Installing Headlight Relays

By: Brian Curry

How would I go about installing headlight relays on my K-Bike?

Necessary parts:

Two relays such as for horns. They do not have to be Bosch but they need to have decent current ratings. Horn relays are fine. The contact current capacity needs to be at least 6 amps and 10 is a lot better. I would not use Radio Shack because I think that what you get at an auto parts place would be more appropriate for the service. (Rough and tumble, and sealed.)

Some 14 gauge wire. Tom Coradeschi recommends 12 gauge be used. It can be used, but is more bulky. Rob used 14 gauge. Do not use 16 gauge. You want heavy wire to carry the current without voltage drop. Use the biggest you feel you can run and can get a fuse for. Also, it should be nice and flexible with lots of strands.

Crimp-on electrical connectors and a tool to _properly_ crimp them or good soldering skills.

Distructions: ;)

Use caution around the battery, it may only be 12 volts, but there is a lot of amp capacity there. Shorting it out, can result in ARCS, SPARKS, SMOKE, and EXPLOSIONs. Not good things at all. Use caution!

You are on your own for removing panels, covers, and the fuel tank. You will need to:

Disconnect the battery, by removing the battery negative lead.

Find a place close to the head light to mount the relays. On a K75S I would probably mount them on the instrument pod support in front of the steering head. This is nice and close to the head light. Mount the relays. Do not drill holes in critical high load bearing frame members.

Run a ground wire to one terminal of the relay coils. I would run the ground from the frame ground under the tank. While this ground could be continued to provide a ground for the head light, I would probably run a second separate ground for the headlight. (Connected later)

I would tap the battery hot lead at the starter relay in the electrical box under the rear of the fuel tank. This is a nice protected location. It is bolted. It is not exposed to road grime or battery acid. FUSE THIS LEAD close to the start relay and inside the electrical box!!! Run the lead up to the relays and connect it to one side of the normally open contacts on each relay. (Doesn't matter which terminal is used of the contact set.)

Cut the leads going to the head lamp socket leaving a ~1" pigtail on the socket. You will have a yellow lead, white lead and a brown lead. The wiring harness brown lead is the ground and does not have to be used, or you could use this for the relay ground and not take the relay ground from the frame ground. If you do not use it, tape it so that it does not flop around and contact the wrong thing accidentally. :( Connect the wiring harness yellow lead to one relay's coil terminal. Connect the previously unused terminal of the relay contact to the yellow pigtail on the head lamp socket. Use the 12/14 gauge wire for the contact terminal to pigtail connection. Connect the wiring harness white lead to the other relay coil's unused terminal. Connect the unused relay contact terminal of this relay to the white pigtail on the head lamp socket. Use the 12/14 gauge wire for this. Connect the new 12/14 gauge ground lead from the frame to the headlight socket brown pigtail.

Put a fuse in the positive lead fuse holder!

Smoke testing:

Reconnect the battery negative lead. (Since you have the wire and the connectors, this is a good time to install a second battery negative lead. I would connect a second one from the battery negative terminal to the same point on the frame as you ran the other grounds from.) Watch and look for smoke. :(

Turn on the ignition. Watch and look for smoke. :(

If you have a Euro headlight switch, turn on the head light.

The low beam should come on. Check the hi-low operation.

You should be able to hear the relays pick up and drop out as you operate the switch.

Reinstall the tank, panels, and covers. Take a ride. Enjoy the light. :) :)

If it does not operate properly, use a indicator light, DVM, or VOM to see what is energized and what is not, and what connections are made up and what is not.

If still lost and not working, send e-mail. ;) ;)


 Hi-Lo 
 Switch       Approximately 16-18 gauge wire.
      o---------------------------------------------------------------|
  _/                                                                  |
                                                                      |
      o--------------------------------------------------|            |
                                                         |            |
                                                         |            |
Note: Before the power                                   |            |
      gets to the Hi-Lo                                  |            |
      switch it also goes                                |            |
      through other switches                             |            |
      and wiring.                                        % Low        |
                                                         | Beam       |
                                                         | Fil.       % High
|            |Beam
                                                         |____________| Fil.
|
                                                        ---   Return/ground
                                                         -
                                                         '

                              ORIGINAL CIRCUIT




      Battery/starter
       relay terminal    Fuse   12/14 gauge wire all of this part
                                of the circuit.
           +  -----------~~~~-------------------------|------------|
                                                      |            |
  Switch                                              |            |
      o-------------------|                           |            |
  _/                      |                           |            |
                          |                           |            |
      o----------|        |                           |            |
                 |        |                           |            |
                 |        |                           |            |
                 |        |                           |            |
                 |        |                           |            |
                 |       / \ Relay   Mech. Conn.     _|_  Relay    |
                 |       \ /  ---------------------  ___  Contact  |
                 |        |                           |            |
                 |        |                           |            |
                / \ Relay |  Mechanical Connection    |           _|_ Relay
                \ / ----- |-------------------------- | --------  ___ Contact
                 |        |                           |            |
                 |________|                           |            |
                 |                                    |            |
                 |                                    % Low        |
                ---                                   | Beam       |
                 -                                    | Fil.       % High
                 '                                    |            | Beam
                                                      |____________| Fil.
                                                      |
                                                     ---   Return/ground
                                                      -
                                                      '

                              REVISED CIRCUIT
Good luck!

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Last Update: 05 October 2007