What causes a dim headlight?
Most people think a dim headlight is caused by a problem on the power side of the circuit where the dim headlight isn’t getting enough power. That’s almost never the cause. The power connectors for headlights are usually well protected against the elements. The connectors often have a silicone seal to keep them dry and corrosion-free. But the ground connection is just the opposite. Carmakers use the body as the return path for the headlight circuit. The ground wire is usually bolted to a metal body component where it is exposed to water and road salt. The water and road salt make that connection a sitting duck for corrosion, which causes increased resistance and reduced electrical flow, which causes a dim headlight.
Melted headlight connector is the exception to the rule
Like I said above, a dim headlight is rarely caused by problems on the power side of the circuit. But lately, some car makers have been having issues with melted headlight connectors and that can cause a dim headlight. The melting is caused by heat from the headlight bulb or a wire gauge that’s too small for the electrical load. So start diagnosing a dim headlight condition by checking the condition of the electrical connector at the bulb. If you find any sign of discoloration or melting, you must replace the connector.
Some companies make ceramic headlight bulb connectors that eliminate the problem of melting caused by overheating. See these posts for more information.
See this post for Replacement headlight connector for H11 bulbs
See this post for replacement headlight connector for H7 bulbs
Why is the dim headlight only on one side?
Carmakers can choose to ground both headlights at a single point, but that means they have to use extra wire to run the ground from one headlight to the other, and it also requires a splice-which cost more money during manufacturing. So it’s not unusual to find a separate ground point for each headlight.
How to fix a dim headlight?
First, check the connector at each bulb. Excessive resistance in the circuit can cause overheating at the headlight which can melt or distort the plastic connector. If you see evidence of melting, you’ll have to fix the underlying problem and replace the headlight connector. Most auto parts stores sell headlight “pigtails” which you splice into the existing wiring harness and seal with heat shrinkable tubing.
Sometimes the silicone seal can fail and cause the terminal to corrode. If you see corrosion, use electrical contact cleaner and a contact file to remove the corrosion. If the corrosion was really bad, you’re best off replacing the connector with a new one. Just search for a headlight pigtail for your vehicle at any auto parts supplier. If you clean the terminals, apply a light film of dielectric grease to prevent future corrosion.
Next, locate the grounding connection for the dim headlight. If
you have a dual filament headlight, you’ll find three wires in the connector. One connector will be for low beams, another for high beams and a third for a common ground wire. That’s the wire you want to trace.
Fix a corroded ground connection
Remove the nut or bolt from the ring terminal. Use sandpaper or a wire brush to remove all traces of rust from the sheet metal and both sides of the ring terminal. Use a new bolt or clean the rust from the old bolt. Apply a light film of dielectric grease to the sheet metal, ring terminal and bolt. That’ll slow down future corrosion. The reattach the ring terminal to the sheet metal.
Still have a dim headlight?
Now it’s time to check if your vehicle uses headlight relays. If so, it’s possible the electrical contacts in the headlight relay have pitted and are now introducing high resistance to the circuit. The easiest way to check for a bad relay is to swap the relay with another one with the same part number. Or, swap the left and right headlight relays.
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©, 2017 Rick Muscoplat