The 25+, "Brake LIght Override"

Continuing on the subject of Taillights and Brake LIghts...

#21. What is “brake light override” circuitry and why do I need it?

Before I explain what it is, let me say that you may or may not need it. That will depend upon the type of tail lights you have and how they are set up.

Brake light override is what happens when the brake light cancels out the turn signal - the brake light “overrides” the turn signal so that the turn signal is no longer visible. And this only happens if the brake light and turn signal functions are being performed by the same bulb/filament, such as when just one #1157 style bulb is being used for tail light, brake light and turn signal. An 1157 has just two filaments inside - one is a lower brightness filament which is for the tail lights, the second is brighter and is both the turn signal and brake light. To prevent override, the car’s turn signal mechanism (usually inside the steering column) turns off the brake light feed to a light whenever the turn signal is turned ON. So if the left turn signal is flashing, the right brake light will still work but the left brake light does not - right side brake light, left side turn signal (and visa versa when the right turn signal is ON).

A car that has separate bulbs or filaments for each function, like Beetles and many foreign cars, and newer US cars and trucks, too, will not need, and therefore will not have, the override circuitry. The easiest way to spot these cars is if they have amber rear turn signals - a dead giveaway that they are separate from the brake lights which must, by law, be red. However, some cars have separate red turn signals. You just have to double check to see which you have.

But this was all figured out by the car manufacturer when they designed the car. The problem comes up when we decide to add turn signals to a car that didn’t originally have them or, more often the case, when we try to change tail lights. 

If your car had separate turn signals before and now you are trying to go to tail lights that require brake light override circuitry, you have a challenge awaiting you. It’s best to know what you have to begin with before you go buying new tail lights and get into trouble. Tell the sales person what you have and figure out whether the new units will work “as-is” or whether additional parts will be needed (such as our Turn Signal Relay Pack which has brake light override circuitry built into it). In worst case scenarios, however, the car has been designed with a special flasher, burnt-out-bulb sensors and god-only-knows-what other special “features” that will make changing tail lights a living hell. Don’t try it and blame the tail light manufacturer if you haven’t done your homework up front.

Going the other way - from a car that had override circuitry to separate brake lights and turn signals - requires bypassing the original turn signal mechanism or at least not running the brake light feed through that mechanism. A little extra work, but nothing like what I talked about above.

Steve Watson, Watson's StreetWorks, http://www.watsons-streetworks.com