Global Machinery Company Sierra 1999 - Manual (Page 124)

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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 and your ignition
is in OFF, LOCK or ACCESSORY. To disable the
chime, rotate the thumbwheel all the way down. 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 come on when:
D the ignition is on,
D the headlamp switch is in automatic headlamp mode,
D the automatic transmission is not in PARK (P),
D the light sensor determines it is daytime and
D the parking brake is released.
When the DRL are on, only your DRL lamps 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 automatic headlamp
system will switch from DRL to the headlamps or the
last chosen headlamp setting that was used.
When you turn the headlamp switch off, the headlamps
will go off, and your DRL lamps will illuminate,
provided it is not dark outside.