I have been playing around with this issue as I find time, which is not often but I bought a truck recently after my honda got stolen and I have had electrical issues. Summary of the problem: Head lights and tail lights work… all right side works fine (blinkers/brake lights)… However on the left side, the front blinker works but blinks really fast and the back left tail light WORKS but the blinker does not, and neither does the back left brake light. I have changed the bulbs and checked to see if all the leads are getting power and they are so I really can’t figure it out… even got under the truck to check out the grounds, cleaned them off and they were fine as well… electrical is not a strong suit of mine… thanks for the help in advance…
What kind of truck first off? Year, make, model.
Sounds like it has separate bulbs for signal and brake/tail, making it likely to be foreign.
The reason it’s flashing really fast is because the rear bulb isn’t working and because of that the flasher relay is seeing more voltage than it should, causing it to cycle on/off at a faster than normal rate. On the side where both front and rear signals are working the flasher sees the right voltage/amperage so it flashes at the normal rate.
You said you checked the voltage at the signal bulb socket and it has power, but even after replacing the bulb it doesn’t work? That doesn’t make sense, unless the socket was really corroded. That should be pretty clear just by looking at it. Are you sure you’re checking the voltage to the right socket? What about when you take the working signal light bulb from the right side and put it on the non-working left side?
If it is corroded mix up some baking soda and water into a paste and use that to clean it up. A dremel with one of the little wire polishing tips works wonders. Flush it out good, put a little dielectric grease in there and it should be fine.
2000 Chevy S-10 Xtreme
I already checked to see if the leads were corroded and they look brand new, which is what gets me, and I know the bulbs are good b/c i just replaced them and you can see when a bulb is blown obviously so… meh… I will try switching the bulbs though, but I don’t think that will solve the problem.
So wait, is it the same bulb for signal/brake/tail? If not which ones are working and which ones aren’t on the side you’re having trouble with.
I heard of similar problems within a Cavalier headlight harness. Make sure you check the wiring as good as you can for corrosion not at the ends but more in the middle. More than likely not a turn signal switch… Goodluck.
OK I JUST WENT OUT AND TOOK THEM APART AND THIS IS WHAT I FOUND… THE LIGHT HAS 3 BULBS… ON TOP A BIG ONE REGULAR TAIL LIGHT BULB AND ITS A DUAL FILAMENT BULB. THIS IS THE TAIL LIGHT, BOTH WORK. THE SECOND BULB DOWN IS A LITTLE ONE, LIKE A SIDE MARKER BULB, THIS IS PART OF THE TAIL LIGHT AND DOESNT REALLY DO ANYTHING. NOW, THE BULB ON TOP AS I SAID IS A DUAL FILAMENT, AND WHEN THE BRAKE IS PUSHED THE SECOND FILAMENT LIGHTS UP TO MAKE IT BRIGHT. ON THE RIGHT SIDE THE SECOND FILAMENT WORKS BUT ON THE LEFT SIDE IT DOES NOT LIGHT UP, THE REASON I HAVE NO BRAKE LIGHT ON THE LEFT SIDE BACK, BUT WHY ISN’T IT LIGHTING UP??? OK LASTLY, WE HAVE THE 3RD BULB AT THE BOTTOM, THIS BULB DOESN’T WORK ON EITHER SIDE BUT IT REALLY HAS NO PURPOSE, BECAUSE THE OTHER TWO LIGHTS ALREADY COVER BRAKE, TURN SIGNAL, AND TAIL. OH AND THE TOP BULB IS ALSO THE TURN SIGNAL AS IT BLINKS WHEN I TRIED THE TURN SIGNAL BUT ON THE RIGHT SIDE ONLY. SO THE LEFT SIDE TOP BULB IS NOT LIGHTING ON THE 2ND FILAMENT OR BLINKING FOR THE BLINKER… THAT SHOULD BETTER DESCRIBE MY PROBLEM, AND NOW I REALLY HAVE NO IDEA WHAT TO DO… THANKS FOR THE HELP
Something is all sorts of fucked up here because no car company builds a tail light with “unused” bulbs. They probably use more than one bulb for the tails or the brakes and the secondary bulbs just aren’t working.
That bulb at the bottom might be your reverse light. Turn the key to on without starting the truck, put it in reverse, and see if that bottom light comes on.
As for your brake light issue, what happens when you swap the known working dual filament bulb from the working right side to the non-working left side? That will prove once and for all if it’s the socket/wiring or the bulb.
I’m guessing it will be the socket/wiring. If you’re 100% sure ALL the contacts in the socket are clean and making good contact and not bent or anything, back probe the socket on the good side and figure out which wires power which filament. Test with just the tail lights on and one should be dead. Test with tails and brakes and that dead one should now have voltage.
Now go to the left side and probe that wire at the back of the socket with tails and brakes on. If it has voltage your problem is the socket itself. If it doesn’t start tracing that wire back and see where it loses power. Tracing it will probably be a bitch without a wiring schematic for your truck.
EDIT:
Looking at that picture I’m pretty sure those bottom bulbs are indeed your reverse lights.
Ok, that will give me somewhere to start… I’ll have to swap the right bulb tomorrow to the left side and see if it works there, but like you said i doubt its the bulbs so I’ll have to test the socket…
Oh, and that makes sense about the reverse lights, b/c I know I have them, I can see them light up when I back up so I’m sure that’s what they are and they work fine but I will double check tomorrow. That should leave my problem at the one bulb/socket on the left side… at least were narrowing it down!
I know this sounds stupid but like I said I changed all the bulbs out when I got the truck but i took them out again just to check and somehow on the bulb on the left somehow the plastic melted over the metal lead so there was no connection happening… something so little that gave me trouble for 3 months… never checked it though cuz the bulbs were brand new… might have been bad before I took it outta the package though who knows… thanks for the help though… coulda spent a lotta money on nothin
Ha… that was why I suggested trying the working bulb in the non-working side, regardless of the “new” bulb. 
That’s what I did…
the boards on GM vehicles are known for that. The resistance builds up and so does the heat. The plastic they used for the tailight assembly had a low melting point.
good to know thanks