Flashing sidemarker lights (and turn signal LED upgrades)

So I've seen a couple of web pages that try to explain how the alternating flash side-marker light on our trucks (and most GMs) work, and I came across this page on Daniel Stern Lighting that actually has a good explanation with pictures.

The side marker bulb socket is isolated from ground, and one bulb lead goes to the running lamp positive circuit. The other lead is tied into the turn signal positive lead.

When the parking lights are on and the turn signal off, it grounds throught the turn signal filament and illuminates the marker lamp. When the turn signal flashes, it interrupts the ground and the marker will flash. When the running lights are off, the process is reversed, with the ground being through the filaments of the running light circuit, and the marker will flash in sync with the turn signal instead of alternately

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(I highly recommend reading the articles on his website, lots of good information there.)

This is why when we try LEDs in our side markers, they don't function properly in most cases.
A standard bulb doesn't care about polarity, so it can pull ground from ether side of bulb.

LEDs ARE polarity sensitive, so you need buy slight more expensive versions that are NOT polarity sensitive.
From some quick looking, it seems that the side markers are cheaper to find in this variety, but your mileage may vary.

So for the same reason we don't want to use HIDs in non-HID headlight fixtures, we need to make sure our side markers still function like stock (safety, that is, and in some areas might result in a fix-it ticket if a cop hasn't met his quota).

I've never had a problem with led side markers.
Maybe yukons are problematic.
I plan to add an extra led 194 to mine.

My markers aren't led but are always on with parking lamps. I've been trying to correct this, any ideas?

Slightly more expensive LEDs that aren't polarity-sensitive? Why not just pull the bulb out of the socket, turn it 180° then plug it back in?

iamdub said:

Slightly more expensive LEDs that aren't polarity-sensitive? Why not just pull the bulb out of the socket, turn it 180° then plug it back in?

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Because reversing the bulb won't fix anything.
It'll just work in only the other half of the time (and at worst not work at all).
Spend a little bit more on the side marker bulbs, OR make sure your LED turn signal bulbs are polarity insensitive, and they will work like they should.

Why cheap out on a small bit of safety equipment that will most likely out last your truck to save a couple of dollars?

It is a cheap and easy way to make sure your front turn signals still function as designed (and potentially keep you from getting a ticket, if you pass a bored cop).

I'm just trying to share what I have learned, so anyone else that wants to upgrade to LEDs in their stock won't have to waste time and money on bulbs that won't do the job correctly.

W8TVI said:

Because reversing the bulb won't fix anything.
It'll just work in only the other half of the time (and at worst not work at all).
Spend a little bit more on the side marker bulbs, OR make sure your LED turn signal bulbs are polarity insensitive, and they will work like they should.

Why cheap out on a small bit of safety equipment that will most likely out last your truck to save a couple of dollars?

It is a cheap and easy way to make sure your front turn signals still function as designed (and potentially keep you from getting a ticket, if you pass a bored cop).

I'm just trying to share what I have learned, so anyone else that wants to upgrade to LEDs in their stock won't have to waste time and money on bulbs that won't do the job correctly.

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I read the first reply and thought this thread was about some kind of 194 bulb side marker that I didn't know about. My mistake. Not discounting your findings or sharing thereof as I may be needing this info for the 4157s in mine.

iamdub said:

I read the first reply and thought this thread was about some kind of 194 bulb side marker that I didn't know about. My mistake. Not discounting your findings or sharing thereof as I may be needing this info for the 4157s in mine.

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I wasn't talking about the DRL bulbs.

I am talking about the turn signals and the "194" side marker light.
You don't have to worry about getting a non-polarized LED for the DRLs.

The only exterior lights on our trucks where you should worry about this type of bulb, is the the front turn signals/side marker bulbs, only because of the way the side marker bulb circuit was designed.

Okay, I'm ignorant on this subject, so I'll ask: how exactly are the side marker bulbs supposed to work? Are they not supposed to blink with the flasher? I've got LEDs in mine, but I'm not sure if they blink or stay constant. Hmmm...

XLDenali06 said:

Okay, I'm ignorant on this subject, so I'll ask: how exactly are the side marker bulbs supposed to work? Are they not supposed to blink with the flasher? I've got LEDs in mine, but I'm not sure if they blink or stay constant. Hmmm...

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Take a look at this video:

I have switchbacks installed for this video, but you can see how the turn signals and side markers are supposed to work.
It is harder to see how they work with the parking lights off, but the side markers blink with the turn signals with the parking lights off.
With the Parking lights on, the side marker goes out when the turn signals flashes.

Daniel Stern Lighting has this page on how these lights work.
The way our trucks are wired, is like the second diagram on that page.

W8TVI said:

Take a look at this video:

I have switchbacks installed for this video, but you can see how the turn signals and side markers are supposed to work.
It is harder to see how they work with the parking lights off, but the side markers blink with the turn signals with the parking lights off.
With the Parking lights on, the side marker goes out when the turn signals flashes.

Daniel Stern Lighting has this page on how these lights work.
The way our trucks are wired, is like the second diagram on that page.

Click to expand...

Thanks. I went out to the car last night to check how mine work, since I installed LEDs. They work like they're supposed to. Fortunately I got the LEDs that don't care about polarity, not even knowing how the side markers work with the turn signals and parking lights. Learned something new thanks to this thread :-D