Turn Signals & Brakes

Most all of the circuits in a car are stand-alone. Yes, they have the battery, charging system and fuse panel in common, but from there on they don't "cross" each other. From the fuse panel on, most circuits are a switch, a load and the ground connected by wires. Simple stuff. A major exception on early American cars and trucks - the ones that DO NOT have separate turn signal lights in the back - are the turn signals and the brake lights.

These two circuit systems start separate, with power coming from the fuse panel through the brake light switch and from the fuse panel through the turn signal flasher, but they both then go to the turn signal switch mechanism inside the steering column. The turn signal switch not only directs the turn signal power to one "side" or the other, it also shuts OFF the brake light power feed to that same side. This is called brake light override circuitry and it prevents the brake light from coming on for the side that the turn signal is flashing - the brake light would "override" the turn signal, effectively turning the flasher off by keeping it from being seen.

Then from the steering column on to the back of the car, the brake light and the turn signal on each side use the VERY SAME wire - one for the right and one for the left. The switch in the column makes sure that the brake and turn are never "on" at the same time and that the wire is acting as a brake light feed or as a turn signal feed but never both.

I bring this up because it is important to realize that, if you are working on this wiring in the back, it involves both circuits. You can't "tap off of" or rerun or modify either of these two wires without affecting both circuits. If you want to deal with the brake lights only, you have to look at the brake light feed UPSTREAM of the steering column.

One visual example I've seen happen is of a guy who puts on his right turn signal, then steps on the brakes and someone notices that his right front turn signal comes on solid. The brake feed is cross feeding to the turn signal and back streaming through the steering column to the front signal. But lots of other strange things can happen, too.

So if you've made some wiring changes in these areas and are seeing peculiar things going on, remember that these circuits aren't completely separate and double check that you made the changes i the right place to the right circuit.

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