Front side marker light wiring surprise

denpo

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Ok, I've been battling with a weird behavior :
Everytime I put the headlights on, both flasher indicators (on the dash) light up.
I've put everything down, disconnected the flasher module, the headlight switch, the rear harness, the dimmer switch, and still, the brown wire (that powers the low headlight) would seem to be connected to the two front flasher wires, from the resistance reading on my voltmeter.

So I looked again in the wiring schematic, and had a big WTF moment :
The front side marker lamps are connected to both. Those light bulbs have their two pins connected to a 12v source. One side the light circuit, the other the flasher circuit!
With my headlight relay switch mod, my no-load flasher and my led gauge light bulb, no wonder it doesn't behave as it does.
When the brown wire get 12v, it goes through the marker light, and then can only escape by creeping up the flasher circuit to the gauge lamp.
If it were a regular bulb, it would slightly glow, with my leds it's enough to light is up completely.
And my flasher, it sees some sort of anti-load, and stop functioning.
My fix will probably be cutting the link and put two separate lighting unit with each its ground. But before I want to understand the logic.

So, you guys who have one that works, in one piece, tell me :

Are the side markers supposed to flash when you turn with the headlight off?
Are the turn indicator on the dash supposed to light up dimmed when the headlights are on?
 
Yes.
No.

I think the Vette-only Electrical Troubleshooting Manuals are still available from Helm, along with the other service manuals. They're invaluable, as they have the wiring diagram for each circuit drawn out separately rather than the standard full-car wiring diagram, along with helpful troubleshooting text. The full-car diagrams are also available.

I found an original 77 ETM on eBay years ago, and it's been a lifesaver on more than one occasion.
 
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Yes.
No.

I think the Vette-only Electrical Troubleshooting Manuals are still available from Helm, along with the other service manuals. They're invaluable, as they have the wiring diagram for each circuit drawn out separately rather than the standard full-car wiring diagram, along with helpful troubleshooting text. The full-car diagrams are also available.

I found an original 77 ETM on eBay years ago, and it's been a lifesaver on more than one occasion.
Thanks Doc for the infos.
 
I just pulled out my 77 ETM. It sounds like your lights are hooked up correctly, but something is bleeding through the circuit, or you have a bad ground somewhere. You might want to try some different marker bulbs to see if that sorts things out.

The brown wire goes to the "park" terminal on the headlight switch, the blue wire goes to the flasher circuit. They're in series with the front turn/marker lights, and the turn indicator lights in the dash for the blue wires and the park light bulbs (the low filament in the taillights, the rear marker lights, and the license plate light).

I'd scan the pages, but it's a very large format book and won't come close to fitting on my scanner.
 
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Please don't bother scanning electrical schematic. I probably have everything but Doc's colored poster.
As far as I understand the side market wiring is some kind of electrical 'trick'.

If the side market lights up along the actual flashers, that mean the signal from the blue line goes through the bulb and them probably goes through the headlight lightbulb to find a ground.

Same for the opposite, if the side market light up when the headlight are on, the current must find its way in the flasher circuit.

On test for a working corvette :
The flasher and speedo/tacho backlight are completely isolated (I'm taking of light insulation here), that mean when the backlight is on, in a dark environement, the flasher indicator should stay pitch black, you shouldn't be able to discern the slightest of green.
Is it really the case?
Because I suspect the flasher lightbulb are actually glowing a little bit but people take that as backlight bleeding.
 
Yeh, I stayed outta this one because I could not come up with the words to describe the operation, much easier to look at the schematic......it's tricky....

:bounce:
 
The filament of the turn signal (front) bulb is where the ground comes from. In my service manual (I have a 75), It states the "Bulb filament provides ground path for marker lamp bulb through the light blue or dark blue/white stripe wires."
 
That rat's nest of a diagram is why the ETM is so helpful.
Hey Sam, dunno why you edited out you previous comments, they were legitimate and sense full. Thanks for chiming in, and while I won't get your awesome colored electrical diagram poster, be assured my weatherstrips won't come from anywhere but your stock. :hi:

The filament of the turn signal (front) bulb is where the ground comes from. In my service manual (I have a 75), It states the "Bulb filament provides ground path for marker lamp bulb through the light blue or dark blue/white stripe wires."
Thanks gkz, that confirm the 'trick' theory.

And thanks Gene for entertaining the hell out of us, as always :clap:
 
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