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.
To find a suspected bad electrical connection, a simple voltage drop test is a quick and conclusive way to do it. You’ll need a digital multimeter; models costing less than $25 work fine. Check the available voltage at the battery with the intended device turned on and the engine turned off. This is a baseline of what voltage is being sent to the device — up to 1 volt less than this delivered to the light is OK.
Next, T-pins are slipped in next to the wires at the back side of the connector at the bulb, touching inner terminals. Another measurement is taken under the same conditions: device on, engine off, and don’t unplug anything.
The difference of the two measurements tells us how well the circuit delivered voltage to the device. A difference much above 1 volt means trouble in the circuit. If good voltage is delivered to a poorly functioning device, this tells us the device has problems.
Next, the multimeter leads can be placed across any section of the circuit, to be used as a difference meter to further narrow down the source of electrical resistance. Electrical resistance impedes voltage, causing heat, which may also be noticeable. Published wiring diagrams and connector locations will likely be needed to repeat these test procedures across more localized areas, zooming in for the specific fault location.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at firstname.lastname@example.org; he cannot make personal replies.