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Headlight Relay Installation, Version 1

Headlight Relay Installation

By Robert Laposta
June 2001

(Editor’s Note: The first article explains this method in general, the second gives more specific instructions)

I just wired up head light relays on my K75. The posted instructions were way too much like rocket science and I didn’t understand it until Scott Conary put it in slightly simpler terms. When I finished I got on photoshop and made this diagram, printed it out to a .pdf file for mass viewing. Hope it helps. Feel free to post.

Scott’s words…

You run a big fat fused power lead from the battery. Split into two, one line for each relay – one relay for hi, one for low. Use automotive type relays rated for 30 amps. I bought mine at Radio Shack.

Let’s play with the high beam, we’ll pretend that its power comes through a white wire (it might, I don’t remember). The brown wire going to the headlight is the ground for both beams.

Each relay has 4 ‘plugs’.

  • One for the BIG power in(A).
  • One for the BIG power(B) out.
  • One for small power in(C).
  • One for ground(D).

(The relays have a nice diagram on their underside and the back of the package)

Cut the white wire going to the headlight. Take the piece running from the bike side and attach to ‘C’. Run a ground line to ‘D’. The relay is now a working switch. When the headlight is turned to hi-beam, power runs to ‘C’ then through the relay and out ‘D’. As it goes through it closes a switch allowing power to run from ‘A’ through to ‘B’. Turn off the hi-beam switch on the handlebar, and power stops flowing through ‘C’ to ‘D’ thus breaking the circuit for the BIG power (‘A’ to ‘B’). Nice.

But you still need to hook up the big power. 🙂 Attach one of the Big lines from the battery to ‘A’. Attach the white wire that is still attached to the headlight to ‘B’.

You might beef up the wire if you’re feeling industrious. While you’re there, also beef up as much of the brown lead to the headlight as you can.

And now your hi-beam is run through a relay. Because you used nice big wires, and it doesn’t have to run through the dirty nasty little switch on the handlebar, you can run much more juice to the headlight.

Now do the same for the low beam.

I used spade type connectors so that I could remove the relays if I wanted to. I mounted them just above the headlight, attaching them to the underside of the top center piece of the fairing (the bit the windscreen attaches to). Fuse the main power lead, and use the thickest wire all around that you think is reasonable. Even if you stick to a stock size bulb, you should see a nice improvement in brightness from the headlight.

Robert Laposta
iBMWr ; IBA #7268 ; LPR #71
’87 K75S – “Maybelline”
’75 Honda CL360, which threw me, wench…
’72 Indian ME-70cc dirt bike


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!


Additional Info on Installing Headlight Relays

By: Brian Curry

Thanks to Alessandro Bruno for raising these issues and prompting some elaboration.

Getting a 12 Volt supply from the Starter relay:
The start relay has two large contacts with bolted connections. The wires have red insulation. From memory, the rearmost connection, goes to the battery. Trace this to confirm it, BMW has been known to make changes. Disconnect the battery negative terminal. (This keeps down the sparks if you make a mistake and short something.) Remove the starter relay wiring bolt. Get a crimp connector that the bolt will fit through and that is properly sized for the wire. Crimp the connector on the wire. Bolt the Headlight relay wiring connector with the original connectors onto the relay. (I use some anti-seize on this type of connection.) You now have 12 volts UN-FUSED available when you reconnect the battery negative.
Fusing the 12V supply:
I found my local auto parts store (Pep Boys) had a real nice “main fuse” holder rated at 30 amps. This should be installed in electrical box, in the wire going to the headlight relays. They also had a 30 amp self resetting “circuit breaker” with the same terminal type and spacing as the 30 amp rated fuse holder. IMO, this is a great application. Since getting to a fuse in the electrical box is a PITA, using a self resetting circuit breaker is a very good move. Or if you don’t trust circuit breakers, use a fuse. They also make them in 30 amp ratings. This is plenty for the headlights and the size wire you are running.
Headlights and Driving lights:
If you will be powering both the headlight and driving lights, run two leads. That way if one shorts, you will not lose all the lights.
Use colored wires:
Use different color wires for different parts of the circuits. Otherwise you have lots of wires all the same color and no idea where they go. If you only have one color put wraps of tape on the wire ends. Different number of wraps on different wires. I suggest Red for the wire from the starter relay to the headlight relays. Then other colors from there.
Relay mounting:
Silicone rubber/caulk can be used to stick relays on the inside of fairing or any other panels. No more rattling. Velcro also works well. Then they are easier to take off the bike if it is sold. Don’t let them flop around!!
Relay numbering:
If you use relays with contacts numbered in accordance with the German Industrial Standards (DIN) here is what the contact numbers mean:85 Relay Coil
86 Relay Coil
The control switch and ground/negative can go to either contact:
15  Relay Contact (Movable) Positive normally after the ignition switch
30  Relay Contact (Movable) Normally connected to the Battery
87  Relay Contact Closed when energized
87a Relay Contact Closed when de-energized

While the 12V power lead can be connected to either contact, it is nice to follow the convention with the lead from the start relay connected to 30 and the headlight lead to 87.

A lot of relays, even those made and used in the US, are starting to be DIN labeled, so even if you don’t get it from BMW or another European car dealer, they may well be numbered. Look. If your relays are not labeled, you will have to spend some time with a meter figuring out what is what.

For those in the US and maybe Canada, Radio Shack has a nice little relay at a good price. It is RS part # 275-226. The price is US$5.99. It could be plugged into a socket, and it has a little tab with hole that could be used for screw mounting. It is rated at 30 amps at 12 volts. The terminals are numbered as noted above, and are sized so that you could use push on connectors. The data shows the coil resistance as 66 ohm, and 160 milliamps which is nothing. It also notes that it will pull in at 6 volts and drop out at 3.6 volts. So, there is a “common” relay source for those in the US.

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