HID or High Intensity Discharge headlamps work
quite differently than the traditional sealed bulb systems. 
Where as in a sealed bulb systems battery voltage is
supplied to the bulb along with a ground and in turn
completes the circuit and the bulb lights.  In the HID
system high voltage is used and in some cases it's a high
voltage AC current.  These high current circuits are
created through the use of a ballast, this ballast
provides the high current to create the arc between
the electrodes in the bulb.

When the request for headlamps is first signaled by the operator a high voltage spike or commonly referred to as an arc is sent to the bulb by way of the starter (usually part of the ballast or can be part of the bulb itself,)  After the bulb receives that initial slam of current an arc is established in the bulb.  The ballast then uses the operating voltage to create a run up power level to bring the light to its full intensity.  The glow will start out dim (in most cases) and soon reach the “steady state” the required voltage level to produce the correct intensity.  This usually only takes a few seconds for this to all happen. 

Signs to change the bulb

As the bulb ages it will become unstable, usually very erratic, and not often just “off”,  Intensity may change a bit but it might not be that easy to spot the difference between a perfectly working bulb and a lamp that isn’t working so well.  Mainly because it’s not uncommon for the actual color or intensity from one side of the car to the other to be different,  One of the big problems in diagnosing these somewhat unpredictable bulb “on/off” conditions is because of the design features and the high voltage used.  When the bulb shuts off on its own the ballast will automatically try to turn the bulb back on within .5 of a second.  So quickly that you’ll not even see the bulb dim or go out between its cycles. As the age of the bulb increases and the frequency of the bulb failing to stay on the ballast can repeat the restart and re-arcing of the lamp as many as 30 times per minute.

Repetitive ballast restarts without enough time between restarts will permanently damage the ballast.  As a kind of built in safety feature when so many restarts are occurring the ballast will go into a state of hibernation (so to speak) and not turn the bulb on at all. That's not to say the damage isn't already done, but if caught at the right time the ballast won't need replaced.

Some of the key signs that should give you a clue to the lamps conditon are:

Flickering light (Usually the first sign of lamp failure)

Age of the vehicle and how often the headlamps are used. (Varies from driver to driver.)

Lamp goes out, then back on (This is when the ballast is detecting excessive and repetitive bulb re-arcing needed.)

Color change (Harder to tell because of the differences in intensity but, a dim pinkish color is a good indicator of the condition of the lamp)

Turning off the lights with the switch resets all the ballast fault circuits back to factory specs.  Which is usually how the mechanic will find the car in the service bay, lights off, working normally when their turned on and no signs of a problem.  (This is when information from the customer will be the utmost help in diagnosing the problem.  Sitting in front of the car on the mechanics stool watching headlights for the next hour or so doesn’t sound like the best use of your time.) (In one descriptive text on diagnosing the HID intermittent problems it was mentioned to observe the problem over the next 100 hours of operation to insure proper diagnostic results.  (Seriously idiotic if you ask me)
If the ballast has been repeatedly sending bulb strikes and, it has been quite some time before the customer got the car into the shop, the odds are it will need to be replaced as well.

But, before replacing the ballast start with replacing the starter/arc tube (the lamp) assembly.  If all is well, the next time the switch is turned on the ballast will charge up and send the arc signal to the starter and hopefully the lamp will light.  If the bulb fails to light or flickers shut it back off.  Repeated resetting of the input power can overheat the internal components or, more than likely... the damage is already done.  When any internal damage is done to the ballast there is no repair for it, just replace it the same time you replace the starter/arc tube.  Keep in mind the variation in headlamp color between the right and left or even to the next car can be quite dramatic.  Although, dramatic in changes of color the intensity is what matters.  (Most HID lamps may appear considerably different in color from one to another near or at the end of the range of usefulness.)

As with many of these systems there are several varied diagnostic procedures (to many to cover in one article) However, these steps provided will give you some clues as to what to look out for when asked to diagnose a HID system.  Anytime you can understand a system better, the better prepared you’ll be to diagnose it. 

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