Mechanics

No Tail Lights or Running Lights

Tail lights, exterior or running lights are lights that are positioned on each corner of your car. These lights are used to make the car more noticeable at night by staying illuminated while the headlights are on. The tail lights can also be controlled separately from the headlights if desired. Tail lights can be used at dusk or in the early morning to increase safety by helping other drivers to see your car more easily. In some states it is required to switch on tail lights in foul weather including fog and rainy conditions.  All headlight switches have the option to turn the running lights on separately. When the tail lights are activated the dash lights will also come on to alert the driver the tail lights have been activated. 

The brightness of the dash lights can be adjusted by turning the rheostat connected to the headlight switch. This is done by turning either the pull-out activation knob or the thumb wheel located above or below the headlight switch bezel. Some manufacturers have located the dash light illumination control remotely from the headlight switch, usually somewhere near the instrument cluster itself. The running light circuit can be controlled electrically through a BCM (body control module) or a LCM (lighting control module). These computer controllers are designed to warn you that a bulb has failed by illuminating a bulb failure warning light on the dash. If your car is not equipped with either of these controllers the headlight switch will perform the actual connection to complete the flow of electricity and illuminate the running lights.

Tail Lights
Tail lights

The interior and exterior lamps are protected by separate fuses. Some cars have a control relay to help shoulder the electrical load. Vehicles are either designed to have the running light bulbs separate from the directional signals while other manufacturers have integrated the running light circuit into the directional bulbs. Much like the brake light systems in American cars the running lights are integrated into the blinker bulbs, while Japanese and European cars have separate lighting systems. Unlike the American and Japanese/European car manufacturers brake light systems we will be able to test the running light systems in all models in a similar manner. Vehicles that have towing capabilities will have a separate wiring system to power trailer lights that are protected by additional fuses located in the fuse panel or the under hood power distribution panel. 

The rear license plate of the car is illuminated by a small bulb which activates when the running lights are on. It is unlawful to operate a car with any lights not working properly and must be corrected in a reasonable amount of time. Some running light systems are equipped with a self controlling timer that will allow the running lights to stay on for a predetermined amount of time after the user has exited the vehicle. This safety device can be disabled in most cases if this option is not desired. In this section we will go over the common failures for the running light control circuits and the simple repairs they will require to fix. Theses malfunctions can range from one or all lights not functioning and we also will cover dimly lit or flickering bulbs.

Before We Begin: Depending on the running light malfunction you will need a new bulb and test light with a possible wiring diagram which can be found in a car repair manual. Also to aid connection problems a small piece of abrasive paper (sand paper) can be helpful. When performing test wear protective gloves and eyewear. (Note: Never connect power wires or terminals to ground, fuse damage can occur).

Troubleshooting Procedures

All Tail lights Not Working

Step 1 - Tail Light System Fuse Inspection - A fuse protects the tail light circuit from damage. If the fuse fails it does not allow electrical current to flow to the tail lights. To inspect the fuse, locate the tail light fuse that protects the circuit in the fuse panel. This panel could be located under the hood in the power distribution center or dash on the drivers side in most cases. Connect a test light clamp to a ground source like an engine accessory mount or dash bracket and move the ignition key to the "on" position. Now with the test light, probe both terminals of the fuse. If one terminal of the fuse illuminates the test light the fuse has failed and replacement is required. 

If the new fuse blows as soon as it is replaced or when the tail light/headlight switch is activated the tail light circuit is shorted to ground and requires troubleshooting to repair the damaged circuit. A system short can occur, but rarely. If no power is detected at the fuse panel (either side of the fuse) use a wiring schematic to trace the power source (wire) and repair as needed. You will find usually when this condition occurs there is a main power relay or fusible link that has failed. To find a wiring diagram a car repair manual is required. When the test light illuminates on both terminals the fuse is "ok" continue to the next procedure.

Step 2 - Test for Power at Headlight/Tail Light Switch - Once the tail light wiring has been located and the fuse condition has been confirmed as ok, use a grounded test light and test for a twelve volt power source at the rear of the headlight switch on one or two of the terminals. (Note: sometimes a headlight switch wiring loom will be the test point because the terminals are not exposed.) The reason for this is to test for power present and available to be connected to the tail light bulbs. The switch must have power to be able to supply power. If no power is confirmed repair the fuse holder or the power feed wire from the fuse panel to the tail light/headlight switch. If power is confirmed at the rear of the switch turn the switch to the "on" position and test the remaining terminals (or wires), power should be present at the remaining terminals if not, the switch has most likely failed and requires replacement. If the headlight switch tests ok continue to the next step.

Step 3 - Non-Relay Controlled Tail Light Systems Only - This test is for vehicles that are not relay controlled lighting systems (use no tail light relay). Remove to expose one or all of the tail light bulb sockets. Using a grounded test light test for power inside the bulb socket. Use caution to not contact the side of the bulb socket with the test light probe, this could cause a fuse to fail. If the test light does not illuminate/power is not present on any of the socket terminals the wiring circuit to the headlight/tail light has failed and requires repair. For this a wiring schematic is required. Trace and test the wiring and recheck system.

Step 4 - Relay Controlled Taillight System Test - Sometimes manufacturers can design a tail light control relay to help shoulder the electrical draw many bulbs can cause on a single circuit or to control the circuit remotely like a BCM (body control module) for example; when the alarm is activated the tail lights will activate for a couple of seconds to inform the user the alarm has been activated. When this wiring system is implemented the headlight/tail light switch is used a the "trigger" circuit and a tail light relay acts as the actual activation switch. To perform a test locate the tail light control relay, usually a diagram is located on the under side relay lid. Most relay control centers are located under the dash or hood of the car. If you cannot locate the relay a car repair manual can be used. If your car is not designed with a control relay refer back to step 3. After the relay is located remove the relay from the relay block or connector. 

With the key in the "on" position use a test light to probe all connector terminals. Two of the terminals should have power, and the remaining two or should be dead. If power is not present at two of the terminals repair the fuse or power feed circuit as needed. If power is present, remove the ground lead from the test light and connect it to a power source for example the positive side of the battery. Next, probe the remaining two terminals in the relay connector while turning the headlight/tail light switch from "on" to "off" and repeat. On one of the terminals that is tested should blink on and off with the headlight/tail light switch. If one of the terminals blinks the relay has probably failed and replacement is required. If no terminals blinks the control switch (headlight/tail light) has most likely failed or the ground circuit connected to it.


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AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2013-08-16)