Relay Kits

 J West Website

Picture from J West Website

When someone buys or has me restore a pair of H1’s or H4’s I always recommend the installation of a relay kit and I’m frequently asked why. There are two reasons to use a relay kit for H1 or H4 headlamps (and other lights with high draws, such as hood mounted Cibies): (1) Assure brightest possible lights (2) Avoid the possibility of overheating the headlight switch

The factory wiring for headlights is of course through the headlight switch (surprise!). The switch is adequate for handling the current delivered to standard headlights. However, high wattage bulbs used by H1’s and H4’s create a higher draw which creates high resistance and heat in the switch. Additionally, the combination of the long distance between the switch and the headlights and the relatively small wire used by the factory also creates high resistance. This results in a negative impact on voltage delivered and creates heat, which in the extreme can cause the headlight switch to burn out.

Voltage drop can have a big impact on the brightness of the headlights also as the effect on decrease in bulb brightness is exponential – a 15% drop in voltage results in a 46% drop in lumens:

  • Early Porsche voltage: 12.8v
  • Assume bulb of brightness: 100 lumens
  • Assume voltage drop of: 15%
  • Delivered voltage: 10.9
  • Drop in lumens (factor is 3.1x): 46%

15% voltage drop decreases bulb brightness from 100 lumens to 54 lumens



Relays help to reduce resistance in two ways: (1) They can be mounted closer to the headlamp so a shorter run of wiring can be used (and while you’re at it you can use a heavier wire), and (2) They become the new – heavier duty – switch eliminating running current through the headlight switch which is then used to switch the relays on and off..

Typical Wiring Diagram

With relays you can run the power lead directly from the battery to the headlights, avoiding the headlight switch. The switch is also wired to the relay and switches it on and off. With the battery in the front you can drastically reduce the length of wiring needed by locating the relays close to the headlights.

wiringSource: HarryD, Portland – Pelican Parts Forum, 2009

You can make up a relay kit yourself with inexpensive components, or buy one from one of many providers. The kit I use and recommend is offered by J West. It’s well made, has easy to follow instructions and at $32.00 is inexpensively priced.

H1’s are real flame throwers when set up properly – set ’em up right and keep ’em bright.

flying a© 2015 ~ Audette Collection ~ Bend, Oregon
Concours Restoration of SWB Headlights & Turn Signals and H1’s ~ Email

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