Cadillac Owners Forum banner
  • Hey everyone! Enter your ride HERE to be a part of September's Ride of the Month Challenge!
1 - 9 of 9 Posts

·
Registered
Joined
·
11 Posts
Discussion Starter · #1 ·
My brake lights don't work but the 3rd light above the trunk works fine. Can I just T-Tap into the brake lights and connect em to the 3rd light for a temporary fix. I don't have the money to take the thing to a dealer. If so, I obviously know black is ground but what would the positive wire color be on the brake lights?

Thank you!
 

·
Registered
2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
Joined
·
3,291 Posts
The brake lights in the ATS are LED and I suspect that they are controlled via the BCM (body control module).
Someone with the service manual would have to verify but if this is the case then you will need to check how this control/drive is handled. LEDs in this application are often driven at higher than normal current but strobed at a rate too high to be visible to the human eye, this allows increased brightness without damaging the LEDs from excessive heating and this is why you will see flickering of LED tail and brake lights when shot with a video camera because the frame rate/shutter isn't synchronized to the LED strobe rate. If the driver is external to the LED module, then feeding them with a continuous 12 volt source will quickly burn out the individual LEDs.

Hopefully someone with a service manual can quickly confirm or disconfirm BCM control, if so there are multiple fuses feeding the BCM and one of those could be the reason for the failed brake lights.

My first experience with GM BCM control of brake lights was with an early 2000s Chevrolet and in that vehicle the brake switch/BCM got out of sync causing the brake lights to be on with the brake off and vice versa which wasn't a real safe situation.

Rodger
 

·
Registered
Joined
·
11 Posts
Discussion Starter · #3 ·
I got ahold of a dealership, the driver side brake light... Blue and white is pos.
Passenger side, purple and blue is pos.
Wired em to my 3rd brake light with T-taps, work if only one is connected but the second I connect the other they go off.

I didn't connect the ground, as I thought the lights were already grounded and I wouldn't need to. Should I do so?
 

·
Registered
2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
Joined
·
3,291 Posts
I believe that those are being fed through current limited drivers in the BCM and when you add the other load to that of the third brake light it is shutting down to protect the BCM and even running with one side brake light plus the high mount center is potentially going to damage the BCM because it is likely using different outputs to drive the traditional brake lights and the center light.

Have you checked the BCM fuses yet? I would start by doing that and hopefully you had an intermittent failure that took out a BCM fuse. They are listed in the owner's manual but don't identify which function is which out of the multiple BCM fuses.

I am afraid that the path you are going down is going to ultimately result in a more expensive repair bill. Current car electrical systems are very different from those of just a few years previously and designed to save energy and weight so many items are controlled via the BCM or other modules and many items are just another module on the CANBUS. You can't just splice and add/share loads like it was done on older vehicles and that is why you see "smart" trailer light adapters because you can't just splice in like the old days.
 

·
Registered
Joined
·
11 Posts
Discussion Starter · #5 ·
I miss the old days lol. I'll undo what I did then, all my fuses are fine I think I have to replace my entire headlamp wiring harness, I have no exterior lights whatsoever.

Previous owner claims the passenger headlamp housing filled with water and shorted so he says I need to replace it in order to gain my brake lights, turn signals and headlights again.

I wired my headlights to the battery 😂 I have to plug turn in when I drive at night and unplug them when I'm done 😂😂😂

Luckily I found the harness for $150 through gmparts. It's a matter of if I can install them or not that'll make or break the bank, as I'm sure it isn't cheap to have the new one installed.
 

·
Registered
2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
Joined
·
3,291 Posts
It is rare to have to replace an entire harness section, even rodent damage can be repaired if not too extensive (been there, done that :( ) Often the problem is corrosion at the connector from water plus road salt and if it isn't too bad you can flush it out and restore the connector to function, WD-40 and a soft bristle brush is excellent for this task. Buy the WD-40 in a gallon container along with a spray bottle which is both far cheaper and far better for a task like this one. CAUTION: WD-40 is primarily a light distillate of petroleum and it is flammable so be careful around ignition sources, in the early days of GM pickup diesels it was briefly recommended as a starting aid for cold engines and unlike using ether it wouldn't explode and potentially cause severe damage to glow plug equipped diesel engines. But in most cases, flaming WD-40 is not a desired outcome!

I would make sure that the old wiring harness really is the issue before replacing it because that is usually a very tedious project. Be sure and take plenty of good photos before you start if you need to replace the harness but given the expense of a replacement plus the difficulty I would repair and splice in to repair a damaged section or failed connector.

For splices you should solder, don't crimp, and use heat shrink tubing to protect the spliced connections. This makes a highly reliable and neat repair. As a hint, typically you will need to splice in multiple wires at a connector so stagger cut the length so that all of the splices are slightly offset from each other, this results in a much smaller repair "bundle" and greatly reduces the chances of two of your repaired wires shorting together in the future if the heat shrink tubing were to fail. If there are any network cables in the harness area, pay particular attention to keeping them routed as they were originally. I really don't know how critical routing is for most of these cables but GM service manuals make note of the importance of not disturbing cable routing and dress for these network cables. I suspect it is only highly critical for the high speed LAN used to connect the ECM, TCM, and stability modules on some vehicles which does require very fast network speed without loss or delay of packets. It will get more interesting as more autonomous features are added greatly increasing the demand placed upon these systems.

The complexities of modern car electrical systems make life much more difficult and is one of the reasons why more stuff goes to the landfill too early which is definitely not "green". A number of vehicles, including quite a few European brands, required dummy resistors to be used when LEDs were used to replace interior incandescent lights because the car computer sensed the LED as an open bulb due to the low current flow and wouldn't allow it to come on so the lucky owners ended up with a hot resistor to go with their replacement LED. LED indicator arrays are driven from a current limited source and if current demand is sensed to be abnormally high the driver will protect itself.

Good luck sorting this out!

Rodger
 

·
Registered
Joined
·
11 Posts
Discussion Starter · #7 ·
Thank you for the response!

I've tried repairing the harness, it's just two wires that are cut from the connector (yellow and gray) but I'm not knowledgeable enough to know how to reconnect it. It honestly may just need put back in the connector and the relay changed... Also my passenger headlamp casing is missing all the connectors I know it will for certain need replaced.
 

·
Registered
2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
Joined
·
3,291 Posts
It sounds like your car had the previous owner from hell. They should make the hood latch system a bit more difficult to figure out in order to prevent clods like that from monkeying around.

Ebay can be a very good source of parts for these cars for things like your headlamp assembly. A lot of junkyards have gone virtual and they sell and ship the parts quickly. I used that method to replace a rodent damaged underhood fuse center in my 2006 GMC pickup several years ago, it was 15% of the price from an online GM parts house and arrived in three days.

From your description, it would be much easier for you to splice (or hire someone local to do the work) rather than replace an entire wiring harness section. You can use the butt splice type crimp connectors to splice wires but I prefer soldering, if you use these crimp type connectors then buy one of the low cost crimp tools to install them. They depend upon the connector biting into the copper wire and if you go that route carefully tape over the connector with good quality electrical tape to seal it once you are sure the connector is solidly installed (tug on the wires to make sure they don't pull back out before taping). Here is an example of that type of splice, choose the connector based upon the wire size: Vinyl Insulated Butt Splices | MPJA.COM And do NOT use those insulation piercing tap type connectors for any exposed automotive work (they were originally developed years ago by 3M and sold as "Scotch Locks"), reliability is far too low for my taste in an environmentally exposed vibration prone automotive environment.

Rodger
 

·
Registered
Joined
·
11 Posts
Discussion Starter · #9 ·
Yeah, I bought the car from an auction real cheap, I looked the guy up that's name was in the title and he was on the FBI wanted list in 2017 so that's wonderful lol.

But, awesome! I'll do that thank you so much for the feedback!
 
1 - 9 of 9 Posts
Top