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Headlight Relay Installation, Version 2

K100/75RT Headlight Relay Installation

by Don Eilenberger, NJ-Shore-BMW-Riders,

(Editor’s Note: this method will work on many kinds of bikes, as you build a custom wire loom that plugs directly into the back of the headlight.)

As has often been discussed on the IBMWR mailing list, addition of headlight relays will give higher voltage at the headlight bulb, resulting in higher output, and lengthen the lifetime of the hi-low-flash headlight switch.

This FAQ details my installation of a relay assembly on a K100RT series.

relay diagram
The relay: available from NAPA auto parts dealers, part number “AR266 Accessory Relay”, this is a heavy duty dual relay assembly (made by Mopar/Chrysler).

Addendum: Robert Jenkins found a male 3 prong connector made by Hella, and is available from: Performance Unlimited Motoring Accessories, Phone: 800-333-PUMA,

Addendum#2: Geoff Adams has discovered the following. “The only male 3-prong connector made by Hella is on one end of #170-66490 which is a polarity reversing adapter used for conversion from sealed beam to the 165 mm H4 on some US vehicles.The other end is a female that plugs on to the H4. To use just the male end with wires attached you would have to cut off the female end. Cost for this part is $11.95 each.”

As can be seen in the diagram, this relay assembly appears ideally suited for use as a dual (hi and low beams) headlight relay. It only requires one power connection (terminal #3), and the case functions as the ground connection. It is diode protected (preventing voltage spikes from damaging delicate electronic components on the bike), and can be easily mounted to any handy bolt or screw on the bike.

My intent in making up this assembly was to have one which: looked ‘factory’; was reliable; was easily taken out of circuit with no tools (in the unlikely event of a failure). This relay assembly, along with some parts also purchased at an auto-supply achieved these goals.

Homemade Male Plug

The other components needed to achieve these goals were both male and female headlight plugs and a power feed wire for the relay assembly.

After considerable searching, I determined that a male version of a 3-prong headlight plug was not something available to the general public. Although I’ve seen them in headlight adaptors (Hella has one), I was unable to locate any place to purchase one. So.. I decided to build my own by modifying a female (readily available) headlight socket.

I made up 3 brass pieces from light gauge sheet brass (about 20 gauge), 1/4″ wide x 3/4″ long. I inserted these into a female headlight plug, and ended up with a male I also reversed two of the connectors in this plug – this is described in detail later in the FAQ. The homemade male plug is illustrated in the photo.maleplug.gif

As can be seen, the plug I used was color coded. In searching for a female headlight plug, I purchased several different headlight plugs from various auto-parts stores.

What I found surprised me – many of them were not dimensionally correct to mate cleanly with a headlight or H4 bulb. The pin spacing was incorrect. The only one I found where the spacing was correct was manufactured by Calterm, part number 08521, “Universal Headlight Socket”. It also had the advantage of using color coded wires, making for ease of wiring. I located it at one chain of local auto-parts stores in one of the usual rotating racks of aftermarket electronic parts.

Complete “Modular” Assembly

assemb.gifThe complete assembly was wired so that it was modular, and easily installed or removed.

To remove the relay assembly from the headlight circuit, all that is necessary is to unplug the stock headlight socket from the male plug that powers the relay switch coils, and remove the female plug from the relay to the headlight bulb, replacing it with the stock headlight plug. Time should not exceed 30 seconds, even in total darkness.

The modular assembly as can be seen is wired with two pigtails (about 12″ long), one (the male plug) of which goes to the existing headlight plug, the other connects to the headlight bulb. The single ‘eye’ connector was used to provide the ground for the headlight (female) plug, and was placed under the mounting bolt for the K-RT right side horn. In addition, there is another ‘eye’ (shown in the circuit diagram – next page) which grounds the shell of the relay – providing the necessary ground to complete the control circuit. This eye exits the wire bundle from the male plug near the relay assembly (not easily visible in this photo).



Wiring Diagram

The diagram above shows the wiring necessary. The colors are correct for the plugs used.

NOTE: The two outer (parallel) connectors in the homemade male plug (which plugs into the wiring harness) had to be swapped for the plug to work correctly as shown.

If I was to use a different plug – one with all the same color wires, it would not have been necessary to reverse the positions of the outer connectors on the male plug, but it would be necessary to note the position of the wiring relative to the plug as is shown above.

Connector Details

plugs.gifWhen assembling the wiring for the relay assembly, only one caution must be taken. As can be seen in the photo, I tried to keep the colors for each filament separate, ie, low beam using yellow wiring and high-beam using red wiring.

To achieve this goal – it was necessary to swap two of the connectors inside the homemade male plug assembly. The female spade connectors within the plug can be removed by using a very tiny screwdriver to depress the retaining tang – which is accessible in a small slot from the front of the plug. The connectors to be swapped (and only on the male plug) are the two outermost ones – the center one stays where it is.

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