K100/75RT Headlight Relay Installation

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

I can be reached at: deilenberger@monmouth.com

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 diagramThe 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...

It's made by Hella, and is available from:

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."

Performance Unlimited Motoring Accessories
Phone: 800-333-PUMA

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
(Page 2)
Complete Modular Assembly
(Page 3)
Wiring Diagram
(Page 4)
Connector Details
(Page 5)
General Wiring, Mounting, Power and Disclaimer
(Page 6)