Auto headlights

[DougG]

I always leave my headlight switch on automatic, as it seems to work quite well that way. But today I was really wondering why it turned my lights on. It was one of those really cold, crisp winter days: -2F, clear blue sky, brilliant sun. It couldn't have been much brighter. But when I pulled into my parking spot I could see by the reflection that my headlights were on!

Anyone figured out what sensors or logic they use to decide if the lights should be on?

Doug G

[markl53]

I've only had my 330i for about a month, but it seems my lights are coming on too soon, when it's still pretty bright out. The manual states that the dealer can adjust the sensitivity and I might ask them to do that after I do a bit more testing. Was this in the morning? It could be there's a "focused" sensor that didn't see the sunlight. On my C240, I used to notice in the late afternoon that even though the lights came on, at times what seemed like bright sun lower in the sky wasn't enough to turn the lights back off.

[LeucX3]

I wonder when Lexus or Infiniti will come up with a feature that turns the lights on or off depending on time of day. Now that would be really cool. But then you'd still need a light sensor if it gets dark during the day. Lord knows BMW won't introduce something like that.

[LDV330i]

My car gets confused when I go under a bridge or an underpass early in the morning or late in the afternoon when it turns on the headlights and then refuses to turn them off. There appears to enough bright sunlight but the car thinks otherwise. :dunno:

[rpeeples]

I always leave my headlight switch on automatic, as it seems to work quite well that way. But today I was really wondering why it turned my lights on. It was one of those really cold, crisp winter days: -2F, clear blue sky, brilliant sun. It couldn't have been much brighter. But when I pulled into my parking spot I could see by the reflection that my headlights were on!

Anyone figured out what sensors or logic they use to decide if the lights should be on?

Doug G

I have this happen too. Clue is "clear blue sky" (read: dark blue sky). My understanding is, the auto-headlight system uses two sensors, one focused straight ahead, and one wide-angle focused straight up overhead (both located in the rearview mirror assembly, above the "clown nose"l. The computer analyzes these two sensors to determine when to turn on the lights. If the sky is too dark - especially on a brilliant clear day with no haze, and the sun low in the sky as it is in winter - the lights will come on.

[DougG]

I have this happen too. Clue is "clear blue sky" (read: dark blue sky). My understanding is, the auto-headlight system uses two sensors, one focused straight ahead, and one wide-angle focused straight up overhead (both located in the rearview mirror assembly, above the "clown nose"l. The computer analyzes these two sensors to determine when to turn on the lights. If the sky is too dark - especially on a brilliant clear day with no haze, and the sun low in the sky as it is in winter - the lights will come on.

An excellent analysis that I'm inclined to agree with, especially coming from someone in the same latitude as me that knows this type of weather well!

DougG in Detroit

[yamato]

I always leave my headlight switch on automatic, as it seems to work quite well that way. But today I was really wondering why it turned my lights on. It was one of those really cold, crisp winter days: -2F, clear blue sky, brilliant sun. It couldn't have been much brighter. But when I pulled into my parking spot I could see by the reflection that my headlights were on!

Anyone figured out what sensors or logic they use to decide if the lights should be on?

Doug G

The exact same thing happened to me this morning. I had to turn the lights off manually. I just assumed the sensors do not work reliably in cold weather.

[jeffreyslc]

I have this happen too. Clue is "clear blue sky" (read: dark blue sky). My understanding is, the auto-headlight system uses two sensors, one focused straight ahead, and one wide-angle focused straight up overhead (both located in the rearview mirror assembly, above the "clown nose"l. The computer analyzes these two sensors to determine when to turn on the lights. If the sky is too dark - especially on a brilliant clear day with no haze, and the sun low in the sky as it is in winter - the lights will come on.
They turn on at dusk (e.g. sun still up, but low on horizon) so cars blinded by the sun can see you better. So the low winter sun is messing it up (actually it is still doing its job).

[pedersenkl]

Same thing is happening on my 2005 330cic. Its just starting to get cold here this week and the last two days both to and from work my headlights have been on when its bright and sunny outside. Really annoying since it seems they should be off. I'm thinking it could be one of a few items:

1. Winter sun
2. Cold weather
3. Sensor problem or location
-Anyone have a picture of where the light sensors are?
-Can we test them with a flashlight to see if the headlights turn off?

I might just have to live with it, since they are probably working as expected.

[LDV330i]

Same thing is happening on my 2005 330cic. Its just starting to get cold here this week and the last two days both to and from work my headlights have been on when its bright and sunny outside. Really annoying since it seems they should be off. I'm thinking it could be one of a few items:

1. Winter sun
2. Cold weather
3. Sensor problem or location
-Anyone have a picture of where the light sensors are?
-Can we test them with a flashlight to see if the headlights turn off?

I might just have to live with it, since they are probably working as expected.The senor read lighting levels, ie. foot-candles. The human eyes is very capable of making adjustments to compensate for the lower light level that we are not readily aware how little light there is. A good example is when you camera decides it need to use flash inside when it may seem brightly lit with lights but in reality the lighting levels are nothing compare to Noon outside.

The sensor is located in the upper part of your windshield behind the rear view mirror.

A flashlight has very low light output that is not going to trick the sensor.

[jeffreyslc]

Same thing is happening on my 2005 330cic. Its just starting to get cold here this week and the last two days both to and from work my headlights have been on when its bright and sunny outside. Really annoying since it seems they should be off. I'm thinking it could be one of a few items:

1. Winter sun
2. Cold weather
3. Sensor problem or location
-Anyone have a picture of where the light sensors are?
-Can we test them with a flashlight to see if the headlights turn off?

I might just have to live with it, since they are probably working as expected.
Was the sun somewhat low on the horizon during your commute? If so, that will cause the auto function to turn your headlights on, so it is normal. Makes you more visible to those blinded by the sun low on the horizon. Even if you use the manual method, you should turn your headlights on when the sun is low for safety.

[FenPhen]

I don't understand why so many people are bothered when an automatic headlight is on when it's not completely dark out. It's automatic. As long as it does turn off at some point, just set it and forget it. :dunno:

As others have said, the automatic headlight mechanism uses two fixed sensors and some logic to determine when the lights should come on. If the low-angle light is much brighter than the high-angle light, the headlights will come on to improve your visibility to other drivers.

The light threshold to turn on the headlights is lower than the threshold to turn off the headlights, just like a thermostat. If it's dusk out or the sun is low, and the headlights aren't on, but you go under a bridge momentarily that triggers the lights on, it's possible for the lights to stay on when you come out from under the bridge. Having the activate and deactivate thresholds at different levels keeps the headlights from unnecessarily flickering. Combine that with a short delay timer, and that keeps the headlights from going nuts if you're say driving under a tree canopy.

To the original poster, did you rule out the possibility that the reflection you saw was daytime running lights?

[Bill97Z]

Mine go on and off a lot too.....mostly on bright sunny days when i am driving on the highway and the trees shade the sun so I am in the shade......they go on.....and then once I am out of the shade they go off.

In comparison the auto lights on my old 97 z28 worked perfectly and turned on just at the right time......but there was no manual override feature....