Headlamp: Description and Operation

The low beam headlamps may be turned on two different ways. First, when the headlamp switch is placed in the ON position. Second, with the headlamp switch placed in the OFF position and the ignition in RUN or CRANK position, during twilight sentinel operation in low light conditions. If the headlight switch is left in the ON position, the headlights are turned off after the ignition switch is turned to the OFF position. If however the headlights are turned on after the ignition switch has been turned OFF, the headlights will remain on until turned off or the battery runs dead. With the headlights in the low beam position, the high beams may be momentarily turned on or flashed with the dimmer switch handle. The flash-to-pass feature is accomplished by pulling the dimmer switch handle toward the driver. The high beams can only turned on when the low beams are operating. The high beams may be switched on by pushing the dimmer switch handle away from the driver until the dimmer switch is activated.

High Beams
The high beam headlamps are illuminated when all of the following relays are energized by the BCM.
1. Daytime running lamps (DRL) relay K131
2. Headlamp relay, LH (high beam) K73
3. Headlamp relay, RH (high beam) K132

The daytime running lamp relay K131 has battery voltage supplied at all times to the switch side of the relay and the coil side of the relay. When the daytime running lamp relay K131 is pulled low by the BCM the relay energizes, the relay's internal switch closes and battery voltage is supplied to the LH high beam lamp. The LH headlamp relay K73 has ground supplied at all times to the switch side of the relay and battery voltage supplied at all times to the coil side of the relay. When the LH headlamp Relay K73 is pulled low by the BCM the relay energizes, the relay's internal switch closes and ground is supplied to the LH headlamp and it illuminates.

The RH headlamp relay K132 has battery voltage supplied at all times to the switch side of the relay and the coil side of the relay. When the RH headlamp relay K132 is pulled low by the BCM the relay energizes, the relay internal switch closes and battery voltage is supplied to the RH headlamp. The RH headlamp is grounded at all times by G103 and the RH headlamp illuminates.

The high beam headlamps can only be turned on when the low beam headlamps are on. The BCM will energize the RH and LH headlamp relays and the daytime running lamp relay when the BCM receives battery voltage from the high beam switch. The LO BEAM Fuse supplies voltage to the turn signal switch only when the low beam headlamps are on. When the turn signal lever is pushed forward, the turn signal switch sends battery voltage to the BCM. When the BCM receives battery voltage from the turn signal switch, it then pulls low the daytime running lamp relay K131 and the LH and RH headlamp relays. The coil side of the relays have battery voltage supplied to them at all times and the relays energize.

Low Beam
When the headlamp switch is in the HEAD position it sends a ground signal to the BCM. The BCM receives the ground signal from the headlamp switch and then sends a ground signal to relay coil of the low beam relay. The headlamp switch also sends a redundant ground signal to relay coil of the low beam relay.

The low beam headlamps are controlled by the low beam relay (K131). The supply voltage for the coil side of the low beam relay is supplied from the BCM exterior lamp power circuit. The low beam relay coil is controlled by a ground from the BCM or the headlamp switch. When the BCM or the headlamp switch grounds relay coil of the low beam relay, the low beam relay coil is energized and the low beam relay internal switch closes.

The low beam relay has battery voltage supplied at all times to the switch side of the relay. When the low beam relay is energized, the internal switch closes and battery voltage is supplied to fuse #22 and fuse #31. Fuse #22 supplies battery voltage to the LH low beam headlamp and fuse #31 supplies battery voltage to the RH low beam headlamp. The LH and RH low beam headlamps are permanently grounded to G103. When battery voltage is applied to the headlamp supply voltage circuit of the LH and RH low beam headlamps, both of the headlamps illuminate.

Headlamps W/High Intensity Discharge (HID)
HID headlamps are optional. Normal operation of the HID system starts at the ballast. The headlamp ballast input connector receives the headlamp low beam supply voltage, which would normally be used to directly supply the low beam headlamp Each ballast has a separate ground. As soon as the input power is applied, the ballast will charge for a brief time, depending on the input voltage level. The ballast is then able to charge the two outputs to the voltage level needed by the starter to strike, or start, the lamp. HID headlamps do not have filaments like traditional bulbs. Instead, the starter uses a high-voltage transformer to convert the input power into a higher voltage. The increased voltage is used in order to create an arc between the electrodes in the bulb. The creation of this arc begins the start up process.

Run Up Of The Lamp (W/HID)
Each ballast requires higher amperage in order to ensure normal startup and run up of the lamp. Run up is the term used to describe the extra power level given to the bulb. The input current during the steady state operation is lower that the start up amperage. After the lamp receives the strike from the starter and the arc is established, the ballast uses its operating voltage in order to provide the run up power needed in order to keep the lamp on. The lamp rapidly increases in intensity from a dim glow to a very high-intensity, bright light called a steady state. Within a few seconds of the arc being established in the bulb, the majority of steady state is complete. 100% of the steady state is completed shortly there after. A high watt power level is necessary in order to bring the lamp to a steady state in such a short period of time. The high watt power level allows the lamp to meet the SAE light vs. time specification.

When To Change The HID Bulb
Bulb failure, end of life occurs when the bulb gets old and becomes unstable. The bulb may begin shutting itself off sporadically and unpredictably at first, perhaps only once during a 24-hour period. When the bulb begins shutting itself off occasionally, the ballast will automatically turn the bulb back on again within 0.5 seconds. The ballast will re-strike the bulb so quickly that the bulb may not appear to have shut off. As the bulb ages, the bulb may begin to shut off more frequently, eventually over 30 times per minute. When the bulb begins to shut off more frequently, the ballast receives excessive, repetitive current input. Repetitive and excessive restarts or re-strikes, without time for the ballast to cool down, will permanently damage the ballast. As a safeguard, when repetitive re-strikes are detected, the ballast will not attempt to re-strike the lamp. The ballast then shuts down and the bulb goes out.

The following symptoms are the noticeable signs of bulb failure:
^ A flickering light, caused in the early stages of bulb failure
^ The lights go out, caused when the ballast detects excessive, repetitive bulb re-strike
^ Color change - the lamp may change to a dim pink glow

Input power to the ballast must be terminated in order to reset the ballast's fault circuitry. In order to terminate the input power to the ballast, turn the lights off and back on again. Turning the lights off and back on again resets all of the fault circuitry within the ballast until the next occurrence of excessive, repetitive bulb re-strikes. When excessive, repetitive bulb re-strikes occur, replace the starter/arc tube assembly. The ballast will begin the start-up process when the starter/arc tube assembly is replaced. Repeatedly resetting the input power can overheat the internal components and cause permanent damage to the ballast. Allow a few minutes of cool-down time in between reset attempts.

Bulb failures are often sporadic at first, and difficult to repeat. Technicians can identify bulb failure by observing if the problem gets progressively worse over the next 100 hours of operation.

Light Color (W/HID)
White light has a different color rating than regular headlamps. The range of white light that is acceptable is broad when compared to halogens. Therefore, some variation in headlight coloring between the right and left headlamp will be normal. One HID at the end of the normal range may appear considerably different in color from one at the other end of the range. Difference in color is normal. Replace the arc tube only if the arc tube is determined to be at the bulb failure stage.

Flash to Pass
The flash to pass feature will operate the high beam headlamps whether the low beam headlamps are ON or OFF. The flash to pass switch has battery voltage supplied to it at all times. Pulling the turn signal lever toward the driver will close the flash to pass switch, sending battery voltage to the BCM. When the BCM receives the flash to pass signal it will energize the RH and LH headlamp relays and the daytime running lamp relay, turning ON the high beam headlamps.