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Lights On/Lights Off

Drivers side daytime light comes on(auto/man) if the sun is out, but not when it’s overcast or dark. Is it the bulb or some kind of computer switch or module?

Might help to tell us what year Blazer you have.

2004 4dr V6

Do you mean the Daytime Running Lights (high beams 50% power, no marker or tail lights) come on when the sun is out? If so, this is normal. When it’s overcast or dark, the headlights, marker, and tail lights should come on.

This is how it works on my 2000 Blazer. Is there anything on the dash blocking the sensor? The sensor on my Blazer is on the left front dash speaker grille.

Here is an excerpt from the 2000 Blazer Owner’s Manual
Automatic Headlamp System
When it is dark enough outside, your automatic headlamp system will turn on your headlamps at the normal brightness along with other lamps such as the taillamps, sidemarker, parking lamps and the instrument panel lights. The radio lights will also be dim.

Your vehicle is equipped with a light sensor on the top of the instrument panel under the radio speaker grill. Be sure it is not covered or the system will be on
whenever the ignition is on. The system may also turn on your headlamps when
driving through a parking garage, heavy overcast weather or a tunnel. This is normal.
There is a delay in the transition between the daytime and nighttime operation of the

Daytime Running Lamps
(DRL) and the automatic headlamp systems so that driving under bridges or bright overhead street lights does not affect the system. The DRL and automatic headlamp system will only be affected when the light sensor sees a change in lighting lasting longer than the delay.

To idle your vehicle with the automatic headlamp system off, set the parking brake while the ignition is off. Then start your vehicle. The automatic headlamp system will stay off until you release the parking brake. As with any vehicle, you should turn on the regular headlamp system when you need it.

Lamps On Reminder
A reminder chime will sound when your headlamps or parking lamps are manually turned on, the driver?s door is opened and your ignition is in OFF, LOCK or ACCESSORY. To turn the chime off, turn the knob all the way to the left or turn the instrument panel dimmer
down to the fully dimmed position. In the automatic mode, the headlamps turn off once the ignition key is in OFF.

Daytime Running Lamps
Daytime Running Lamps (DRL) can make it easier for others to see the front of your vehicle during the day. DRL can be helpful in many different driving conditions, but they can be especially helpful in the short periods after dawn and before sunset.

The DRL system will make your headlamps come on at reduced brightness when:
the ignition is on,
the headlamp switch is in automatic headlamp mode,
the sensor detects daytime light,
an automatic transmission is not in PARK §, and
the parking brake is released.

When the DRL are on, only your headlamps will be on. The taillamps, sidemarker and other lamps won?t be on. The instrument panel won?t be lit up either. When it begins to get dark, the headlamps will automatically switch from DRL to the regular headlamps.

The DRL system on some vehicles may turn off temporarily while the turn signals are activated. To idle an automatic transmission vehicle with the DRL off, put the transmission in PARK §. To idle a manual transmission vehicle with the DRL off, set the parking brake. The DRL will stay off until you shift out of PARK § or release the parking brake. As with any vehicle, you should turn on the regular headlamp system when you need it.

The drivers side only works after the sun come out. It does not work at night. Finally at night even using the manual switch the drivers side will not come on.

Try changing the bulb? 6 years is about right for a new one.