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August 25, 2013

Headlight remains stubbornly dim

I just replaced the left headlight cartridge on my car and it’s still dim. I’m sure it was the correct bulb. What else can I do to fix this?

Not knowing the type of car you have is a mixed blessing. Rather than being able to provide some specific testing information, I get a chance to approach this in a generic manner, which should be of interest to a wider audience, for a variety of problems.

When an electrical device doesn’t work well, or at all, there are three general fault possibilities besides the device itself being faulty. The most likely cause is a poor connection somewhere in the circuit connecting the device to its power supply, as is likely the case with your headlight. If the headlight didn’t work at all, an open circuit — such as a broken wire, unplugged connector, failed fuse or bulb — could be the cause. The least likely cause of no operation is a short circuit. This results in electricity taking an unintended path into or from the circuit, possibly blowing a fuse or causing unwanted operation.

Since you have already renewed the bulb, and bulbs generally work or they don’t, we’ll assume it’s OK. Let’s check all lights on the vehicle exterior for proper operation. For example, if both left and right low beams were dim but the high beams were OK, we’d know the fault was in a common part of the low beam positive circuit, perhaps within the high/low switch or relay, or in a common connector. If all headlights were found to be dim, this places the fault closer to the trunk of the tree, possibly within the headlight switch or a main power connector.

The third possibility, and likely your situation, is some or all lights in that corner of the car also function poorly. This means the ground connection, a screw attaching wires to the car body that is often shared by the group of lights, is loose or dirty. Since most of the lights in that corner have differing positive circuits leading to them, these would not be of concern.

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