Ask The Experts: Trouble Shooting Taillight/Brake Light Problems

It is not uncommon for vintage cars and trucks to have brake lights that are brighter on one side than the other. There can be several reasons why this happens and we asked our experts at Ron Francis Wiring to help us wade through the troubleshooting procedures.

Vintage project cars and trucks have a host of problems that need to be sorted out before you can consider them roadworthy. Among those issues are lighting and electrical woes. This past weekend we noticed a common ailment in our very own project vehicle, a 1960 C20 pickup truck. When the brake lights were actuated, the right side was vivid and bright like a warning light should be. The left side, on the other hand, was weak and barely visible.

We decided to call in our favorite electrical professionals at Ron Francis Wiring to get some helpful tips on how to handle this common brake light problem. Scott Bowers at Ron Francis Wiring has been a frequent contributor to the Rod Authority site as an automotive electrical expert, so we decided to go back to the well and draw out a bucket of knowledge before we disassembled the entire electrical system on our project vehicle.

Taillight assemblies usually live in harsh conditions, especially in pickup trucks. Corrosion, water, and other elements can affect the lifespan of a lighting system. The dull finish on the taillight housing minimizes the reflective ability of the light.

 

“I cannot stress enough that troubleshoot electrical systems should be done in a systematic and thorough manner,” said Bowers. “You need to qualify the problem by starting with the obvious. Check the power. Using a multimeter, check the two poles in the socket on each side. If the voltage on both sides are equal, the problem is somewhere else. If there is a drop between the two sockets, there is probably a problem in the wiring that provides the power.” Shorts or electrical disruption can be isolated by checking fuses, then connectors and lastly, the wiring.

1157 Taillight Bulb

The most common bulb used in vintage automotive tail light and brake light applications was the 1157 bulb.

With the running lights on, the low filament emits a lower candlepower light, but when the brakes are applied or the turn signals are actuated, the high filament emits a much higher candlepower light – providing a good contrast between the low and high filaments.

For safety, this is an important consideration so other drivers on the road could differentiate between the two levels of light.

On many vintage vehicles the taillights and brakes lights are combined in one assembly. This type of taillight unit uses one bulb with two filaments. Such is the case with our project truck, which uses a very common 1157 two filament lamp. One filament is for the taillights and the other is for the brake lights.

“These type of vehicles use a simple mechanical switch that is mounted on a bracket near the brake pedal arm. When the brake pedal is depressed, the switch is released and completes a circuit that turns the brake light on,” said Bowers. “When the pedal is released back to its normal position, the switch is pressed in and the brake light goes off.” If the brake lights are not working and the taillights are operating correctly, the mechanical switch is most likely the culprit.

Following Bower’s principle of checking the obvious items in a systemic fashion, the next step is to check the ground. “A loose light socket can cause a bad ground,” added Bowers. “If the ground path is broken, the current can’t return and the bulb won’t work properly.”

“The most common issues with brake lights tend to be simple to fix,” said Bowers. “Check the light bulb to make sure that one of the filaments has not come loose.” In some early vehicles, it is possible to install a dual filament bulb incorrectly where the brighter light filament functions as the taillight. When the brake is activated, the lower light filament is on and you won’t be able to see it. In this case, the bulb just needs to be installed in the correct position.

Bowers suggests checking the voltage at both poles on each taillight, looking for a drop in voltage between the two taillights.

Taillights have a difficult life and tend to live in harsh environments, as in the case of our project truck. Double check the bulbs and make sure to look for corrosion on the connectors. Bowers reminded us to check the switches when tracking down faults. Loose connection or even a worn out switch can have a troubleshooter searching in the wrong place for hours. “Don’t take for granted that the turn signal switch is working correctly,” he added.

Lastly, Bowers warned us to check the taillight lenses. If one is an OE 50-year-old stock lens and the other is a newer aftermarket replacement, light may cast differently between the two lenses.

In our case, Project Geronimo was plagued by a bad ground. Once the socket was cleaned up and the bulb reinstalled, the brake light worked just like the other side. It pays to call the experts.

Mission accomplished. We corrected a bad ground problem and the light intensity improved on the passenger side brake light.

Article Sources

About the author

Bobby Kimbrough

Bobby grew up in the heart of Illinois, becoming an avid dirt track race fan which has developed into a life long passion. Taking a break from the Midwest dirt tracks to fight evil doers in the world, he completed a full 21 year career in the Marine Corps.
Read My Articles

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