Strange Turn Signal / Marker Lamp Electrical Issue - Need Help

sneaky98gt

Member
Nov 5, 2013
54
1
8
Hello all. I'm having a werid electrical issue with my 2003 Duramax. I'm usually a pretty good troubleshooter, but electrical stuff is sometimes my kryptonite, so I need some help.

First off, here's a quick video showing what's going on.

https://www.youtube.com/watch?v=fQeIrjCUoKk

Basically, with the parking lights on, the turn signal alternates flashing with the side marker lamp. With the parking lights off, the turn signal and side marker lamp flash together. The video doesn't show this, but with just the parking lights on (no signals), the parking lamp and side marker lamp light up as they are supposed to.

All 4 corners do exactly this, and they do it with both the multi-function switch, and the hazard switch. I have verified that all bulbs are good. Activating the brakes doesn't change anything. I unplugged all of the connectors at the rear junction box, and the front lights still do the exact same thing with the rears unplugged.

Before I dive too deep into this, can someone confirm if this is normal operation? A few of my coworkers seem to think this may be normal, but I've always understood an alternating turn signal to indicate (no pun intended...) some sort of problem. And in my experiences on other vehicles, the side marker lights aren't supposed to flash.

Here's my general thoughts:

-No change in the fronts by unplugging the rear junction box would mean there isn't a physical issue in the rear of the truck (short or grounding).
-The issue happening with both the turn signal and hazard switch indicates that it isn't the multifunction switch.
-The fact that all 4 corners are doing exactly the same thing leads me to believe that it isn't an individual physical issue like a bad ground or shorted wire (would seem awfully improbably that 4 different circuits had 4 different, identical failures), but rather something more systemic. Like a flasher relay, or bad BCM, etc.

And that's where my expertise runs out. I have no clue where to go from here, and I hate throwing parts at something. Any input?

Thanks!
 

M.A.M.

Member
Jan 9, 2016
403
2
18
Ohio
I’d check operation on a totally separate truck. I just about think that’s normal operation.


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sneaky98gt

Member
Nov 5, 2013
54
1
8
I’d check operation on a totally separate truck. I just about think that’s normal operation.
For like 5 days now, I've been scanning the roadways as I drive for another same-body-style truck to see if it does the same or not, lol.

Unfortunately, haven't seen one yet, don't have any close friends with one, and haven't found a single video online yet verifying it.

What do you see with your headlights on with turn signals? Different? How about DRL's? anything different there?
Everything else works exactly as it's supposed to. High and low headlights, DRLs, fog lights, mirror lights, cab lights, license plate lights, interior lights all work perfectly.

I've read through dozens of old forum posts regarding strange tail light / turn signal issues, and virtually always they have some other lights that are acting weirdly at the same time (i.e. bad ground). But this is almost like it's programmed to work this way, as nothing else is amiss. I'm still not convinced that it's supposed to be this way. I think power from the turn signal circuit is feeding through the marker lamps, for whatever reason.
 

zakkb787

<that’s not me...
Sep 29, 2014
2,133
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38
Granite Falls NC
99.9999999% sure that’s normal. I’d check on my 06 but I don’t have marker bulbs in mine anymore. If you think about it, it makes sense. During the daylight there doesn’t need to be a differentiation of pulse because there’s no other amber illuminated. Night time, you already have an amber light, so they are opposite to attract more attention. Might not make sense but it does in my head.
 

JoshH

Daggum farm truck
Staff member
Vendor/Sponsor
Feb 14, 2007
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Texas!!!
It is normal. The reason being is because the side marker light is a single filament bulb that grounds back through the turn signal. When the lights are on and the turn signal activates, it puts voltage to the side marker bulb’s ground which turns it off as the turn signal turns on. When the light isn’t on, the turn signal powers both on and the side marker light grounds back through the wire that normally powers it as a side marker light with the park lights.
 

TheBac

Why do I keep doing this?
Staff member
Apr 19, 2008
13,375
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Mid Michigan
Lights off --- they work in unison
Lights on --- they work opposite each other
Completely normal.
 
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sneaky98gt

Member
Nov 5, 2013
54
1
8
99.9999999% sure that’s normal. I’d check on my 06 but I don’t have marker bulbs in mine anymore. If you think about it, it makes sense. During the daylight there doesn’t need to be a differentiation of pulse because there’s no other amber illuminated. Night time, you already have an amber light, so they are opposite to attract more attention. Might not make sense but it does in my head.
Lights off --- they work in unison
Lights on --- they work opposite each other
Completely normal.
Well, I suppose it's never too late to learn something new. I've had, and been around, a lot of vehicles and trailers, and alternating lights always meant that there was something wrong somewhere, usually a bad ground. But, it sure looks like this is how it's supposed to be.

It is normal. The reason being is because the side marker light is a single filament bulb that grounds back through the turn signal. When the lights are on and the turn signal activates, it puts voltage to the side marker bulb’s ground which turns it off as the turn signal turns on. When the light isn’t on, the turn signal powers both on and the side marker light grounds back through the wire that normally powers it as a side marker light with the park lights.
So, I found a wiring diagram, and had some of the electrical engineers I work with help me work through it. What we found was.....exactly what you described. I never would have guessed that it was intentionally wired this way.

In theory, if I wanted it work differently (looking at the left front lamp), I could cut and ground the brown wire coming from the side marker lamp, which would make it always flash with the turn signal. Or, I could cut and ground the light blue wire coming from the side marker lamp, which would make it always on with the parking lights, and not flash at all.

I also combed through the tail light diagrams, and they're wired the exact same way....which means that everything I'm seeing....is exactly how it's supposed to be....:doh:

I feel kinda dumb, but I do appreciate the help. :thumb:
 

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