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carhelpguy, ASE Certified Tech.
Category: Chevy
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Experience:  Over 15 years of experience working on GM, Ford, Chrysler, Toyota, and Honda. Diesel and Electrical specialist.
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2007 Cobalt: auto mode, the headlights and parking lights are..daytime

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2007 Cobalt: If lights are in auto mode, the headlights and parking lights are always on and the dash/radio is dimmed as if it were night. If auto lights are turned off then dash/radio lights are bright like they should be in daytime. Light sensor on dash checks ok. Wires to light sensor read about 5V which I think is the correct reference voltage. Daytime Running Lights (DRL) fuse checks ok. Is there another relay I should be checking?  What other components are in this circuit that I can check?  It's really hard to read the dash and radio readouts when the console lights are dimmed during the day; I need to fix this.
Thank you for your question, I hope I can help.

My name is XXXXX XXXXX I will be assisting you with your question.

Taking a look at your concern I have a couple of questions for you. When you say that the sensor checks out fine, how did you check the sensor?

With the sensor disconnected, and testing between the 2 terminals, having a 5 volt reading is good. What you will want to do is plug the sensor in, backprobe the LT/GN with Black tracer wire in the connector and test the voltage with the sensor covered and with a flashlight on it.

The voltage should change between 1.7 volts and 4.5 volts. If not then the sensor is faulty and will have to be replaced.

Let me know if this helps.
Customer: replied 5 years ago.
I rechecked the light sensor. With no sensor attached, the connector reads 4.97V, with the sensor attached and in full sun the connection reads 0.67V, with the sensor attached and in darkness the connection reads 4.67V.

Everything checks good at the sensor but the lights remain on full headlights and parking lights with the dash dimmed.
Thanks for the update, this tells me that the BCM is seeing the input from the light sensor. Given that the problem is either the BCM is faulty or the BCM thinks the headlights are turned on manually.

Do you have access to a scan tool to read BCM data? If you do check the day/night parameter of the light sensor when covering the sensor, and then shining a light on it. Also check the headlight switch parameter to see if it reads on all the time.

If you do not have access to a scanner then gain access to the headlight switch connector and test circuit 103 (white wire) in terminal 2 of connector C1. Test the voltage with the switch plugged in the same way you tested the sensor. With the headlight switch in the off position there should be 12 volts on that circuit. With the switch turned on it should drop to 0 volts.

Another thing to look for is to see how long the headlights stay on after you turn the engine off and remove the key. If they stay on for several minutes before turning off then there is a problem with either the switch or that circuit mentioned above. The voltage tests will also tell you if there is a problem there.

If all circuits are testing like they should then the problem is most likely a BCM.
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Customer: replied 5 years ago.

You've given me some great information and it led me to working on the BCM connections which apparently fixed the problem. It would be nice to know more about the "DRL ambient light sensor low reference" but you've certainly done your part.

More Details:

I have an older/simpler scan tool that didn't give me those parameters so I checked the BCM harness. Near the end of trying all of the switch combinations, it became evident that the auto lights and DRL were once again working.

It could be that removing and reseating the C1 and C2 connectors had something to do with fixing everything. Maybe it was nothing more than a poor connection.

This is what I found:

The "headlamp switch headlamp on signal" reads as expected. When auto headlights are off, it's 12V in auto and parking position and it's 0V when in headlights position. (With auto headlights on, and under bright light for 15-20sec, the headlamps went off and the voltage went from 0V to 12V.)

The "headlamp switch signal" is 0V in headlamp position, 12V in parking position. If auto lights are on it's 0V in darkness and 12V after 15-20 sec in bright light; if auto lights are off it's 12V.

The "DRL ambient light sensor signal" appears to match what I read at the sensor.

The one that confuses me is the "DRL ambient light sensor low reference" pin. It reads only 0.01V. Shouldn't that read higher?

I am glad to hear the concern has been repaired. It is very possible that the connection was loose causing the concern.

As for the low reference pin of the sensor. All the low reference circuit does is ground the light sensor. Whenever you run across a circuit labeled low reference all that means is a ground circuit that runs through a module, but basically they are the same as if they run straight to the frame.

The way the circuit works is the sensor will change resistance internally, dropping the 5 volt reference signal to a certain voltage telling the module what it is looking for.

A special note about low reference. Never test the resistance to ground of a low reference with the key on or the module powered up as it will always give you false high resistance readings. (most flow charts have you test the resistance but do not tell you to have the module powered down before you do).

I am glad I had the chance to help you with this concern. If you have any more problems do not hesitate to let us know. Have a great day.
Customer: replied 5 years ago.
Thanks for the explanation of the low reference pin and the proper way to test its connection to ground. I probably would have tested it the wrong way.
Not a problem, I am glad to help out anytime. Thank you for using Just Answer and have a great day.

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