LED headlight upgrade

[Paul Workman]

Originally Posted by XfireZ51

Just a question. Is there really a point to having fogs ON w the HI Beams?

Normally, the power for the fog light comes from the switch on the dash, through the fog light relay coil, then through the high beam filament to ground (and back to the battery via the chassis).

Between the fog light and the high beam filament is another (hot) wire connected to the high beam switch. When that high beam switch is closed, voltage is equal on BOTH sides of the fog light relay coil - NO difference in potential - and current through the relay coil stops and the fog light circuit opens. The path for high beam current comes directly from that high beam switch to the wire connected between the fog relay and the high beam filaments.

The substitution of an LED in place of the high beam filament changes the scheme significantly by increasing the resistance in the circuit significantly.

LEDs draw a tiny fraction of the current of their FILAMENT counterpart. And, their (forward direction) opposition to current flow (i.e., "impedance") is very low; so low that additional resistance or a current regulator is typically installed in series with the LED to allow it to work within its limited (current) parameters.

It is that current limiting additional resistance necessary for the LED that causes a problem for the fog lights. Where the headlight filament resistance may have been less than 10Ω to ground, the resistance in the LED path may be several 100s of ohms!

Solution: either install a (LOAD) resistor to ground with similar impedance as the original filament the LED replaced, OR provide a alternate path to ground for the fog light filament (relay).

LOAD resistor:

Restores the original control and functionality of the lights
Generates significant heat that has to be dissipated, when the high beams are on.
Adds external connections that are exposed to the outside environment

Alternate ground for the fog light relay:

Compared to the OE configuration, it reduces overall current draw when the high beams are ON
Results in the fog lights remaining ON along with the high beams*.