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jborland77 · Jul 10, 2012, 10:09 AM

My automobile turn signal lights and brake lights share the same filament in bulb. Turn signals work so bulb is okay. Brake lights do not work. Have checked the switch at brake pedal and it is okay. What else could it be?

Stratmando · Jul 10, 2012, 10:25 AM

I take it you can place into gear, if so the switch is working and correct about the filiment with many vehicles, don't know what you have.
I think it is in the signalhead, try the 4 way flashers, it uses the same filiments.
Does confirm wiring good, and bulbs good.
Fuse?
You checked brake switch for continuity, OR voltage in and out?

jborland77 · Jul 10, 2012, 10:51 AM

There is voltage at the brake switch so the fuse must be okay. We use a wire to jump across the terminals at the switch which should activate the brake lights, but they do not turn on.
How are the brake lights influenced by the 4 way flashers? Yes, the flashers and brake lights share the same filament but the brake lights do not flash.

Stratmando · Jul 10, 2012, 11:37 AM

The Hazzard, turn signal and Flashers use the same filiment. Usually
The Hazzard is on a constantly live circuit and fuse, and the signals may be on an Ignition switched circuit and different fuse.
Both go through the Signalhead(Turn Signal), if the Internal switching is bad, the brake lights may not work.

jborland77 · Jul 10, 2012, 11:43 AM

Cool! I will try replacing the hazard flasher and see if that fixes the problem. Thank you!

Stratmando · Jul 10, 2012, 03:53 PM

No, that won't fix the problem. Hazzard uses the same filiment.
I am saying it is likely at the signalhead.
What Make, Model, and year is it? Testing can be done at the connector after locating the diagram.

jborland77 · Jul 10, 2012, 04:03 PM

Sorry to be so dense but what is the signalhead?

This is an old, beat-up mid-70s Dodge pickup truck. The brake lights worked up until recently but have stopped working. As I've said, the turn signals and tail lights still work fine. Thanks again for trying to help me.

Stratmando · Jul 10, 2012, 04:46 PM

Those didn't have the Transmission needing brakes to be pushed in to engage.
The Signalhead is your turn signal lever, and associated switching of Stop/Turn/Hazzard.
What happens when you apply the flashers, do all bulbs flash(at least the 4 corners)?
The following wires will be at the SignaHeadl:
A Constant Hot, an Ignition Switched Hot, wiring to your Brake/Turn Filiment, your front turn signal lights, and the dash indicator lights.

jborland77 · Jul 10, 2012, 05:04 PM

Thank you for so carefully explaining the location and configuration of the signalhead.

Yes, when we apply the hazard lights all the bulbs flash. One strange thing is that the hazard lights only work when one of the turn signals is engaged (left or right blinker). Maybe that's a clue that there is something wrong inside the signalhead.

Stratmando · Jul 10, 2012, 06:26 PM

If the Flashers only work with the turn signal set at left or right, hazzard fuse may be bad, and you are getting power from signal fuse when switched. You should be able to hear the Flasher, and Hazzard Flasher to locate.
You can jump out the Flashers like the Brake switch for a test, Remove flasher, insert jumper.
Now the Hazzards or signal will remain on, if needing to test.
Check for power to both flasher sockets with key on.

jborland77 · Jul 10, 2012, 09:42 PM

Hazard flasher getting power from the turn signal fuse. What an interesting analysis! That makes so much sense. We will take another look at the flashers and try your jumper test. Once again, thanks for your thoughtful analyses and for taking the time to answer my questions.

Stratmando · Jul 11, 2012, 06:10 AM

Did you find Both Flashers?

CaptainRich · Jul 11, 2012, 07:23 AM

https://www.askmehelpdesk.com/cars-t...le-303903.html

Tell me what you drive and I'll see if the schematic for your issue is available.

Stratmando · Jul 11, 2012, 07:47 AM

Mid '70s Dodge Pickup?
Exact year would help.

jborland77 · Jul 11, 2012, 09:02 AM

Originally Posted by CaptainRich

https://www.askmehelpdesk.com/cars-t...le-303903.html

Tell me what you drive and I'll see if the schematic for your issue is available.

1977 Dodge 1/2 ton 150 pick-up truck

CaptainRich · Jul 12, 2012, 05:32 AM

Sorry, I looked and didn't find any schematics for that year range. Hands-on diag is the only way to go. I think the only way power goes through the brake light switch to the lamps is through the turn signal switch. Fault tracing this from the brake lamp switch, jumpered or not, will reveal an open circuit somewhere along the path.

jborland77 · Jul 12, 2012, 01:36 PM

Thanks for trying. I really appreciate the help given me by you and Stratmando.

Related to all this, I still have a question about automotive lighting. When two functions (brakes & turn signals) share the same bulb there must be a switching mechanism somewhere in the wiring diagram. The switch would have two inputs (sends from the turn signals and from the brakes) and one output, to the lamp inside the taillight assembly. It doesn't seem like it would work just to splice all three of these wires together. Where and how does this switching take place?

As before, thank you for your answers to my questions.

Stratmando · Jul 12, 2012, 02:55 PM

It ALL happens in the Turn Signal switch.
It will have a constant hot for flashers, a switched ignition wire, a wire to each bulb filiment, and 2 for the indicators.
I believe it is 1 of 2 things.
A fuse or
Signal head.
A manual can save you time and money for the rest of the time you own this truck. Will save others time as well. If you find a schematic and can post it, I can walk you through it.