The Stovebolt Site Forums The Shop Area The Electrical Bay Tail light confusion

My truck was converted to 12v some time ago by a previous owner, and has a 60's GM steering column. The turn signals, brake lights and tail lights have always work fine.

I'm rewiring the truck now with an aftermarket harness. When I pulled apart one of my tailights to hook up the new wires, I noticed there are two bulbs, each with one element. So my first question is, how then do the lights work? How is it that there are only two bulbs with one filament each, yet I have tail lights, brake lights and turn signals? It just doesn't seem to make sense to me. I guess the truck didn't come from the factory with turn signals, but I'm just not sure how the previous owner made this work with only two bulbs. What bulb does what?

Secondly, my new harness has wires for the tailights and turn signals, but not for the brake lights. How does that work, does it just run off of the tailights?

And thridly, of the two bulbs that are in a stock 1940 tailight housing, which one is which?

Thanks!

A wire from the brake light switch runs to the turn signal switch in the '60s column. Wires run from there to the bright bulbs. The dimmer bulbs are the tail lights.

Ahhh... okay, so the brake lights and turn signals share a bulb? And the tail lights are their own bulb? So how do the brake lights and turn signals share a bulb when the bulb only has one element? How do they flash at the same time you are applying the brake? I would have thought there would be at least one bulb in the housing with two elements, and separate wires for each function.

With the original two bulb tail light you might have to mount a separate light to use as a turn signal. The problem with your housing is a bright bulb won't fit without bending the bracket the sockets mount to. I use a #81 6cp bulb for my tail light and a #1129 21cp bulb for the brake light. I believe originally a #63 3cp bulb was used for the tail light and the #81 was the brake light. As I mentioned the bracket must be bent some to use the #1129 because it's longer and won't clear the lens. I don't know what the 12v equivalent bulbs would be.

Well, it's using the original housing, and there are two small identical looking bulbs mounted vertically inside on a plate, that looks like it might be stock. However this truck does have working turn signals. All the wiring is currently ripped out, so unfortunately I can't just test it to see what is what. I'm just trying to figure out how to connect the new harness to the rear of the truck. The harness has wires for "tail" and turn signals", but nothing for "brake" (there is a wire labeled third brake light, but I don't have one so I just tucked that wire away). So based on Cletis' response, the turn signals and brake lights share a bulb, and the tail light has it's own bulb. I'm just curious how this actually works, does the bulb get less voltage for the turn signals and then more voltage for the brakes so they are brighter? I don't think it would shut off the brake lights so the turn signals can come on. How does it work when you use both the brakes and turn signals at the same time.

And inside a stock 1940 tail light housing, which of the two vertically mounted bulbs is which?

Alright, I did some searching, and I think the last post in this thread explains it. https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=91403#Post91403

So if I'm understanding correctly, when you apply the brakes, the bulb is lit, when the brakes are applied and you use the turn signal, it just cuts out the power off and on for that side, alternating between brake and signal? Meanwhile the opposite side brake light stays lit? That makes sense, I think.

yes, you understand correctly Steve, the signal mechanism is "in the middle of" the brake circuit and the brake bulb serves both functions, so the wire labeled "turn" is for both signal and brake, just need to get the right to the right and left to left

this 55-59 diagram might help understand the layout, note the signal unit gets power from the brake light switch

Bill

Okay, I get it now, thanks! So inside the housing, which one is the turn/brake light? Is the top bulb or the bottom bulb?

the brightest one will be the brake/turn, should be a marking on the bulbs

Bill

I was just out taking out all my old nasty rear end wiring, and when I started putting everything back together when I put the left blinkers on, the right ones blinked as well. I had cut out the resistor! The problem was, that when the blinker and brake wires met and the blinkers were applied, the current was jumping to the other side via the brake wire.

Long story short, make sure you put a resister back in if one was there, or you will just be scratching your head in confusion (I would know...)

Dalyman, You have me confused. There should be no resistor in that circuit.

yes, what Cletis sez, the only "resistor" on the truck is the ignition resistor - check the diagram again

Bill

lol yeah... haha sorry it was a diode used in the horrible wiring on my truck. it works for the purpose it is intended for and will be solid when I redo it until I get the money to put a new harness in

Ok so in case anyone ever looks through this conversation again, I fixed the problem.

This is what I think was the problem, and how I fixed it:
As it was, the brake and turn signals worked independently, but when both were on only brake showed. The problem was the brake wire was ran from the switch straight to the rear end. It SHOULD have been ran from the brake switch back to the column switch where it would blink the current when the brake and turn signal are on. It could not work with 2 filaments (tail being the other).

Soooooo, I made a bracket that sticks out farther than the lights on it now, and mounted an amber marker light to it. The marker light is now the turn, and both tail and brake are in the original lights (trailer lights). It works great, and looks good too. Thanks for your help with all my shenanigans.