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Chevy Suburban: light switch set to automatic, headlights..faulty

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Chevy Suburban 2001 - with...
Chevy Suburban 2001 - with light switch set to automatic, headlights have started to stay on at full brightness during the day when only the DRLs should be on. Seems the photocell sensing daylight is faulty, or perhaps the associated control ciruitry. What do I need to do to trouble-shoot? Thanks.

Graham
Submitted: 7 years ago.Category: Chevy
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Answered in 4 hours by:
11/2/2010
Chevy Mechanic: Dave Nova, Chevrolet Technician replied 7 years ago
Dave Nova
Dave Nova, Chevrolet Technician
Category: Chevy
Satisfied Customers: 19,452
Experience: ASE Master Certification. GM World Class Certification
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Hello and welcome to justanswer.

 

First thing you will want to do is to make sure the ambient light sensor in the center of the dash (on top) is not covered or obstructed. Then make sure the sensor is in place, you should see a small plastic dome shaped sensor. If you do not see it, then it has fallen into the dash. Let me know what you find there and then we can continue with diagnosis.

Dave Nova
Dave Nova, Chevrolet Technician
Category: Chevy
Satisfied Customers: 19,452
Experience: ASE Master Certification. GM World Class Certification
Verified
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Customer reply replied 7 years ago

Thanks Dave,

 

No sign of an ambient light sensor on top of the dash and there is no hole for it. Should it come up through the vent near the windshield, or where should it be? I think the problem may have started after I cleaned the windshield and dash board with a towel, so it's possible I've pushed something out of place, but I can't see what or where.

Chevy Mechanic: Dave Nova, Chevrolet Technician replied 7 years ago

The ambient light sensor is located up in the center of the defrost grille area.

 

graphic

 

The upper dash panel has to be removed to service this sensor.

 

Remove the end covers and 2 phllips screws on each end of the dash.

 

graphic

 

Remove the passenger side dash vents by prying them out of the dash. Use a flatbladed screwdriver with the tip wrapped in tape to keep from scratching the dash. Remove the screws behind each vent, then reach through the vent opening and depress the retainer tabs that hold the grab handle into the dash. Pull back on the handle while pressing the release tabs.

 

graphic

 

Shift the shifter all the way to Low 1 range and remove the gauge cluster bezel by pulling straight back. Then remove the screws around the perimeter behind the bezel.

 

graphic

 

Then the upper dash panel can be lifted and pulled straight back. Reach underneath to access the ambient light sensor. It fits into the hole in the dash and secures with a 1/4 turn.

 

graphic

 

 

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Customer reply replied 7 years ago

Thanks, XXXXX XXXXX makes it clear where the sensors are. I was looking for one central sensor with clear plastic and was confused by the two black plastic domes which I thought were clips or spigots holding the grille in position. I confirm that both the ambient light sensor and sun load sensor are in position and not covered up. So what's next?

Chevy Mechanic: Dave Nova, Chevrolet Technician replied 7 years ago

You will need to test voltage output from the ambient light sensor. If you have a scan tool like the dealers, you can read the voltage from the scan tool. Since you do not, read the voltage at the BCM connector. You will leave everything plugged in, just use the voltmeter probes to insert into the back side of the BCM connector to read voltage. The BCM is located under the drivers side of the dash, remove the panel under the steering column for access.

 

graphic

 

Measure voltage at the purple connector, pin A4 gray wire, and pin A6 white wire as shown in the diagram below. With the key on and daylight hitting the sensor, you should have below 1.3 volts. Cover the sensor and you should have above 2.2 volts. If these readings are incorrect, replace the ambient light sensor. If the readings are correct, replace the BCM.

 

graphic

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Customer reply replied 7 years ago

Thanks, I'll let you know what I find after checking the voltage later today.

Chevy Mechanic: Dave Nova, Chevrolet Technician replied 7 years ago
That will be great. Thanks for the update.
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