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Driving lights not working??

12K views 8 replies 4 participants last post by  Yellowelephant 
#1 ·
The driving lights that run down the side of the headlights on my 13 XTS aren't working. Everything else is working fine (headlights, turn signals and fog lights)

Any know what could be the problem?

Thanks!
 
#3 ·
Hello Sublime69,

I am very sorry to hear that you are experiencing a concern with the headlights on your XTS and I sincerely apologize for any inconvenience. I see you are currently soliciting advice from fellow owners on the forum, but if you would like to have your concerns addressed by your local GM dealership, I would be more than happy to assist you.
If this is something that would interest you, please feel free to send me a private message with your full contact information, VIN, current vehicle mileage, and the name of your preferred dealership.
I am looking forward to hearing from you.

Best wishes,

Steven A.
Cadillac Customer Care
 
#4 ·
Good Morning Sublime69,

We’re sorry to hear about your experiences with your headlights and we understand how this situation can be frustrating to you. We would be happy to connect you with a certified GM dealership to take a look at your issue with your lights. To proceed, please send me your full contact, VIN, current mileage and your preferred GM dealership in a private message. We look forward to hearing from you soon.

Best,

Jerel J.
Cadillac Customer Care
 
#7 · (Edited)
1. Yes.
2. Just about everything not connected to the engine and drivetrain.
3. Maybe. What kind of mechanical and electrical skills do you have? Do you have a factory service manual? Do you have any warranty left?
4. Under the dash, if I remember correctly (I'll verify this tonight.) A new part (not real expensive) will probably need programming to ensure the most up-to-date software is installed.

Answer #3 and we'll go from there.

CC
 
#8 · (Edited)
Follow-up:

After some back-and-forth PMs, we agreed the best approach for this problem would be to have the BCM scanned to isolate the problem, rather than install a used BCM that was nearly as expensive (<$100) as a new one --which would still need to be programmed by a dealership anyway.

Back in the good old days, when you wanted to turn on the radio or electrically roll down the windows, you pressed a switch. The switch completed a ground path and power flowed to the radio or the motor that drove the window glass down. When these functions broke, the first thing you checked was the operation of the switch and verified power was being applied where it was supposed to be. It was easy and fun, especially if there was beer involved.

Things have changed. Beer is hand-crafted and cars sense things. These are the good old days. While troubleshooting is still just that, the way parts of a car are turned on and off is with ones and zeros and power sources. The power won't flow until the ones and zeros tell it to. There's a lot going on, what with heads-up displays, digital dashes, CUE displays that want to compete with your time monitoring the road. Nothing is easy when it comes to diagnosing even a seat heater problem. Or is it? With the right tools, isolating an electrical issue can be performed while drinking a cup of coffee. Maybe these are the good old days! (If you're a professional tech.)

Current Cadillacs are divided into logical systems that use dedicated modules communicating back and forth, sharing their sensor information to help each other out. With so many modules competing at different speeds, sharing so much information from one another, something has to maintain law and order, or the data steams crash to a halt (no pun intended.) The traffic cop who controls the flow is the BCM --or Body Control Module, for you non-geeks.

The XTS has high and low-speed data buses (depending on priority, like a Collision Alert! message on the dash) to pass information back and forth between the many modules that control various functions of the vehicle. If a module needs to know the temperature, the BCM figures out who knows the current temp, then routes it to the awaiting module. This happens at an insane amount of speed.) The BCM is the intersection, or gateway, since it resides right in the middle of all of the data, and controls what information goes where and when.

Some work examples for the BCM's resume, include when a door is ajar, its door module senses the door switch open and places a message on a bus (a data path) intended for the instrument panel cluster. The BCM basically signals the IP cluster "You've got mail!" and the result is a message telling you it might be a good idea to try pulling that door shut again.

If you switch between high and low-beam headlights to verify that really is a clown standing in the middle of your driveway, the BCM receives the command from the steering wheel-mounted switch and activates the proper circuit in the headlight, using internal switching. (Maybe it should be re-named the "Bad Clown Monitor?")

The Grand Entrance at night isn't immune from the BCM's charms either. When you marvel at the choreographed lighting sequence the XTS presents to you, thank the BCM; it's making sure the interior and exterior lights illuminate (and dim) in the proper order.

When you press the fob to open the trunk or remotely start the vehicle, the BCM is processing the command from the receiver module and sending it to the trunk latch or ignition circuit, giving you that satisfying click.

Speaking of the ignition circuit. In the good old days, the key was attached to a mechanical electrical switch, which completed a path and applied current to the starter circuit. The current ignition switch creates a start-up signal, which makes the BCM get busy in a hurry.

The downside to all of this technology is you can't always use three beers, a couple of screwdrivers and a multi-meter to diagnose electrical problems in most cases. Now, it can take nearly a six-pack to pass the time constructively while waiting as a software update slowly creeps through a car's systems, before the reason you wanted beer to enhance the working parts of this job even begins.

As it relates to this thread's original problem, a little symptom recognition and elaboration is in order. The LEDs on the sides of the headlights should both operate when the headlamp bulb is illuminated. Since they aren't, the problem is more likely caused by something common to both headlights, since electrically, they have their own dedicated wiring and none are tied to the other electrically -but not functionally, since they're supposed to work at the same time.

So what's common? Well, power and ground, but as covered, they each have a power and ground circuit, so if power was missing on both sides of the front end, it stands to reason more circuitry would probably be affected.

--You'll see me write words like, "probably and "likely" a lot when I diagnose a problem remotely. This is because troubleshooting isn't a spectator sport. You really have to be there to make sense of the symptoms. (Even your sense of smell - when things get really ugly.) There are no absolutes when remotely troubleshooting, so if you see someone replies with, "Oh yeah, that's the left-handed, electric dog polisher enable switch! I've seen that before." Don't take it as gospel and order a switch online. It might be the tail-wag sensing circuit is defective. You have to look at the symptoms to list the possible causes and their most likely rate of failure.

Troubleshooting is basic problem-solving, and it's vital to avoid "tunnel vision" --especially when the "quick fix" is anything but. System knowledge pays off big-time. There's too much complexity for any one human brain to remember without exploding, so factory service manuals are essential. They're (supposed to be ) the most up-to-date and detailed documentation available.

Back on point:

The headlamp wiring for each circuit (High/Low/DRLs) goes to the BCM. Main vehicle power is available at the BCM. When the headlamps are needed, the BCM makes it happen. With the same problem on two lights, it could be: The LEDs in each light have gone to silicon heaven, or the BCM is faulty. LEDs are pretty reliable, as opposed to the incandescent bulbs of yore. Since this owner just acquired this vehicle with over 200k miles on it, the previous owner might know if both sides expired simultaneously or not, but he doesn't have that luxury. A little educated guessing is in order along with a fresh beer for reflection.

Life is one big crap shoot. The chances of a single circuit in the BCM failing are higher than two headlamps failing identically, so that's a reasonable area to begin troubleshooting. This is where the Big Decisions start. Why disassemble the front end to get at the headlamp connectors (identical corrosion from visual inspection?) --or remove dash panels to probe the pins on the BCM that control the lights? If one area isn't the cause, you have to take the other interference off to inspect the other. And unless your car is destined for a Mad Maxx cameo, you have to put the panels back on, doubling your chances for a retaining clip failure or scratching a part accidentally.

"I'll take, "What To Do Next?, Alex."

The Jeopardy Daily-Double right answer is:

"What is scanning the BCM, Alex?"

You win! Scan the module, so individual BCM functions can be activated, --if no overt fault code is immediately detected wagging its nagging finger toward the BCM. If the BCM is bad, an up-to-date BCM can be installed, with a minimum of prying things off the car. I'm a big believer in the "If it works, don't fix it" mantra, and don't want to take apart more than I have to when working on a car.

BCMs require programming for the specific car after installation, so you kill two birds with one stone. In the OP's situation, without access and procedures to use the proper diagnostic tools, a dealer visit is certainly justified. The OP wanted to know if a used BCM he located was a good idea, and I advised him against it. The price between new and used was $15, and without a way to determine of the BCM was faulty, the part might not be needed. And it need to be programmed anyway.

A quick diagnostic scan eliminates a lot of wasted time and money for plugging and chugging parts as many DIYs end up doing. It needn't be as traumatic or costly as having to sell your firstborn male child. Don't go down this path! You can buy the part online and have a dealer install it (with prior permission from the Service Manager) often saving enough to pay for the labor!

This is probably more than most want to ever know about the XTS's BCM, hid behind your left knee from the driver's seat, but I wanted to follow through on the thread so there aren't any loose ends if he comes back from the dealer's scan and reports what happened. Now that we have an idea of how the circuit operates, the explanation should make more sense.

CC
 
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