Flashing DRLs for emergency lighting

[Gryphon762mm]

Since there are no decent locations in the Sierra headlight assemblies to install hideaway strobes or LEDs (corner lights are amber lenses so you have no choice in color), I modified my DRLs to flash when my emergency lights are activated. If anyone is interested in how it was done I could do a little writeup.

 

Ok so there seems to be enough interest that I can add something here. First let me say that I have a Sierra with LED DRLs; Silverados have halogen DRLs and I'm not sure how they're handled so some of this information may not apply. I believe they're wired identically but I'll need Silverado owners to confirm.

 

Materials needed:

 

2 x SPDT 12VDC changeover relays

LED flasher unit

Switching system

Wire, female quick disconnects, splices

Cable ties

 

Let's start with a bit of theory. DRLs are constantly on when the vehicle is out of park, so using them as emergency flashing lights presents a challenge. How do you flash something that is already on? We need to interrupt the power to the lamps first so they are normally dark, and connect a flasher module to apply its own power in the pattern we choose.

 

This involves cutting into the factory wiring. I initially didn't want to muck with the BCM but after examining it, that was the best place to do this. The wiring involved two cuts in the factory harness and isn't any different than an installer adding an alarm system or other ancillary accessory, and in fact is much less invasive.

 

Locate the Body Control Module (BCM) under the driver's dash. It is identifiable as a bank of large plastic colored plugs with a TON of wires coming out if it. We are interested in the BLACK and BROWN connectors. Remove both of them from the BCM. Next, carefully cut away the fabric tape wrapped around each pack of wires to expose a good 4-6 inches worth. The tape is especially strong so scissors, side cutters, or a sharp knife is in order. Be careful not to cut into any of the wires while doing this.

 

Thanks to icecrm and his thread Brighter LED DRLs? Yup! for the following schematics as they helped tremendously with my project.

 

 

 

 

We can ignore some of the information above as that's for a different mod. Specifically all we need are the two wires providing +12VDC to the DRLs which are the GRAY/DARK BLUE in the X4 BLACK connector, and the DARK BLUE/BROWN wire in the X5 BROWN connector. Note that although both are indicated as being PIN 7, they are NOT in the same positions on the connectors. Identify them strictly by color. Once you've located these two wires in the harness cut them in the middle of what you managed to expose. Make sure to leave sufficient length at both ends in order to re-splice back to factory configuration or to make corrections.

 

Splice a short 6 inch length of wire (16-18 ga should do) to each of the cut wire ends. It should start looking something like this.

 

 

Next we need a pair of SPDT changeover relays. I got a pair at Princess Auto for ten bucks a piece. They came with a socket and pigtail but I chose to connect to the relays directly with female disconnects (if you choose this option use insulated disconnects). Connect the BCM (plug) side of one wire to the 87a terminal of the relay. Connect the DRL (chassis) side of the same wire to the 30 terminal of the relay. Repeat for the second wire.

 

Next run a minimum of four conductors from your emergency equipment to the dash section. I used a single CAT5 cable and a 3-conductor 18ga strobe cable. One conductor is going to be used to drive the relay coils to do the changeover, one is for the ground (you can choose to ground at the dash but I have a central ground point at my amp that is much more accessible), and two are to drive the DRLs from whatever flasher you are using.

 

Relays don't use much amperage at all so you can use small conductors for driving the coils. I used one pair of the CAT5 for this although I probably could have gotten away with a single conductor. I chose CAT5 because I plan to tap into various other wires for inputs on my Whelen CanTrol but that's a different story. I used the black 18ga conductor in the strobe cable for the relay coil ground.

 

You will need to switch the relays with a constant +12VDC to turn off the DRLs to allow your flasher to override. If you are using some sort of switchbox in conjunction with an LED flasher, you can use the same output from the switch that powers the flasher to trip the relays. I used an LED output on the CanTrol and configured it for relay and it works great. Connect your constant +12VDC source to both terminals 85 on the relays, and ground to terminals 86. You can activate both at once, but we need two relays to be able to supply independent flashing current to each side. If you used only one relay, both DRLs would flash simultaneously.

 

Take the two outputs from your flasher module (or LED outputs from my CanTrol) and connect them to the 87 terminals on the relays.

 

 

 

Tuck the wiring and relays up into the dash area and secure with cable ties.

 

That's basically it. Now when you apply +12VDC to the relays, they will cut off the current from the BCM to the DRLs leaving them dark. At the same time your flasher is activated and supplies current through the relay and controls the DRLs. This will allow the DRLs to flash both in park when not illuminated, and when driving, causing it to look something like this:

 

https://youtu.be/a3fevjv93mc

[nibroc92]

I just installed a set of hideaways in the headlights of a 2014 Silverado the trick to it is to get a light that is long enough to reach through both walls of the headlight

 

I'm interested to see how you did wig wags

[precision45]

Since there are no decent locations in the Sierra headlight assemblies to install hideaway strobes or LEDs (corner lights are amber lenses so you have no choice in color), I modified my DRLs to flash when my emergency lights are activated. If anyone is interested in how it was done I could do a little writeup.

 

I'm interested...

[NitroDatsik]

I wanna know.

 

2014 Silverado 2WT CC Summit White 4.3L V6

DROPKICK Edition

IG: jorge_dropkick_rangel

[NitroDatsik]

I just installed a set of hideaways in the headlights of a 2014 Silverado the trick to it is to get a light that is long enough to reach through both walls of the headlight

 

I'm interested to see how you did wig wags

What kind of lights did you use?

 

2014 Silverado 2WT CC Summit White 4.3L V6

DROPKICK Edition

IG: jorge_dropkick_rangel

[nibroc92]

I'm a dealer for federal signal and Whelen so I had some lights sitting around that I test fit the federal lights have a longer lighthead. Sound off has a longer light too as well as Whelen they just get a little pricey

[emailj]

Very interested in how you did this, pics and write up would be groovy. Would love to add wig wags using existing factory headlights. Guessing I need a module of some sort, I love Whelen products!

[SULLVERADO]

anyway to just disable DRL?

[Gryphon762mm]

Done (see post 1). Hope this helps you guys out a bit.

[SULLVERADO]

Done (see post 1). Hope this helps you guys out a bit.

this is how to disable them? id like to disable drls and just have power from the switch only

[SULLVERADO]

sorry i mean to specify i have a silverado

[Gryphon762mm]

No, the writeup is for what the post is titled as - flashing LEDs for emergency lighting. Disabling DRLs is prohibited in Canada anyways, it is mandatory safety equipment. I know of people that have gotten ticketed for having them burnt out. Why would you want to disable them? They've saved my ass on the highway a couple of times when I went to pass and they helped judge the speed of oncoming traffic. I think they should be mandatory everywhere.

[emailj]

I haven't had DRL's on my vehicles since 2012 and never been ticketed or even questioned. Just sayin' And thanks for the write up on the flashy flashy. I may have to attempt this... one day.

[SULLVERADO]

I live in Eastern canada. I just run park lights and I've seen MANY RCMP vehciles just running park lights.. lol my 2010 had the drl fuse the day it came home. so id like to do the same to my 2014.

[Gryphon762mm]

That's because park lights count as DRLs.