Many classic car owners deal with “Voltage drop” in their electrical systems. This means that while there maybe be 13+ Volts at the alternator or battery terminal, the number drops when measured at the headlamp plug. Voltage drops can be caused by a number of things, such as corroded connectors, weak ground, etc.
The factory harnesses also were excessively long and undersized, often running power into the dash, through the headlight switch, and then back out to the headlamps. This distance creates an increase in resistance while also increasing the odds for a bad or corroded connection.
Voltage drop can severely impair headlamp output. The relationship between voltage drop and light output is not linear, it’s exponential. For example, when voltage drops to 90 percent, bulb output drops to just 67 percent of what it should be. Let’s compare what the lumen output would be in that situation: