I'm getting conflicting answers when I search the forum about what is in the center of my defrost vent. I have a 1999 Trailblazer with the electronic HVAC (set a temp) controls. One post says that it's the ambient light sensor (GM 12450120) for the automatic headlights, another says it's the Sun Load sensor (GM 13578464). I looked over at the passenger side speaker cover and there's something there that's been pushed down and is moving around, probably part of my DRL issues if it's the ambient light sensor. No OnStar until 2000 so it's not a microphone. I'm prepping the rig so the wife can drive it cross country and she has higher standards than I do when it comes to things like driving lights and HVAC controls. I'm used to dim lights and using the window to regulating temperatures and she's used to HID and computer controlled to an exact temperature. I get 23 year old Blazer and a 3/4 ton work truck, she gets a new Charger, happy wife and all that, right? The Charger is going to be shipped and she won't drive the quad cab long bed 3/4 ton for some strange reason.

 

According to my 99 Service manual, the Ambient Light Sensor is located on the Top, LH side of the IP (instrument panel) and the sun-load sensor is in the middle of the defrost vent panel.

 

That's fantastic! I'm a little confused on the Ambient being in the upper LH of the instrument panel though. Could that be the cutout in the dash speaker cover? There's nothing topside on the driver's side. Could it be the 'Left Hand' while facing the truck from the front? Either way, it looks like I should probably start looking around for a factory service manual if I plan on keeping it (and I do, this little $600 dog hauler has won me over these past several years despite it's quirks). Thank you for answering!

 

Yes, I just looked. The ambient light sensor is in the speaker grill and it looks to be the one on the drivers side.

I don't know what I would have done without my service manual. Be warned though, some of the procedures assume little things like the engine being out of the vehicle or that you have access to all the special tools. LOL I have a hard copy. It comes in three volumes! I got it off eBay ages ago.

 

Well, no joy on any of the lights after replacing the ambient light sensor... Scanner is still showing a B2647 after replacement and clearing the code.

 

I scanned this for you (attached). This is the diagnostic list for this error. Maybe it will help you track down the problem. Maybe it is a bad connector or bad wire. The circuit looks pretty simple. Just be careful to ground yourself to discharge any static on your hands and on your multimeter before touching the leads to the BCM.

 

While I'm not looking at a Trailblazer manual, from what I see B2647 is AMBIENT LIGHT SENSOR LOW. In GM speak "low" = ground. Based off your symptoms in another post, it would make me think that you either have a short to ground or a problem with your BCM. Since I also saw these problems started happening after you had your dash apart, combined with your symptoms I would really lean towards a short more than a BCM problem. Logic: Your trouble code combined with the fact your door circuitry is finding a path to complete the circuit when it shouldn't. I'd recommend you purchase a subscription for your truck's service manual at https://www.alldata.com This will not only provide you with trouble shooting steps for the code, but will also provide color wire diagrams you can follow.

If you don't have already, I'd get a DVOM and a test lamp.


EDIT: Looks like Christine and I were responding at the same time. Following the steps she provided should point you in the right direction.

 

Thank you! I'm looking at a set of used service manuals now. It's crazy to think that you started working on yours about the same time I joined but you have progressed WAY beyond where I am and I used to think I was handy around vehicles. LOL, my own fault for getting old and accepting the quirks instead of fixing them. But again, thank you!

 

She's a rockstar! I'm fully accepting the fact that I have let my automotive knowledge collect too much dust while she's been sharpening her tools these past years. The majority of my tools are sitting in a storage container somewhere in Texas right now but who doesn't need another volt meter and test light, right? I also think I'm going to have to bite the bullet and get the 3 volume service manual set for this rig as it approaches it's 25th birthday. It's been a good rig for quite a few years at this point and being rust free, the only thing that should kill it at this point is a lack of maintenance by me. Thank you as well for at least being honest and replying that it's probably a grounding issue likely caused when I pulled/installed the dash. It's not what I wanted to hear but I think I already knew it. At least now I have an idea of where, what, and how to test with the values provided by Christine.

 

So i finally got my digital volt meter in (I know it took awhile but I'm cheap and ordered off Amazon) and went to check my bcm and... should there be an open port? From driver side view looking in from bottom to top its: open, light blue, brown. Please tell me that's for something like a sunroof or factory amplifier or basically ANYTHING I don't have! I am 99.9% sure that I didn't miss any connections when I reinstalled the dash (heater core replacement) but that empty plug has me worried now. I don't WANT to pull the dash again but if that's what happened, I'm going to kick myself.

Say it ain't so!