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Auto headlights don't come on 2008 Chevy Trailblazer LT. What's the problem?

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Hey Ray , this is just a little more complicated then my partner suggests , The BCM - body control module an the HDM - headlamp driver module play a role in how this works ! An there are DTC'S that can set in the BCM ! There are 16 different codes possibly ! You can not use a code reader to read BCM codes , you need a professional grade scan tool to check these . I have factory service information , DTC descriptions , diagnostic flow charts etc.
The headlamp driver module (HDM) is an electronic module that provides electrical power to the vehicle low beam headlamps. The HDM is controlled by a pulse width modulated (PWM) ground signal from the body control module (BCM). The BCM determines the desired lamp intensity using the ambient light sensor, multifunction switch and the park brake switch as inputs. In general, the HDM will operate in 1 of 3 modes:
?€¢
Reduced Intensity Mode: This mode is used by features that do not require or allow maximum low beam headlamp intensity such as daytime running lights (DRL) (nominal: 85% duty cycle).


?€¢
Full Intensity Mode: This mode is used by features that require full low beam headlamp intensity such as low beam headlamp operation in manual or auto modes (nominal: 100% duty cycle).


?€¢
Off Mode: This is the default mode of the system that will occur when none of the features described above are active. The BCM will deactivate its PWM output (0% duty cycle) during this mode.


Don't get me wrong here , it could very well be the light sensor but me I would like to know for sure before I just start replacing parts !
DTC B2645 00: Ambient Light Sensor Circuit .
The ambient light sensor is a light sensitive transistor that varies its voltage signal to the body control module (BCM) in response to changes to the outside (ambient) light level. When the BCM receives this signal it will either turn on the daytime running lamps (DRL) through the HDM using the reduced output or the headlamps at full intensity for auto headlamp operation. Any function or condition that turns on the headlights will cancel the daytime running lamps operation. The daytime running lamps (DRL) are the low beam headlights at a reduced intensity. With the headlight switch in the OFF position, the headlights will either be turned ON or OFF, after an approximate 8 second delay depending on whether daylight or low light conditions are sensed. When the BCM signals the HDM for DRL operation in daylight conditions, the HDM illuminates the low beam headlights at a reduced intensity. The DRL will operate when the ignition switch is in the RUN position, the gear selector is not in the PARK position and the parking brake is released. When these conditions have been met and the ambient light sensor indicates daytime conditions, the DRL will illuminate.

Posted on Apr 15, 2015

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Maybe bad sensor as seen on the following video



And see how he troubleshoot it

Posted on Apr 15, 2015

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SOURCE: replace low beam headlight on 2003 chevy trailblazer

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SOURCE: Change headlight on 2008 Chevy Malibu

Hi there,

Well first you can please follow the below steps -
1. Make sure the ignition key is in the off position and the headlight switch is off. Release the hood latch.
2. Open the hood and locate the top of the headlight assembly where you intend to change the headlight. If a plastic shroud is present, locate the center bolt and remove it with a ratchet and a socket. Lift the shroud out of the way to access the headlight assembly retaining brackets. Some later models may feature a snap-release shroud requiring no tools to remove.
3. Locate the brackets on the top rear of each side of the headlight assembly. Lift them all the way up to align the hole on the bottom of each bracket to the retaining tabs on the assembly.
4. Pull the assembly forward away from the Malibu, but be careful not to scratch the bumper. There are three guide pins that require unseating. The guide pins are what aim the headlights and it may take some wiggling to get the pins from their seats.
5. Support the assembly with one hand and then reach behind the unit with the other. The low beam bulb is on the outside, the high beam bulb is on the inside. You can also replace the parking light if desired. Locate the wire harness connection. Un-clip the wire harness from the back of the bulb socket. Remove the bulb by turning it counterclockwise 1/3 of a turn to align the release tabs of the ring and the assembly. Wiggle the bulb out of the assembly.
6. Place the latex gloves on your hands before opening the package of the replacement bulb. Oil in our skin can damage the glass bulbs of halogen headlights.
7. Carefully insert the new bulb into the ring and then align the tabs of the ring to the tabs of the headlight assembly. Push inward and then twist clockwise 1/3 of a turn to lock the bulb in place. Reattach the wire harness. Rest the headlight assembly inside its cavity and test the operation of the headlight before securing the headlight assembly in place. Be sure to turn the lights and ignition back off.
8. Align the three guide pins to their seats and push the assembly back into them gently until they snap into place. Replace the retaining brackets by pushing them downward to lock into place. Replace shroud, if applicable. Remove the tools and old bulb and close the hood. Post comments for further help or information......

Good Luck!!

Posted on Mar 27, 2010

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