a thunk. It’s driving me nuts at 561 miles on the odometer.

  3. David Hodge says:

    Great stuff from the both of you. Thanks for the primer – digging in deep this weekend w/a 1988 Jeep YJ.

  4. Joshua says:

    I have a 2000 Silverado 1500 and my white signal bulbs went out and when I went to replace them the sockets broke. I got the right sockets and replaced them and the bulbs. Now all of my signal lights flash twice as fast to tell me something is wrong. The lights still don’t work. I checked every fuse in both of my fuse panels and tried switching the wires around but they still don’t work. I need help.

  5. Margaret says:

    I have a 2005 Jeep Grand Cherokee Limited when I’m braking the turn singals will blink fast regardless if I’m turning right or left. If I am not braking the work just fine. I was told to replace the multi function switch but I am not positive that it is the correct solution. Has anyone heard of this happening before?

  6. MKRBRKR says:

    Ok, here’s one for ya’. I have a ’68 ford F100. My problem is, when the turn signal switch is activated, that particular side goes Out. (eg. left turn signal, the left side goes dim, ditto with Right…) I have changed the turn signal switch, flashers, fuses, lamp switch, etc. Everything else works great ( hazards, headlights, brake lights, etc.)

  7. Ralph McCall says:

    I have a 1997 Jeep Cherokee, the right rear turn signal will not light, I replaced the relay and that was not the problem, the reason I changed it, when I turned the right signal on it blinked very fast, the new one blinks slow, but my right rear will not come on, I checked it with a probe 3 tester and I have a ground on both side

  8. Bill Boehme says:

    2018 Chevrolet HD3500 – Normal situation upper and lower bulb flash with turn signal on. Current situation shows upper bulb normal and lower bulb on but does not flash. When brakes are applied, the upper bulb normal but lower bulb does not come on.

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