No low beams or Daytime lights

flhtrider

Original poster
Member
Apr 16, 2012
4
I have a 2004 Trailblazer EXT 4X4. The other night while driving my headlights went out. I was in the parking lot of my destination, luckily. I played with the switch changing from off to auto to manual, nothing. Being not far from home I decided to try to get home. As i was driving the lights came back on. I also noticed that there were no headlights,brake lights, or reverse lights upon locking or unlocking the vehicle. The next day I went out to go to work and they were working, on the way they went out again, and came back on after a bit. When got off work I went to the internet to find answers. After reading things from trailvoy, and other sites, I checked the relays by switching #45 and #46, nothing. Checked all the fuses in both fuse boxes front and rear, all good. As I scratched my head over this, something told me to cover the ambient light sensor. Upon covering the sensor I had low beams, of course no dtl's. I also had the light that come when you unlock (keyfob), but not when I locked it. The temporary fix is good for now but, I would like to fix it right. Since the light do work with the als covered could it still be the wires to the headlights, or maybe the ALS? Any further help with this problem would be greatly appreciated :crazy:
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
I would certainly guess the sensor, or the connection to the BCM that reads its output.
 

flhtrider

Original poster
Member
Apr 16, 2012
4
the roadie said:
I would certainly guess the sensor, or the connection to the BCM that reads its output.

Are you speaking of the 21 or so pin connecter?When I looked at the driver's side passenger seat, under the seat fuse box, it looked like it was brand new no dirt, every clean.:thumbsup:
 

The_Roadie

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Nov 19, 2011
9,957
Portland, OR
The harness from the BCM to the sensor is long and goes through other connections. Tough to inspect. You may need a tech with a GM Tech II tool to read the sensor output data. Unless you have a meter and a bit of troubleshooting experience to see what the voltage is at the sensor itself and see if that gets back to the measurement system in the BCM.

IIRC, the lights you mention should ALL flash to confirm lock/unlock, no matter whether it's daytime or night. Can anybody else check this to confirm?

There may be two issues, and the BCM is at the heart of both of them. Even if the ALS has a problem, turning the headlight switch to manual ON should override that even in the day and get you your headlights back. At the very least I would remove and reseat the connector at the BCM. And also the headlight control switch. You should be able to get at that using the little access hatch in the dash that's visible when the door is open. Use a knife to pry it off.
 

flhtrider

Original poster
Member
Apr 16, 2012
4
the roadie said:
The harness from the BCM to the sensor is long and goes through other connections. Tough to inspect. You may need a tech with a GM Tech II tool to read the sensor output data. Unless you have a meter and a bit of troubleshooting experience to see what the voltage is at the sensor itself and see if that gets back to the measurement system in the BCM.

IIRC, the lights you mention should ALL flash to confirm lock/unlock, no matter whether it's daytime or night. Can anybody else check this to confirm?

There may be two issues, and the BCM is at the heart of both of them. Even if the ALS has a problem, turning the headlight switch to manual ON should override that even in the day and get you your headlights back. At the very least I would remove and reseat the connector at the BCM. And also the headlight control switch. You should be able to get at that using the little access hatch in the dash that's visible when the door is open. Use a knife to pry it off.

I will try what you suggest, and let you know the results. Thanks for you help:thumbsup:
 

flhtrider

Original poster
Member
Apr 16, 2012
4
I took the connectors loose at the bcm and the light swith, and then pluged them back in and everything seems to work as it should. although sometimes when I lock it remotely the headlights don't come on. But I now have headlights and daytime lights. Thenk again :thumbsup:
 

MRaymond1

Member
Apr 8, 2012
12
This issue just started with my 2007 Trailblazer. My vehicle is doing exactly the same thing. The low beams and daytime lights do not work even when the headlight switch is activated. The high beams do work. Reviewing these suggestions, I'm stuck in acronym limbo. What and where is the BCM? I understood everything else.
 

MRaymond1

Member
Apr 8, 2012
12
the roadie said:
It's almost always the relay. The BCM is the Body Control Module, under the rear seat adjacent to the rear fuse box. We have an acronym list somewhere around here.

Problem solved. Relay 46 under the hood marked HDM. I'm guessing that it means Headlight Dimmer Module (daytime running lights are dimmer than normal low beams).

$35.00 at Autozone. I could have gotten it cheaper on line but for some reason I need headlights. Thank You to everyone here.

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The_Roadie

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Nov 19, 2011
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Portland, OR
You got it. The reason it's more expensive than the normal $5-8 gray relay is it's solid state (electronic) and not the cheap mechanical guts. The reason it's electronic is that they way it accomplishes the DRL (daytime running light) dimming is it turns the headlights on and off at a rate faster than the eye can see (faster than 100 times a second) and mechanical relays would die in hours and make a lot of buzzing noise doing it. In electronic terms this is called a PWM (pulse width modulation) signal, and lots of functions in modern vehicles are run this way. Digital HVAC system blower speed, gauge and switch light dimming, electroviscous fan clutch engagement.
 
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